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Removed LaTeX version of reference manual. Added ref/ref.ps.

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Guido van Rossum
1996-10-22 20:00:02 +00:00
parent 6a05f951cd
commit 1f17543ee7
20 changed files with 16382 additions and 6770 deletions
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31
Doc/Makefile
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@@ -6,9 +6,14 @@
# This is a bit of a mess. The main documents are:
# tut -- Tutorial (file tut.tex)
# lib -- Library Reference (file lib.tex, inputs lib*.tex)
# ref -- Language Reference (file ref.tex, inputs ref*.tex)
# ext -- Extending and Embedding (file ext.tex)
#
# The Reference Manual is now maintained as a FrameMaker document.
# See the subdirectory ref; PostScript is included as ref/ref.ps.
# (In the future, the Tutorial will also be converted to FrameMaker;
# the other documents will be maintained in a text format such
# as LaTeX or perhaps TIM.)
#
# The main target "make all" creates DVI and PostScript for these
# four. You can also do "make lib" (etc.) to process individual
# documents.
@@ -74,20 +79,19 @@ DOCDESTDIR= $LIBDEST/doc
# Main target
all: all-ps
all-dvi: tut.dvi lib.dvi ref.dvi ext.dvi
all-ps: tut.ps lib.ps ref.ps ext.ps
all-dvi: tut.dvi lib.dvi ext.dvi
all-ps: tut.ps lib.ps ext.ps
# Individual document fake targets
tut: tut.ps
lib: lib.ps
ref: ref.ps
ext: ext.ps
# CWI Quarterly document fake target
qua: qua.ps
# Dependencies
tut.dvi lib.dvi ref.dvi ext.dvi: myformat.sty fix_hack
tut.dvi lib.dvi ext.dvi: myformat.sty fix_hack
# Tutorial document
tut.dvi: tut.tex
@@ -97,18 +101,6 @@ tut.dvi: tut.tex
tut.ps: tut.dvi
$(DVIPS) tut >tut.ps
# Reference document
ref.dvi: ref.tex ref1.tex ref2.tex ref3.tex ref4.tex ref5.tex ref6.tex \
ref7.tex ref8.tex
touch ref.ind
$(LATEX) ref
./fix_hack ref.idx
$(MAKEINDEX) ref
$(LATEX) ref
ref.ps: ref.dvi
$(DVIPS) ref >ref.ps
# LaTeX source files for the Python Library Reference
LIBFILES = lib.tex \
libintro.tex libobjs.tex libtypes.tex libexcs.tex libfuncs.tex \
@@ -216,11 +208,6 @@ l2htut: tut.dvi
@rm -rf python-tut
mv tut python-tut
l2href: ref.dvi
$(L2H) $(L2HARGS) ref.tex
@rm -rf python-ref
mv ref python-ref
l2hext: ext.dvi
$(L2H) $(L2HARGS) ext.tex
@rm -rf python-ext

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\documentstyle[twoside,11pt,myformat]{report}
\title{Python Reference Manual}
\input{boilerplate}
% Tell \index to actually write the .idx file
\makeindex
\begin{document}
\pagenumbering{roman}
\maketitle
\input{copyright}
\begin{abstract}
\noindent
Python is a simple, yet powerful, interpreted programming language
that bridges the gap between C and shell programming, and is thus
ideally suited for ``throw-away programming'' and rapid prototyping.
Its syntax is put together from constructs borrowed from a variety of
other languages; most prominent are influences from ABC, C, Modula-3
and Icon.
The Python interpreter is easily extended with new functions and data
types implemented in C. Python is also suitable as an extension
language for highly customizable C applications such as editors or
window managers.
Python is available for various operating systems, amongst which
several flavors of {\UNIX} (including Linux), the Apple Macintosh O.S.,
MS-DOS, MS-Windows 3.1, Windows NT, and OS/2.
This reference manual describes the syntax and ``core semantics'' of
the language. It is terse, but attempts to be exact and complete.
The semantics of non-essential built-in object types and of the
built-in functions and modules are described in the {\em Python
Library Reference}. For an informal introduction to the language, see
the {\em Python Tutorial}.
\end{abstract}
\pagebreak
{
\parskip = 0mm
\tableofcontents
}
\pagebreak
\pagenumbering{arabic}
\include{ref1} % Introduction
\include{ref2} % Lexical analysis
\include{ref3} % Data model
\include{ref4} % Execution model
\include{ref5} % Expressions and conditions
\include{ref6} % Simple statements
\include{ref7} % Compound statements
\include{ref8} % Top-level components
\input{ref.ind}
\end{document}

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\documentstyle[twoside,11pt,myformat]{report}
\title{Python Reference Manual}
\input{boilerplate}
% Tell \index to actually write the .idx file
\makeindex
\begin{document}
\pagenumbering{roman}
\maketitle
\input{copyright}
\begin{abstract}
\noindent
Python is a simple, yet powerful, interpreted programming language
that bridges the gap between C and shell programming, and is thus
ideally suited for ``throw-away programming'' and rapid prototyping.
Its syntax is put together from constructs borrowed from a variety of
other languages; most prominent are influences from ABC, C, Modula-3
and Icon.
The Python interpreter is easily extended with new functions and data
types implemented in C. Python is also suitable as an extension
language for highly customizable C applications such as editors or
window managers.
Python is available for various operating systems, amongst which
several flavors of {\UNIX} (including Linux), the Apple Macintosh O.S.,
MS-DOS, MS-Windows 3.1, Windows NT, and OS/2.
This reference manual describes the syntax and ``core semantics'' of
the language. It is terse, but attempts to be exact and complete.
The semantics of non-essential built-in object types and of the
built-in functions and modules are described in the {\em Python
Library Reference}. For an informal introduction to the language, see
the {\em Python Tutorial}.
\end{abstract}
\pagebreak
{
\parskip = 0mm
\tableofcontents
}
\pagebreak
\pagenumbering{arabic}
\include{ref1} % Introduction
\include{ref2} % Lexical analysis
\include{ref3} % Data model
\include{ref4} % Execution model
\include{ref5} % Expressions and conditions
\include{ref6} % Simple statements
\include{ref7} % Compound statements
\include{ref8} % Top-level components
\input{ref.ind}
\end{document}

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\chapter{Introduction}
This reference manual describes the Python programming language.
It is not intended as a tutorial.
While I am trying to be as precise as possible, I chose to use English
rather than formal specifications for everything except syntax and
lexical analysis. This should make the document more understandable
to the average reader, but will leave room for ambiguities.
Consequently, if you were coming from Mars and tried to re-implement
Python from this document alone, you might have to guess things and in
fact you would probably end up implementing quite a different language.
On the other hand, if you are using
Python and wonder what the precise rules about a particular area of
the language are, you should definitely be able to find them here.
It is dangerous to add too many implementation details to a language
reference document --- the implementation may change, and other
implementations of the same language may work differently. On the
other hand, there is currently only one Python implementation, and
its particular quirks are sometimes worth being mentioned, especially
where the implementation imposes additional limitations. Therefore,
you'll find short ``implementation notes'' sprinkled throughout the
text.
Every Python implementation comes with a number of built-in and
standard modules. These are not documented here, but in the separate
{\em Python Library Reference} document. A few built-in modules are
mentioned when they interact in a significant way with the language
definition.
\section{Notation}
The descriptions of lexical analysis and syntax use a modified BNF
grammar notation. This uses the following style of definition:
\index{BNF}
\index{grammar}
\index{syntax}
\index{notation}
\begin{verbatim}
name: lc_letter (lc_letter | "_")*
lc_letter: "a"..."z"
\end{verbatim}
The first line says that a \verb@name@ is an \verb@lc_letter@ followed by
a sequence of zero or more \verb@lc_letter@s and underscores. An
\verb@lc_letter@ in turn is any of the single characters `a' through `z'.
(This rule is actually adhered to for the names defined in lexical and
grammar rules in this document.)
Each rule begins with a name (which is the name defined by the rule)
and a colon. A vertical bar (\verb@|@) is used to separate
alternatives; it is the least binding operator in this notation. A
star (\verb@*@) means zero or more repetitions of the preceding item;
likewise, a plus (\verb@+@) means one or more repetitions, and a
phrase enclosed in square brackets (\verb@[ ]@) means zero or one
occurrences (in other words, the enclosed phrase is optional). The
\verb@*@ and \verb@+@ operators bind as tightly as possible;
parentheses are used for grouping. Literal strings are enclosed in
quotes. White space is only meaningful to separate tokens.
