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<title>Python XPCOM Package Tutorial</title>
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<body>
<h1>Python XPCOM Package Tutorial</h1>
<p>This is a quick introduction to the Python XPCOM Package. We assume that you have a good understanding of Python and <a href="http://www.mozilla.org/projects/xpcom/">XPCOM</a>,
and have experience both using and implementing XPCOM objects in some other
language (e.g., C++ or JavaScript). We <b><i>do not</i></b> attempt to
provide a tutorial to XPCOM or Python itself, only to using Python <i>and</i>
XPCOM.</p>
<p>This tutorial contains the following sections:</p>
<ul>
<li><a href="#Using">Using XPCOM Objects and Interfaces</a> - when you wish to
<i>use</i> a component written by anyone else in any XPCOM supported
language.</li>
<li><a href="#Implementing">Implementing XPCOM Objects and Interfaces</a> -
when you wish to implement a component for use by anyone else in any xpcom
supported language.</li>
<li><a href="#Parameters">Parameters and Types</a> - useful information
regarding how Python translates XPCOM types, and handles byref parameters.</li>
</ul>
<p>For anything not covered here, try the <a href="advanced.html">advanced
documentation</a>, and if that fails, use the source, Luke!</p>
<h2><a name="Using">Using XPCOM object and interfaces.</a></h2>
<p>The techniques for using XPCOM in Python have been borrowed from JavaScript -
thus, the model described here should be quite familiar to existing JavaScript
XPCOM programmers.</p>
<h3>xpcom.components module</h3>
<p>When using an XPCOM object, the primary module used is the <u><i>xpcom.components</i></u>
module.&nbsp; Using this module, you can get a Python object that supports any
scriptable XPCOM interface. Once you have this Python object, you can
simply call XPCOM methods on the object, as normal.</p>
<p>The <u><i>xpcom.components</i></u> module defines the following public
members:</p>
<table border="1" width="100%">
<tr>
<td width="16%"><b>Name</b></td>
<td width="84%"><b>Description</b></td>
</tr>
<tr>
<td width="16%">classes</td>
<td width="84%">A mapping (dictionary-like object) used to get XPCOM
&quot;classes&quot;.&nbsp; These are indexed by XPCOM contract ID, just
like the JavaScript object of the same name.&nbsp;&nbsp;
<p>Example:</p>
<pre>cls = components.classes[&quot;@mozilla.org/sample;1&quot;]
ob = cls.createInstance() # Now have an nsISupports</pre>
</td>
</tr>
<tr>
<td width="16%">interfaces</td>
<td width="84%">An object that exposes all XPCOM interface IDs (IIDs).&nbsp;
Like the JavaScript object of the same name, this object uses
&quot;dot&quot; notation, as demonstrated below.
<p>Example:</p>
<pre>ob = cls.createInstance(components.interfaces.nsISample)
# Now have an nsISample</pre>
</td>
</tr>
</table>
<p>For many people, this is all you need to know. Consider the Mozilla Sample Component.&nbsp; The Mozilla Sample
Component has a contract ID of <i>@mozilla.org/sample;1</i>,
and implements the <i>nsISample</i> interface.</p>
<p>Thus, a complete Python program that uses this component is shown below.</p>
<pre>from xpcom import components
cls = components.classes[&quot;@mozilla.org/sample;1&quot;]
ob = cls.createInstance() # no need to specify an IID for most components
# nsISample defines a &quot;value&quot; property - let's use it!
ob.value = &quot;new value&quot;
if ob.value != &quot;new value&quot;:
print &quot;Eeek - what happened?&quot;</pre>
<p>And that is it - a complete Python program that uses XPCOM.</p>
<h2><a name="Implementing">Implementing XPCOM Objects and Interfaces.</a></h2>
<p>Implementing XPCOM objects is almost as simple as using them. The
basic strategy is this:</p>
<ol>
<li>Create a standard Python source file, with a standard Python class.</li>
<li>Add some special <a href="#Attributes"> attributes</a> to your class for use by the Python XPCOM
framework. This controls the XPCOM behavior of your object.</li>
<li>Implement the XPCOM <a href="#Properties"> properties</a> and methods of your classes as normal.</li>
<li>Put the Python source file in the Mozilla <i> components</i> directory.</li>
<li>Run <i> regxpcom.</i></li>
</ol>
<p>Your component is now ready to be used.</p>
<h3><a name="Attributes">Attributes</a></h3>
<p>There are two classes of attributes: those used at runtime to define the object
behavior and those used at registration time to control object
registration.&nbsp; Not all objects require registration, thus not all
Python XPCOM objects will have registration-related attributes.</p>
<table border="1" width="100%">
<tr>
<td width="17%"><b>Attribute</b></td>
<td width="83%"><b>