Rules are normally contained on a single line; rules with many
alternatives may be formatted alternatively with each line after the
first beginning with a vertical bar.
In lexical definitions (as the example above), two more conventions
are used: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of \ASCII{}
characters. A phrase between angular brackets (\verb@<...>@) gives an
informal description of the symbol defined; e.g. this could be used
to describe the notion of `control character' if needed.
\index{lexical definitions}
\index{ASCII}
Even though the notation used is almost the same, there is a big
difference between the meaning of lexical and syntactic definitions:
a lexical definition operates on the individual characters of the
input source, while a syntax definition operates on the stream of
tokens generated by the lexical analysis. All uses of BNF in the next
chapter (``Lexical Analysis'') are lexical definitions; uses in
subsequent chapters are syntactic definitions.

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\chapter{Lexical analysis}
A Python program is read by a {\em parser}. Input to the parser is a
stream of {\em tokens}, generated by the {\em lexical analyzer}. This
chapter describes how the lexical analyzer breaks a file into tokens.
\index{lexical analysis}
\index{parser}
\index{token}
\section{Line structure}
A Python program is divided in a number of logical lines. The end of
a logical line is represented by the token NEWLINE. Statements cannot
cross logical line boundaries except where NEWLINE is allowed by the
syntax (e.g. between statements in compound statements).
\index{line structure}
\index{logical line}
\index{NEWLINE token}
\subsection{Comments}
A comment starts with a hash character (\verb@#@) that is not part of
a string literal, and ends at the end of the physical line. A comment
always signifies the end of the logical line. Comments are ignored by
the syntax.
\index{comment}
\index{logical line}
\index{physical line}
\index{hash character}
\subsection{Explicit line joining}
Two or more physical lines may be joined into logical lines using
backslash characters (\verb/\/), as follows: when a physical line ends
in a backslash that is not part of a string literal or comment, it is
joined with the following forming a single logical line, deleting the
backslash and the following end-of-line character. For example:
\index{physical line}
\index{line joining}
\index{line continuation}
\index{backslash character}
%
\begin{verbatim}
if 1900 < year < 2100 and 1 <= month <= 12 \
and 1 <= day <= 31 and 0 <= hour < 24 \
and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
return 1
\end{verbatim}
A line ending in a backslash cannot carry a comment; a backslash does
not continue a comment (but it does continue a string literal, see
below).
\subsection{Implicit line joining}
Expressions in parentheses, square brackets or curly braces can be
split over more than one physical line without using backslashes.
For example:
\begin{verbatim}
month_names = ['Januari', 'Februari', 'Maart', # These are the
'April', 'Mei', 'Juni', # Dutch names
'Juli', 'Augustus', 'September', # for the months
'Oktober', 'November', 'December'] # of the year
\end{verbatim}
Implicitly continued lines can carry comments. The indentation of the
continuation lines is not important. Blank continuation lines are
allowed.
\subsection{Blank lines}
A logical line that contains only spaces, tabs, and possibly a
comment, is ignored (i.e., no NEWLINE token is generated), except that
during interactive input of statements, an entirely blank logical line
terminates a multi-line statement.
\index{blank line}
\subsection{Indentation}
Leading whitespace (spaces and tabs) at the beginning of a logical
line is used to compute the indentation level of the line, which in
turn is used to determine the grouping of statements.
\index{indentation}
\index{whitespace}
\index{leading whitespace}
\index{space}
\index{tab}
\index{grouping}
\index{statement grouping}
First, tabs are replaced (from left to right) by one to eight spaces
such that the total number of characters up to there is a multiple of
eight (this is intended to be the same rule as used by {\UNIX}). The
total number of spaces preceding the first non-blank character then
determines the line's indentation. Indentation cannot be split over
multiple physical lines using backslashes.
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.
\index{INDENT token}
\index{DEDENT token}
Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again. The numbers pushed on
the stack will always be strictly increasing from bottom to top. At
the beginning of each logical line, the line's indentation level is
compared to the top of the stack. If it is equal, nothing happens.
If it is larger, it is pushed on the stack, and one INDENT token is
generated. If it is smaller, it {\em must} be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is
generated. At the end of the file, a DEDENT token is generated for
each number remaining on the stack that is larger than zero.
Here is an example of a correctly (though confusingly) indented piece
of Python code:
\begin{verbatim}
def perm(l):
# Compute the list of all permutations of l
if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
\end{verbatim}
The following example shows various indentation errors:
\begin{verbatim}
def perm(l): # error: first line indented
for i in range(len(l)): # error: not indented
s = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:]) # error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r # error: inconsistent dedent
\end{verbatim}
(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer --- the indentation of
\verb@return r@ does not match a level popped off the stack.)
\section{Other tokens}
Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
exist: identifiers, keywords, literals, operators, and delimiters.
Spaces and tabs are not tokens, but serve to delimit tokens. Where
ambiguity exists, a token comprises the longest possible string that
forms a legal token, when read from left to right.
\section{Identifiers}
Identifiers (also referred to as names) are described by the following
lexical definitions:
\index{identifier}
\index{name}
\begin{verbatim}
identifier: (letter|"_") (letter|digit|"_")*
letter: lowercase | uppercase
lowercase: "a"..."z"
uppercase: "A"..."Z"
digit: "0"..."9"
\end{verbatim}
Identifiers are unlimited in length. Case is significant.
\subsection{Keywords}
The following identifiers are used as reserved words, or {\em
keywords} of the language, and cannot be used as ordinary
identifiers. They must be spelled exactly as written here:
\index{keyword}
\index{reserved word}
\begin{verbatim}
and elif global not try
break else if or while
class except import pass
continue finally in print
def for is raise
del from lambda return
\end{verbatim}
% When adding keywords, pipe it through keywords.py for reformatting
\section{Literals} \label{literals}
Literals are notations for constant values of some built-in types.
\index{literal}
\index{constant}
\subsection{String literals}
String literals are described by the following lexical definitions:
\index{string literal}
\begin{verbatim}
stringliteral: shortstring | longstring
shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
shortstringitem: shortstringchar | escapeseq
longstringitem: longstringchar | escapeseq
shortstringchar: <any ASCII character except "\" or newline or the quote>
longstringchar: <any ASCII character except "\">
escapeseq: "\" <any ASCII character>
\end{verbatim}
\index{ASCII}
In ``long strings'' (strings surrounded by sets of three quotes),
unescaped newlines and quotes are allowed (and are retained), except
that three unescaped quotes in a row terminate the string. (A
``quote'' is the character used to open the string, i.e. either
\verb/'/ or \verb/"/.)
Escape sequences in strings are interpreted according to rules similar
to those used by Standard C. The recognized escape sequences are:
\index{physical line}
\index{escape sequence}
\index{Standard C}
\index{C}
\begin{center}
\begin{tabular}{|l|l|}
\hline
\verb/\/{\em newline} & Ignored \\
\verb/\\/ & Backslash (\verb/\/) \\
\verb/\'/ & Single quote (\verb/'/) \\
\verb/\"/ & Double quote (\verb/"/) \\
\verb/\a/ & \ASCII{} Bell (BEL) \\
\verb/\b/ & \ASCII{} Backspace (BS) \\
%\verb/\E/ & \ASCII{} Escape (ESC) \\
\verb/\f/ & \ASCII{} Formfeed (FF) \\
\verb/\n/ & \ASCII{} Linefeed (LF) \\
\verb/\r/ & \ASCII{} Carriage Return (CR) \\
\verb/\t/ & \ASCII{} Horizontal Tab (TAB) \\
\verb/\v/ & \ASCII{} Vertical Tab (VT) \\
\verb/\/{\em ooo} & \ASCII{} character with octal value {\em ooo} \\
\verb/\x/{\em xx...} & \ASCII{} character with hex value {\em xx...} \\
\hline
\end{tabular}
\end{center}
\index{ASCII}
In strict compatibility with Standard C, up to three octal digits are
accepted, but an unlimited number of hex digits is taken to be part of
the hex escape (and then the lower 8 bits of the resulting hex number
are used in all current implementations...).
All unrecognized escape sequences are left in the string unchanged,
i.e., {\em the backslash is left in the string.} (This behavior is
useful when debugging: if an escape sequence is mistyped, the
resulting output is more easily recognized as broken. It also helps a
great deal for string literals used as regular expressions or
otherwise passed to other modules that do their own escape handling.)
\index{unrecognized escape sequence}
\subsection{Numeric literals}
There are three types of numeric literals: plain integers, long
integers, and floating point numbers.
\index{number}
\index{numeric literal}
\index{integer literal}
\index{plain integer literal}
\index{long integer literal}
\index{floating point literal}
\index{hexadecimal literal}
\index{octal literal}
\index{decimal literal}
Integer and long integer literals are described by the following
lexical definitions:
\begin{verbatim}
longinteger: integer ("l"|"L")
integer: decimalinteger | octinteger | hexinteger
decimalinteger: nonzerodigit digit* | "0"
octinteger: "0" octdigit+
hexinteger: "0" ("x"|"X") hexdigit+
nonzerodigit: "1"..."9"
octdigit: "0"..."7"
hexdigit: digit|"a"..."f"|"A"..."F"
\end{verbatim}
Although both lower case `l' and upper case `L' are allowed as suffix
for long integers, it is strongly recommended to always use `L', since
the letter `l' looks too much like the digit `1'.
Plain integer decimal literals must be at most 2147483647 (i.e., the
largest positive integer, using 32-bit arithmetic). Plain octal and
hexadecimal literals may be as large as 4294967295, but values larger
than 2147483647 are converted to a negative value by subtracting
4294967296. There is no limit for long integer literals apart from
what can be stored in available memory.
Some examples of plain and long integer literals:
\begin{verbatim}
7 2147483647 0177 0x80000000
3L 79228162514264337593543950336L 0377L 0x100000000L
\end{verbatim}
Floating point literals are described by the following lexical
definitions:
\begin{verbatim}
floatnumber: pointfloat | exponentfloat
pointfloat: [intpart] fraction | intpart "."
exponentfloat: (intpart | pointfloat) exponent
intpart: digit+
fraction: "." digit+
exponent: ("e"|"E") ["+"|"-"] digit+
\end{verbatim}
The allowed range of floating point literals is
implementation-dependent.
Some examples of floating point literals:
\begin{verbatim}
3.14 10. .001 1e100 3.14e-10
\end{verbatim}
Note that numeric literals do not include a sign; a phrase like
\verb@-1@ is actually an expression composed of the operator
\verb@-@ and the literal \verb@1@.
\section{Operators}
The following tokens are operators:
\index{operators}
\begin{verbatim}
+ - * / %
<< >> & | ^ ~
< == > <= <> != >=
\end{verbatim}
The comparison operators \verb@<>@ and \verb@!=@ are alternate
spellings of the same operator.
\section{Delimiters}
The following tokens serve as delimiters or otherwise have a special
meaning:
\index{delimiters}
\begin{verbatim}
( ) [ ] { }
, : . " ` '
= ;
\end{verbatim}
The following printing \ASCII{} characters are not used in Python. Their
occurrence outside string literals and comments is an unconditional
error:
\index{ASCII}
\begin{verbatim}
@ $ ?
\end{verbatim}
They may be used by future versions of the language though!

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\chapter{Execution model}
\index{execution model}
\section{Code blocks, execution frames, and name spaces} \label{execframes}
\index{code block}
\indexii{execution}{frame}
\index{name space}
A {\em code block} is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code blocks may
textually contain other code blocks. Code blocks may invoke other
code blocks (that may or may not be textually contained in them) as
part of their execution, e.g. by invoking (calling) a function.
\index{code block}
\indexii{code}{block}
The following are code blocks: A module is a code block. A function
body is a code block. A class definition is a code block. Each
command typed interactively is a separate code block; a script file is
a code block. The string argument passed to the built-in function
\verb@eval@ and to the \verb@exec@ statement are code blocks.
And finally, the
expression read and evaluated by the built-in function \verb@input@ is
a code block.
A code block is executed in an execution frame. An {\em execution
frame} contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
name spaces, the local and the global name space, that affect
execution of the code block.
\indexii{execution}{frame}
A {\em name space} is a mapping from names (identifiers) to objects.
A particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called {\em binding} a name (to an object); changing the mapping of
a name is called {\em rebinding}; removing a name is {\em unbinding}.
Name spaces are functionally equivalent to dictionaries.
\index{name space}
\indexii{binding}{name}
\indexii{rebinding}{name}
\indexii{unbinding}{name}
The {\em local name space} of an execution frame determines the default
place where names are defined and searched. The {\em global name
space} determines the place where names listed in \verb@global@
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
\indexii{local}{name space}
\indexii{global}{name space}
\stindex{global}
Whether a name is local or global in a code block is determined by
static inspection of the source text for the code block: in the
absence of \verb@global@ statements, a name that is bound anywhere in
the code block is local in the entire code block; all other names are
considered global. The \verb@global@ statement forces global
interpretation of selected names throughout the code block. The
following constructs bind names: formal parameters, \verb@import@
statements, class and function definitions (these bind the class or
function name), and targets that are identifiers if occurring in an
assignment, \verb@for@ loop header, or \verb@except@ clause header.
A target occurring in a \verb@del@ statement is also considered bound
for this purpose (though the actual semantics are to ``unbind'' the
name).
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (which is actually the
global name space of the module \verb@__builtin__@). When a name is not
found at all, the \verb@NameError@ exception is raised.%
\footnote{If the code block contains {\tt exec} statements or the
construct {\tt from \ldots import *}, the semantics of names not
explicitly mentioned in a {\tt global} statement change subtly: name
lookup first searches the local name space, then the global one, then
the built-in one.}
\bimodindex{__builtin__}
\stindex{from}
\stindex{exec}
\stindex{global}
\ttindex{NameError}
The following table lists the meaning of the local and global name
space for various types of code blocks. The name space for a
particular module is automatically created when the module is first
referenced. Note that in almost all cases, the global name space is
the name space of the containing module --- scopes in Python do not
nest!
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
Code block type & Global name space & Local name space & Notes \\
\hline
Module & n.s. for this module & same as global & \\
Script & n.s. for \verb@__main__@ & same as global & \\
Interactive command & n.s. for \verb@__main__@ & same as global & \\
Class definition & global n.s. of containing block & new n.s. & \\
Function body & global n.s. of containing block & new n.s. & (2) \\
String passed to \verb@exec@ statement
& global n.s. of containing block
& local n.s. of containing block & (1) \\
String passed to \verb@eval()@
& global n.s. of caller & local n.s. of caller & (1) \\
File read by \verb@execfile()@
& global n.s. of caller & local n.s. of caller & (1) \\
Expression read by \verb@input@
& global n.s. of caller & local n.s. of caller & \\
\hline
\end{tabular}
\end{center}
\bimodindex{__main__}
Notes:
\begin{description}
\item[n.s.] means {\em name space}
\item[(1)] The global and local name space for these can be
overridden with optional extra arguments.
\item[(2)] The body of lambda forms (see section \ref{lambda}) is
treated exactly the same as a (nested) function definition. Lambda
forms have their own name space consisting of their formal arguments.
\indexii{lambda}{form}
\end{description}
The built-in functions \verb@globals()@ and \verb@locals()@ returns a
dictionary representing the current global and local name space,
respectively. The effect of modifications to this dictionary on the
name space are undefined.%
\footnote{The current implementations return the dictionary actually
used to implement the name space, {\em except} for functions, where
the optimizer may cause the local name space to be implemented
differently, and \verb@locals()@ returns a read-only dictionary.}
\section{Exceptions}
Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors or other exceptional
conditions. An exception is {\em raised} at the point where the error
is detected; it may be {\em handled} by the surrounding code block or
by any code block that directly or indirectly invoked the code block
where the error occurred.
\index{exception}
\index{raise an exception}
\index{handle an exception}
\index{exception handler}
\index{errors}
\index{error handling}
The Python interpreter raises an exception when it detects an run-time
error (such as division by zero). A Python program can also
explicitly raise an exception with the \verb@raise@ statement.
Exception handlers are specified with the \verb@try...except@
statement.
Python uses the ``termination'' model of error handling: an exception
handler can find out what happened and continue execution at an outer
level, but it cannot repair the cause of the error and retry the
failing operation (except by re-entering the the offending piece of
code from the top).
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
Exceptions are identified by string objects or class instances. Two
different string objects with the same value identify different
exceptions. An exception can be raised with a class instance. Such
exceptions are caught by specifying an except clause that has the
class name (or a base class) as the condition.
When an exception is raised, an object (maybe \verb@None@) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
exception handler as additional information. For exceptions raised
with a class instance, the instance is passed as the ``parameter''.
For example:
\begin{verbatim}
>>> class Error:
... def __init__(self, msg): self.msg = msg
...
>>> class SpecificError(Error): pass
...
>>> try:
... raise SpecificError('broken')
... except Error, obj:
... print obj.msg
...
broken
\end{verbatim}
See also the description of the \verb@try@ and \verb@raise@
statements.

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\chapter{Compound statements}
\indexii{compound}{statement}
Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb@if@, \verb@while@ and \verb@for@ statements implement
traditional control flow constructs. \verb@try@ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses'. A clause
consists of a header and a `suite'. The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a
clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb@if@ clause a following \verb@else@ clause would
belong:
\index{clause}
\index{suite}
\begin{verbatim}
if test1: if test2: print x
\end{verbatim}
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb@print@ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
\end{verbatim}
Summarizing:
\begin{verbatim}
compound_stmt: if_stmt | while_stmt | for_stmt
| try_stmt | funcdef | classdef
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb@NEWLINE@ possibly followed
by a \verb@DEDENT@.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb@else@' problem is solved in Python by requiring
nested \verb@if@ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb@if@ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
("elif" condition ":" suite)*
["else" ":" suite]
\end{verbatim}
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb@if@ statement is executed or evaluated). If
all conditions are false, the suite of the \verb@else@ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
\section{The {\tt while} statement}
\stindex{while}
\indexii{loop}{statement}
The \verb@while@ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
while_stmt: "while" condition ":" suite
["else" ":" suite]
\end{verbatim}
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb@else@ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
\section{The {\tt for} statement}
\stindex{for}
\indexii{loop}{statement}
The \verb@for@ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
\begin{verbatim}
for_stmt: "for" target_list "in" condition_list ":" suite
["else" ":" suite]
\end{verbatim}
The condition list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb@else@ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb@else@
clause if there was no next item.
\stindex{break}
\stindex{continue}
The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.
The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb@range()@ returns a sequence of
integers suitable to emulate the effect of Pascal's
\verb@for i := a to b do@;
e.g. \verb@range(3)@ returns the list \verb@[0, 1, 2]@.
\bifuncindex{range}
\index{Pascal}
{\bf Warning:} There is a subtlety when the sequence is being modified
by the loop (this can only occur for mutable sequences, i.e. lists).
An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means that
if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a temporary
copy using a slice of the whole sequence, e.g.
\index{loop!over mutable sequence}
\index{mutable sequence!loop over}
\begin{verbatim}
for x in a[:]:
if x < 0: a.remove(x)
\end{verbatim}
\section{The {\tt try} statement} \label{try}
\stindex{try}
The \verb@try@ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
["else" ":" suite]
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb@try@ statement: \verb@try...except@ and
\verb@try...finally@. These forms cannot be mixed.
The \verb@try...except@ form specifies one or more exception handlers
(the \verb@except@ clauses). When no exception occurs in the
\verb@try@ clause, no exception handler is executed. When an
exception occurs in the \verb@try@ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
except clause with a condition, that condition is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception, or (for exceptions
that are classes) it is a base class of the exception, or it is a
tuple containing an item that is compatible with the exception. Note
that the object identities must match, i.e. it must be the same
object, not just an object with the same value.
\kwindex{except}
If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb@try@ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. When the end of this suite
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the \verb@sys@ module:
\verb@sys.exc_type@ receives the object identifying the exception;
\verb@sys.exc_value@ receives the exception's parameter;
\verb@sys.exc_traceback@ receives a traceback object (see section
\ref{traceback}) identifying the point in the program where the
exception occurred.
\bimodindex{sys}
\ttindex{exc_type}
\ttindex{exc_value}
\ttindex{exc_traceback}
\obindex{traceback}
The optional \verb@else@ clause is executed when no exception occurs
in the \verb@try@ clause. Exceptions in the \verb@else@ clause are
not handled by the preceding \verb@except@ clauses.
\kwindex{else}
The \verb@try...finally@ form specifies a `cleanup' handler. The
\verb@try@ clause is executed. When no exception occurs, the
\verb@finally@ clause is executed. When an exception occurs in the
\verb@try@ clause, the exception is temporarily saved, the
\verb@finally@ clause is executed, and then the saved exception is
re-raised. If the \verb@finally@ clause raises another exception or
executes a \verb@return@, \verb@break@ or \verb@continue@ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb@return@ or \verb@break@ statement is executed in the
\verb@try@ suite of a \verb@try...finally@ statement, the
\verb@finally@ clause is also executed `on the way out'. A
\verb@continue@ statement is illegal in the \verb@try@ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
\stindex{break}
\stindex{continue}
\section{Function definitions} \label{function}
\indexii{function}{definition}
A function definition defines a user-defined function object (see
section \ref{types}):\footnote{The new syntax to receive arbitrary
keyword arguments is not yet documented in this manual. See chapter
12 of the Tutorial.}
\obindex{user-defined function}
\obindex{function}
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (defparameter ",")* ("*" identifier [, "**" identifier]
| "**" identifier
| defparameter [","])
defparameter: parameter ["=" condition]
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
\end{verbatim}
A function definition is an executable statement. Its execution binds
the function name in the current local name space to a function object
(a wrapper around the executable code for the function). This
function object contains a reference to the current global name space
as the global name space to be used when the function is called.
\indexii{function}{name}
\indexii{name}{binding}
The function definition does not execute the function body; this gets
executed only when the function is called.
When one or more top-level parameters have the form {\em parameter =
condition}, the function is said to have ``default parameter values''.
Default parameter values are evaluated when the function definition is
executed. For a parameter with a default value, the correponding
argument may be omitted from a call, in which case the parameter's
default value is substituted. If a parameter has a default value, all
following parameters must also have a default value --- this is a
syntactic restriction that is not expressed by the grammar.%
\footnote{Currently this is not checked; instead,
{\tt def f(a=1,b)} is interpreted as {\tt def f(a=1,b=None)}.}
\indexiii{default}{parameter}{value}
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, first missing arguments for which a
default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
\index{parameter}
\index{argument}
\indexii{parameter}{formal}
\indexii{parameter}{actual}
\begin{itemize}
\item
If there are no formal parameters, there must be no arguments.
\item
If the formal parameter list does not end in a star followed by an
identifier, there must be exactly as many arguments as there are
parameters in the formal parameter list (at the top level); the
arguments are assigned to the formal parameters one by one. Note that
the presence or absence of a trailing comma at the top level in either
the formal or the actual parameter list makes no difference. The
assignment to a formal parameter is performed as if the parameter
occurs on the left hand side of an assignment statement whose right
hand side's value is that of the argument.
\item
If the formal parameter list ends in a star followed by an identifier,
preceded by zero or more comma-followed parameters, there must be at
least as many arguments as there are parameters preceding the star.
Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb@()@, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
\end{itemize}
Note that the `variable length parameter list' feature only works at
the top level of the parameter list; individual parameters use a model
corresponding more closely to that of ordinary assignment. While the
latter model is generally preferable, because of the greater type
safety it offers (wrong-sized tuples aren't silently mistreated),
variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\indexii{lambda}{form}
\section{Class definitions} \label{class}
\indexii{class}{definition}
A class definition defines a class object (see section \ref{types}):
\obindex{class}
\begin{verbatim}
classdef: "class" classname [inheritance] ":" suite
inheritance: "(" [condition_list] ")"
classname: identifier
\end{verbatim}
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list
should evaluate to a class object. The class's suite is then executed
in a new execution frame (see section \ref{execframes}), using a newly
created local name space and the original global name space.
(Usually, the suite contains only function definitions.) When the
class's suite finishes execution, its execution frame is discarded but
its local name space is saved. A class object is then created using
the inheritance list for the base classes and the saved local name
space for the attribute dictionary. The class name is bound to this
class object in the original local name space.
\index{inheritance}
\indexii{class}{name}
\indexii{name}{binding}
\indexii{execution}{frame}

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\chapter{Top-level components}
The Python interpreter can get its input from a number of sources:
from a script passed to it as standard input or as program argument,
typed in interactively, from a module source file, etc. This chapter
gives the syntax used in these cases.
\index{interpreter}
\section{Complete Python programs}
\index{program}
While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb@sys@
(various system services), \verb@__builtin__@ (built-in functions,
exceptions and \verb@None@) and \verb@__main__@. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
\bimodindex{__main__}
\bimodindex{__builtin__}
The syntax for a complete Python program is that for file input,
described in the next section.
The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb@__main__@.
\index{interactive mode}
\bimodindex{__main__}
Under {\UNIX}, a complete program can be passed to the interpreter in
three forms: with the {\bf -c} {\it string} command line option, as a
file passed as the first command line argument, or as standard input.
If the file or standard input is a tty device, the interpreter enters
interactive mode; otherwise, it executes the file as a complete
program.
\index{UNIX}
\index{command line}
\index{standard input}
\section{File input}
All input read from non-interactive files has the same form:
\begin{verbatim}
file_input: (NEWLINE | statement)*
\end{verbatim}
This syntax is used in the following situations:
\begin{itemize}
\item when parsing a complete Python program (from a file or from a string);
\item when parsing a module;
\item when parsing a string passed to the \verb@exec@ statement;
\end{itemize}
\section{Interactive input}
Input in interactive mode is parsed using the following grammar:
\begin{verbatim}
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
\end{verbatim}
Note that a (top-level) compound statement must be followed by a blank
line in interactive mode; this is needed to help the parser detect the
end of the input.
\section{Expression input}
\index{input}
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb@eval()@ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb@input()@ must have the following form:
\bifuncindex{input}
\begin{verbatim}
input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb@raw_input()@ or the \verb@readline()@ method
of file objects.
\obindex{file}
\index{input!raw}
\index{raw input}
\bifuncindex{raw_index}
\ttindex{readline}

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\chapter{Introduction}
This reference manual describes the Python programming language.
It is not intended as a tutorial.
While I am trying to be as precise as possible, I chose to use English
rather than formal specifications for everything except syntax and
lexical analysis. This should make the document more understandable
to the average reader, but will leave room for ambiguities.
Consequently, if you were coming from Mars and tried to re-implement
Python from this document alone, you might have to guess things and in
fact you would probably end up implementing quite a different language.
On the other hand, if you are using
Python and wonder what the precise rules about a particular area of
the language are, you should definitely be able to find them here.
It is dangerous to add too many implementation details to a language
reference document --- the implementation may change, and other
implementations of the same language may work differently. On the
other hand, there is currently only one Python implementation, and
its particular quirks are sometimes worth being mentioned, especially
where the implementation imposes additional limitations. Therefore,
you'll find short ``implementation notes'' sprinkled throughout the
text.
Every Python implementation comes with a number of built-in and
standard modules. These are not documented here, but in the separate
{\em Python Library Reference} document. A few built-in modules are
mentioned when they interact in a significant way with the language
definition.
\section{Notation}
The descriptions of lexical analysis and syntax use a modified BNF
grammar notation. This uses the following style of definition:
\index{BNF}
\index{grammar}
\index{syntax}
\index{notation}
\begin{verbatim}
name: lc_letter (lc_letter | "_")*
lc_letter: "a"..."z"
\end{verbatim}
The first line says that a \verb@name@ is an \verb@lc_letter@ followed by
a sequence of zero or more \verb@lc_letter@s and underscores. An
\verb@lc_letter@ in turn is any of the single characters `a' through `z'.
(This rule is actually adhered to for the names defined in lexical and
grammar rules in this document.)
Each rule begins with a name (which is the name defined by the rule)
and a colon. A vertical bar (\verb@|@) is used to separate
alternatives; it is the least binding operator in this notation. A
star (\verb@*@) means zero or more repetitions of the preceding item;
likewise, a plus (\verb@+@) means one or more repetitions, and a
phrase enclosed in square brackets (\verb@[ ]@) means zero or one
occurrences (in other words, the enclosed phrase is optional). The
\verb@*@ and \verb@+@ operators bind as tightly as possible;
parentheses are used for grouping. Literal strings are enclosed in
quotes. White space is only meaningful to separate tokens.
Rules are normally contained on a single line; rules with many
alternatives may be formatted alternatively with each line after the
first beginning with a vertical bar.
In lexical definitions (as the example above), two more conventions
are used: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of \ASCII{}
characters. A phrase between angular brackets (\verb@<...>@) gives an
informal description of the symbol defined; e.g. this could be used
to describe the notion of `control character' if needed.
\index{lexical definitions}
\index{ASCII}
Even though the notation used is almost the same, there is a big
difference between the meaning of lexical and syntactic definitions:
a lexical definition operates on the individual characters of the
input source, while a syntax definition operates on the stream of
tokens generated by the lexical analysis. All uses of BNF in the next
chapter (``Lexical Analysis'') are lexical definitions; uses in
subsequent chapters are syntactic definitions.

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\chapter{Lexical analysis}
A Python program is read by a {\em parser}. Input to the parser is a
stream of {\em tokens}, generated by the {\em lexical analyzer}. This
chapter describes how the lexical analyzer breaks a file into tokens.
\index{lexical analysis}
\index{parser}
\index{token}
\section{Line structure}
A Python program is divided in a number of logical lines. The end of
a logical line is represented by the token NEWLINE. Statements cannot
cross logical line boundaries except where NEWLINE is allowed by the
syntax (e.g. between statements in compound statements).
\index{line structure}
\index{logical line}
\index{NEWLINE token}
\subsection{Comments}
A comment starts with a hash character (\verb@#@) that is not part of
a string literal, and ends at the end of the physical line. A comment
always signifies the end of the logical line. Comments are ignored by
the syntax.
\index{comment}
\index{logical line}
\index{physical line}
\index{hash character}
\subsection{Explicit line joining}
Two or more physical lines may be joined into logical lines using
backslash characters (\verb/\/), as follows: when a physical line ends
in a backslash that is not part of a string literal or comment, it is
joined with the following forming a single logical line, deleting the
backslash and the following end-of-line character. For example:
\index{physical line}
\index{line joining}
\index{line continuation}
\index{backslash character}
%
\begin{verbatim}
if 1900 < year < 2100 and 1 <= month <= 12 \
and 1 <= day <= 31 and 0 <= hour < 24 \
and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
return 1
\end{verbatim}
A line ending in a backslash cannot carry a comment; a backslash does
not continue a comment (but it does continue a string literal, see
below).
\subsection{Implicit line joining}
Expressions in parentheses, square brackets or curly braces can be
split over more than one physical line without using backslashes.
For example:
\begin{verbatim}
month_names = ['Januari', 'Februari', 'Maart', # These are the
'April', 'Mei', 'Juni', # Dutch names
'Juli', 'Augustus', 'September', # for the months
'Oktober', 'November', 'December'] # of the year
\end{verbatim}
Implicitly continued lines can carry comments. The indentation of the
continuation lines is not important. Blank continuation lines are
allowed.
\subsection{Blank lines}
A logical line that contains only spaces, tabs, and possibly a
comment, is ignored (i.e., no NEWLINE token is generated), except that
during interactive input of statements, an entirely blank logical line
terminates a multi-line statement.
\index{blank line}
\subsection{Indentation}
Leading whitespace (spaces and tabs) at the beginning of a logical
line is used to compute the indentation level of the line, which in
turn is used to determine the grouping of statements.
\index{indentation}
\index{whitespace}
\index{leading whitespace}
\index{space}
\index{tab}
\index{grouping}
\index{statement grouping}
First, tabs are replaced (from left to right) by one to eight spaces
such that the total number of characters up to there is a multiple of
eight (this is intended to be the same rule as used by {\UNIX}). The
total number of spaces preceding the first non-blank character then
determines the line's indentation. Indentation cannot be split over
multiple physical lines using backslashes.
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.
\index{INDENT token}
\index{DEDENT token}
Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again. The numbers pushed on
the stack will always be strictly increasing from bottom to top. At
the beginning of each logical line, the line's indentation level is
compared to the top of the stack. If it is equal, nothing happens.
If it is larger, it is pushed on the stack, and one INDENT token is
generated. If it is smaller, it {\em must} be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is
generated. At the end of the file, a DEDENT token is generated for
each number remaining on the stack that is larger than zero.
Here is an example of a correctly (though confusingly) indented piece
of Python code:
\begin{verbatim}
def perm(l):
# Compute the list of all permutations of l
if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
\end{verbatim}
The following example shows various indentation errors:
\begin{verbatim}
def perm(l): # error: first line indented
for i in range(len(l)): # error: not indented
s = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:]) # error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r # error: inconsistent dedent
\end{verbatim}
(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer --- the indentation of
\verb@return r@ does not match a level popped off the stack.)
\section{Other tokens}
Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
exist: identifiers, keywords, literals, operators, and delimiters.
Spaces and tabs are not tokens, but serve to delimit tokens. Where
ambiguity exists, a token comprises the longest possible string that
forms a legal token, when read from left to right.
\section{Identifiers}
Identifiers (also referred to as names) are described by the following
lexical definitions:
\index{identifier}
\index{name}
\begin{verbatim}
identifier: (letter|"_") (letter|digit|"_")*
letter: lowercase | uppercase
lowercase: "a"..."z"
uppercase: "A"..."Z"
digit: "0"..."9"
\end{verbatim}
Identifiers are unlimited in length. Case is significant.
\subsection{Keywords}
The following identifiers are used as reserved words, or {\em
keywords} of the language, and cannot be used as ordinary
identifiers. They must be spelled exactly as written here:
\index{keyword}
\index{reserved word}
\begin{verbatim}
and elif global not try
break else if or while
class except import pass
continue finally in print
def for is raise
del from lambda return
\end{verbatim}
% When adding keywords, pipe it through keywords.py for reformatting
\section{Literals} \label{literals}
Literals are notations for constant values of some built-in types.
\index{literal}
\index{constant}
\subsection{String literals}
String literals are described by the following lexical definitions:
\index{string literal}
\begin{verbatim}
stringliteral: shortstring | longstring
shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
shortstringitem: shortstringchar | escapeseq
longstringitem: longstringchar | escapeseq
shortstringchar: <any ASCII character except "\" or newline or the quote>
longstringchar: <any ASCII character except "\">
escapeseq: "\" <any ASCII character>
\end{verbatim}
\index{ASCII}
In ``long strings'' (strings surrounded by sets of three quotes),
unescaped newlines and quotes are allowed (and are retained), except
that three unescaped quotes in a row terminate the string. (A
``quote'' is the character used to open the string, i.e. either
\verb/'/ or \verb/"/.)
Escape sequences in strings are interpreted according to rules similar
to those used by Standard C. The recognized escape sequences are:
\index{physical line}
\index{escape sequence}
\index{Standard C}
\index{C}
\begin{center}
\begin{tabular}{|l|l|}
\hline
\verb/\/{\em newline} & Ignored \\
\verb/\\/ & Backslash (\verb/\/) \\
\verb/\'/ & Single quote (\verb/'/) \\
\verb/\"/ & Double quote (\verb/"/) \\
\verb/\a/ & \ASCII{} Bell (BEL) \\
\verb/\b/ & \ASCII{} Backspace (BS) \\
%\verb/\E/ & \ASCII{} Escape (ESC) \\
\verb/\f/ & \ASCII{} Formfeed (FF) \\
\verb/\n/ & \ASCII{} Linefeed (LF) \\
\verb/\r/ & \ASCII{} Carriage Return (CR) \\
\verb/\t/ & \ASCII{} Horizontal Tab (TAB) \\
\verb/\v/ & \ASCII{} Vertical Tab (VT) \\
\verb/\/{\em ooo} & \ASCII{} character with octal value {\em ooo} \\
\verb/\x/{\em xx...} & \ASCII{} character with hex value {\em xx...} \\
\hline
\end{tabular}
\end{center}
\index{ASCII}
In strict compatibility with Standard C, up to three octal digits are
accepted, but an unlimited number of hex digits is taken to be part of
the hex escape (and then the lower 8 bits of the resulting hex number
are used in all current implementations...).
All unrecognized escape sequences are left in the string unchanged,
i.e., {\em the backslash is left in the string.} (This behavior is
useful when debugging: if an escape sequence is mistyped, the
resulting output is more easily recognized as broken. It also helps a
great deal for string literals used as regular expressions or
otherwise passed to other modules that do their own escape handling.)
\index{unrecognized escape sequence}
\subsection{Numeric literals}
There are three types of numeric literals: plain integers, long
integers, and floating point numbers.
\index{number}
\index{numeric literal}
\index{integer literal}
\index{plain integer literal}
\index{long integer literal}
\index{floating point literal}
\index{hexadecimal literal}
\index{octal literal}
\index{decimal literal}
Integer and long integer literals are described by the following
lexical definitions:
\begin{verbatim}
longinteger: integer ("l"|"L")
integer: decimalinteger | octinteger | hexinteger
decimalinteger: nonzerodigit digit* | "0"
octinteger: "0" octdigit+
hexinteger: "0" ("x"|"X") hexdigit+
nonzerodigit: "1"..."9"
octdigit: "0"..."7"
hexdigit: digit|"a"..."f"|"A"..."F"
\end{verbatim}
Although both lower case `l' and upper case `L' are allowed as suffix
for long integers, it is strongly recommended to always use `L', since
the letter `l' looks too much like the digit `1'.
Plain integer decimal literals must be at most 2147483647 (i.e., the
largest positive integer, using 32-bit arithmetic). Plain octal and
hexadecimal literals may be as large as 4294967295, but values larger
than 2147483647 are converted to a negative value by subtracting
4294967296. There is no limit for long integer literals apart from
what can be stored in available memory.
Some examples of plain and long integer literals:
\begin{verbatim}
7 2147483647 0177 0x80000000
3L 79228162514264337593543950336L 0377L 0x100000000L
\end{verbatim}
Floating point literals are described by the following lexical
definitions:
\begin{verbatim}
floatnumber: pointfloat | exponentfloat
pointfloat: [intpart] fraction | intpart "."
exponentfloat: (intpart | pointfloat) exponent
intpart: digit+
fraction: "." digit+
exponent: ("e"|"E") ["+"|"-"] digit+
\end{verbatim}
The allowed range of floating point literals is
implementation-dependent.
Some examples of floating point literals:
\begin{verbatim}
3.14 10. .001 1e100 3.14e-10
\end{verbatim}
Note that numeric literals do not include a sign; a phrase like
\verb@-1@ is actually an expression composed of the operator
\verb@-@ and the literal \verb@1@.
\section{Operators}
The following tokens are operators:
\index{operators}
\begin{verbatim}
+ - * / %
<< >> & | ^ ~
< == > <= <> != >=
\end{verbatim}
The comparison operators \verb@<>@ and \verb@!=@ are alternate
spellings of the same operator.
\section{Delimiters}
The following tokens serve as delimiters or otherwise have a special
meaning:
\index{delimiters}
\begin{verbatim}
( ) [ ] { }
, : . " ` '
= ;
\end{verbatim}
The following printing \ASCII{} characters are not used in Python. Their
occurrence outside string literals and comments is an unconditional
error:
\index{ASCII}
\begin{verbatim}
@ $ ?
\end{verbatim}
They may be used by future versions of the language though!

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\chapter{Execution model}
\index{execution model}
\section{Code blocks, execution frames, and name spaces} \label{execframes}
\index{code block}
\indexii{execution}{frame}
\index{name space}
A {\em code block} is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code blocks may
textually contain other code blocks. Code blocks may invoke other
code blocks (that may or may not be textually contained in them) as
part of their execution, e.g. by invoking (calling) a function.
\index{code block}
\indexii{code}{block}
The following are code blocks: A module is a code block. A function
body is a code block. A class definition is a code block. Each
command typed interactively is a separate code block; a script file is
a code block. The string argument passed to the built-in function
\verb@eval@ and to the \verb@exec@ statement are code blocks.
And finally, the
expression read and evaluated by the built-in function \verb@input@ is
a code block.
A code block is executed in an execution frame. An {\em execution
frame} contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
name spaces, the local and the global name space, that affect
execution of the code block.
\indexii{execution}{frame}
A {\em name space} is a mapping from names (identifiers) to objects.
A particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called {\em binding} a name (to an object); changing the mapping of
a name is called {\em rebinding}; removing a name is {\em unbinding}.
Name spaces are functionally equivalent to dictionaries.
\index{name space}
\indexii{binding}{name}
\indexii{rebinding}{name}
\indexii{unbinding}{name}
The {\em local name space} of an execution frame determines the default
place where names are defined and searched. The {\em global name
space} determines the place where names listed in \verb@global@
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
\indexii{local}{name space}
\indexii{global}{name space}
\stindex{global}
Whether a name is local or global in a code block is determined by
static inspection of the source text for the code block: in the
absence of \verb@global@ statements, a name that is bound anywhere in
the code block is local in the entire code block; all other names are
considered global. The \verb@global@ statement forces global
interpretation of selected names throughout the code block. The
following constructs bind names: formal parameters, \verb@import@
statements, class and function definitions (these bind the class or
function name), and targets that are identifiers if occurring in an
assignment, \verb@for@ loop header, or \verb@except@ clause header.
A target occurring in a \verb@del@ statement is also considered bound
for this purpose (though the actual semantics are to ``unbind'' the
name).
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (which is actually the
global name space of the module \verb@__builtin__@). When a name is not
found at all, the \verb@NameError@ exception is raised.%
\footnote{If the code block contains {\tt exec} statements or the
construct {\tt from \ldots import *}, the semantics of names not
explicitly mentioned in a {\tt global} statement change subtly: name
lookup first searches the local name space, then the global one, then
the built-in one.}
\bimodindex{__builtin__}
\stindex{from}
\stindex{exec}
\stindex{global}
\ttindex{NameError}
The following table lists the meaning of the local and global name
space for various types of code blocks. The name space for a
particular module is automatically created when the module is first
referenced. Note that in almost all cases, the global name space is
the name space of the containing module --- scopes in Python do not
nest!
\begin{center}
\begin{tabular}{|l|l|l|l|}
\hline
Code block type & Global name space & Local name space & Notes \\
\hline
Module & n.s. for this module & same as global & \\
Script & n.s. for \verb@__main__@ & same as global & \\
Interactive command & n.s. for \verb@__main__@ & same as global & \\
Class definition & global n.s. of containing block & new n.s. & \\
Function body & global n.s. of containing block & new n.s. & (2) \\
String passed to \verb@exec@ statement
& global n.s. of containing block
& local n.s. of containing block & (1) \\
String passed to \verb@eval()@
& global n.s. of caller & local n.s. of caller & (1) \\
File read by \verb@execfile()@
& global n.s. of caller & local n.s. of caller & (1) \\
Expression read by \verb@input@
& global n.s. of caller & local n.s. of caller & \\
\hline
\end{tabular}
\end{center}
\bimodindex{__main__}
Notes:
\begin{description}
\item[n.s.] means {\em name space}
\item[(1)] The global and local name space for these can be
overridden with optional extra arguments.
\item[(2)] The body of lambda forms (see section \ref{lambda}) is
treated exactly the same as a (nested) function definition. Lambda
forms have their own name space consisting of their formal arguments.
\indexii{lambda}{form}
\end{description}
The built-in functions \verb@globals()@ and \verb@locals()@ returns a
dictionary representing the current global and local name space,
respectively. The effect of modifications to this dictionary on the
name space are undefined.%
\footnote{The current implementations return the dictionary actually
used to implement the name space, {\em except} for functions, where
the optimizer may cause the local name space to be implemented
differently, and \verb@locals()@ returns a read-only dictionary.}
\section{Exceptions}
Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors or other exceptional
conditions. An exception is {\em raised} at the point where the error
is detected; it may be {\em handled} by the surrounding code block or
by any code block that directly or indirectly invoked the code block
where the error occurred.
\index{exception}
\index{raise an exception}
\index{handle an exception}
\index{exception handler}
\index{errors}
\index{error handling}
The Python interpreter raises an exception when it detects an run-time
error (such as division by zero). A Python program can also
explicitly raise an exception with the \verb@raise@ statement.
Exception handlers are specified with the \verb@try...except@
statement.
Python uses the ``termination'' model of error handling: an exception
handler can find out what happened and continue execution at an outer
level, but it cannot repair the cause of the error and retry the
failing operation (except by re-entering the the offending piece of
code from the top).
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
Exceptions are identified by string objects or class instances. Two
different string objects with the same value identify different
exceptions. An exception can be raised with a class instance. Such
exceptions are caught by specifying an except clause that has the
class name (or a base class) as the condition.
When an exception is raised, an object (maybe \verb@None@) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
exception handler as additional information. For exceptions raised
with a class instance, the instance is passed as the ``parameter''.
For example:
\begin{verbatim}
>>> class Error:
... def __init__(self, msg): self.msg = msg
...
>>> class SpecificError(Error): pass
...
>>> try:
... raise SpecificError('broken')
... except Error, obj:
... print obj.msg
...
broken
\end{verbatim}
See also the description of the \verb@try@ and \verb@raise@
statements.

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\chapter{Compound statements}
\indexii{compound}{statement}
Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The \verb@if@, \verb@while@ and \verb@for@ statements implement
traditional control flow constructs. \verb@try@ specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses'. A clause
consists of a header and a `suite'. The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a
clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which \verb@if@ clause a following \verb@else@ clause would
belong:
\index{clause}
\index{suite}
\begin{verbatim}
if test1: if test2: print x
\end{verbatim}
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
\verb@print@ statements are executed:
\begin{verbatim}
if x < y < z: print x; print y; print z
\end{verbatim}
Summarizing:
\begin{verbatim}
compound_stmt: if_stmt | while_stmt | for_stmt
| try_stmt | funcdef | classdef
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
\end{verbatim}
Note that statements always end in a \verb@NEWLINE@ possibly followed
by a \verb@DEDENT@.
\index{NEWLINE token}
\index{DEDENT token}
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling \verb@else@' problem is solved in Python by requiring
nested \verb@if@ statements to be indented).
\indexii{dangling}{else}
The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.
\section{The {\tt if} statement}
\stindex{if}
The \verb@if@ statement is used for conditional execution:
\begin{verbatim}
if_stmt: "if" condition ":" suite
("elif" condition ":" suite)*
["else" ":" suite]
\end{verbatim}
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section \ref{Booleans} for
the definition of true and false); then that suite is executed (and no
other part of the \verb@if@ statement is executed or evaluated). If
all conditions are false, the suite of the \verb@else@ clause, if
present, is executed.
\kwindex{elif}
\kwindex{else}
\section{The {\tt while} statement}
\stindex{while}
\indexii{loop}{statement}
The \verb@while@ statement is used for repeated execution as long as a
condition is true:
\begin{verbatim}
while_stmt: "while" condition ":" suite
["else" ":" suite]
\end{verbatim}
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the \verb@else@ clause, if present, is
executed and the loop terminates.
\kwindex{else}
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
\stindex{break}
\stindex{continue}
\section{The {\tt for} statement}
\stindex{for}
\indexii{loop}{statement}
The \verb@for@ statement is used to iterate over the elements of a
sequence (string, tuple or list):
\obindex{sequence}
\begin{verbatim}
for_stmt: "for" target_list "in" condition_list ":" suite
["else" ":" suite]
\end{verbatim}
The condition list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the \verb@else@ clause, if
present, is executed, and the loop terminates.
\kwindex{in}
\kwindex{else}
\indexii{target}{list}
A \verb@break@ statement executed in the first suite terminates the
loop without executing the \verb@else@ clause's suite. A
\verb@continue@ statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the \verb@else@
clause if there was no next item.
\stindex{break}
\stindex{continue}
The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.
The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function \verb@range()@ returns a sequence of
integers suitable to emulate the effect of Pascal's
\verb@for i := a to b do@;
e.g. \verb@range(3)@ returns the list \verb@[0, 1, 2]@.
\bifuncindex{range}
\index{Pascal}
{\bf Warning:} There is a subtlety when the sequence is being modified
by the loop (this can only occur for mutable sequences, i.e. lists).
An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means that
if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a temporary
copy using a slice of the whole sequence, e.g.
\index{loop!over mutable sequence}
\index{mutable sequence!loop over}
\begin{verbatim}
for x in a[:]:
if x < 0: a.remove(x)
\end{verbatim}
\section{The {\tt try} statement} \label{try}
\stindex{try}
The \verb@try@ statement specifies exception handlers and/or cleanup
code for a group of statements:
\begin{verbatim}
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
["else" ":" suite]
try_fin_stmt: "try" ":" suite
"finally" ":" suite
\end{verbatim}
There are two forms of \verb@try@ statement: \verb@try...except@ and
\verb@try...finally@. These forms cannot be mixed.
The \verb@try...except@ form specifies one or more exception handlers
(the \verb@except@ clauses). When no exception occurs in the
\verb@try@ clause, no exception handler is executed. When an
exception occurs in the \verb@try@ suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
except clause with a condition, that condition is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception, or (for exceptions
that are classes) it is a base class of the exception, or it is a
tuple containing an item that is compatible with the exception. Note
that the object identities must match, i.e. it must be the same
object, not just an object with the same value.
\kwindex{except}
If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire \verb@try@ statement
raised the exception).
When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. When the end of this suite
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the \verb@sys@ module:
\verb@sys.exc_type@ receives the object identifying the exception;
\verb@sys.exc_value@ receives the exception's parameter;
\verb@sys.exc_traceback@ receives a traceback object (see section
\ref{traceback}) identifying the point in the program where the
exception occurred.
\bimodindex{sys}
\ttindex{exc_type}
\ttindex{exc_value}
\ttindex{exc_traceback}
\obindex{traceback}
The optional \verb@else@ clause is executed when no exception occurs
in the \verb@try@ clause. Exceptions in the \verb@else@ clause are
not handled by the preceding \verb@except@ clauses.
\kwindex{else}
The \verb@try...finally@ form specifies a `cleanup' handler. The
\verb@try@ clause is executed. When no exception occurs, the
\verb@finally@ clause is executed. When an exception occurs in the
\verb@try@ clause, the exception is temporarily saved, the
\verb@finally@ clause is executed, and then the saved exception is
re-raised. If the \verb@finally@ clause raises another exception or
executes a \verb@return@, \verb@break@ or \verb@continue@ statement,
the saved exception is lost.
\kwindex{finally}
When a \verb@return@ or \verb@break@ statement is executed in the
\verb@try@ suite of a \verb@try...finally@ statement, the
\verb@finally@ clause is also executed `on the way out'. A
\verb@continue@ statement is illegal in the \verb@try@ clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
\stindex{return}
\stindex{break}
\stindex{continue}
\section{Function definitions} \label{function}
\indexii{function}{definition}
A function definition defines a user-defined function object (see
section \ref{types}):\footnote{The new syntax to receive arbitrary
keyword arguments is not yet documented in this manual. See chapter
12 of the Tutorial.}
\obindex{user-defined function}
\obindex{function}
\begin{verbatim}
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (defparameter ",")* ("*" identifier [, "**" identifier]
| "**" identifier
| defparameter [","])
defparameter: parameter ["=" condition]
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
\end{verbatim}
A function definition is an executable statement. Its execution binds
the function name in the current local name space to a function object
(a wrapper around the executable code for the function). This
function object contains a reference to the current global name space
as the global name space to be used when the function is called.
\indexii{function}{name}
\indexii{name}{binding}
The function definition does not execute the function body; this gets
executed only when the function is called.
When one or more top-level parameters have the form {\em parameter =
condition}, the function is said to have ``default parameter values''.
Default parameter values are evaluated when the function definition is
executed. For a parameter with a default value, the correponding
argument may be omitted from a call, in which case the parameter's
default value is substituted. If a parameter has a default value, all
following parameters must also have a default value --- this is a
syntactic restriction that is not expressed by the grammar.%
\footnote{Currently this is not checked; instead,
{\tt def f(a=1,b)} is interpreted as {\tt def f(a=1,b=None)}.}
\indexiii{default}{parameter}{value}
Function call semantics are described in section \ref{calls}. When a
user-defined function is called, first missing arguments for which a
default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
\indexii{function}{call}
\indexiii{user-defined}{function}{call}
\index{parameter}
\index{argument}
\indexii{parameter}{formal}
\indexii{parameter}{actual}
\begin{itemize}
\item
If there are no formal parameters, there must be no arguments.
\item
If the formal parameter list does not end in a star followed by an
identifier, there must be exactly as many arguments as there are
parameters in the formal parameter list (at the top level); the
arguments are assigned to the formal parameters one by one. Note that
the presence or absence of a trailing comma at the top level in either
the formal or the actual parameter list makes no difference. The
assignment to a formal parameter is performed as if the parameter
occurs on the left hand side of an assignment statement whose right
hand side's value is that of the argument.
\item
If the formal parameter list ends in a star followed by an identifier,
preceded by zero or more comma-followed parameters, there must be at
least as many arguments as there are parameters preceding the star.
Call this number {\em N}. The first {\em N} arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is \verb@()@, if
there is just one extra argument, it is a singleton tuple.
\indexii{variable length}{parameter list}
\end{itemize}
Note that the `variable length parameter list' feature only works at
the top level of the parameter list; individual parameters use a model
corresponding more closely to that of ordinary assignment. While the
latter model is generally preferable, because of the greater type
safety it offers (wrong-sized tuples aren't silently mistreated),
variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section \ref{lambda}.
\indexii{lambda}{form}
\section{Class definitions} \label{class}
\indexii{class}{definition}
A class definition defines a class object (see section \ref{types}):
\obindex{class}
\begin{verbatim}
classdef: "class" classname [inheritance] ":" suite
inheritance: "(" [condition_list] ")"
classname: identifier
\end{verbatim}
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list
should evaluate to a class object. The class's suite is then executed
in a new execution frame (see section \ref{execframes}), using a newly
created local name space and the original global name space.
(Usually, the suite contains only function definitions.) When the
class's suite finishes execution, its execution frame is discarded but
its local name space is saved. A class object is then created using
the inheritance list for the base classes and the saved local name
space for the attribute dictionary. The class name is bound to this
class object in the original local name space.
\index{inheritance}
\indexii{class}{name}
\indexii{name}{binding}
\indexii{execution}{frame}

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\chapter{Top-level components}
The Python interpreter can get its input from a number of sources:
from a script passed to it as standard input or as program argument,
typed in interactively, from a module source file, etc. This chapter
gives the syntax used in these cases.
\index{interpreter}
\section{Complete Python programs}
\index{program}
While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for \verb@sys@
(various system services), \verb@__builtin__@ (built-in functions,
exceptions and \verb@None@) and \verb@__main__@. The latter is used
to provide the local and global name space for execution of the
complete program.
\bimodindex{sys}
\bimodindex{__main__}
\bimodindex{__builtin__}
The syntax for a complete Python program is that for file input,
described in the next section.
The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of \verb@__main__@.
\index{interactive mode}
\bimodindex{__main__}
Under {\UNIX}, a complete program can be passed to the interpreter in
three forms: with the {\bf -c} {\it string} command line option, as a
file passed as the first command line argument, or as standard input.
If the file or standard input is a tty device, the interpreter enters
interactive mode; otherwise, it executes the file as a complete
program.
\index{UNIX}
\index{command line}
\index{standard input}
\section{File input}
All input read from non-interactive files has the same form:
\begin{verbatim}
file_input: (NEWLINE | statement)*
\end{verbatim}
This syntax is used in the following situations:
\begin{itemize}
\item when parsing a complete Python program (from a file or from a string);
\item when parsing a module;
\item when parsing a string passed to the \verb@exec@ statement;
\end{itemize}
\section{Interactive input}
Input in interactive mode is parsed using the following grammar:
\begin{verbatim}
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
\end{verbatim}
Note that a (top-level) compound statement must be followed by a blank
line in interactive mode; this is needed to help the parser detect the
end of the input.
\section{Expression input}
\index{input}
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to \verb@eval()@ must have the following form:
\bifuncindex{eval}
\begin{verbatim}
eval_input: condition_list NEWLINE*
\end{verbatim}
The input line read by \verb@input()@ must have the following form:
\bifuncindex{input}
\begin{verbatim}
input_input: condition_list NEWLINE
\end{verbatim}
Note: to read `raw' input line without interpretation, you can use the
built-in function \verb@raw_input()@ or the \verb@readline()@ method
of file objects.
\obindex{file}
\index{input!raw}
\index{raw input}
\bifuncindex{raw_index}
\ttindex{readline}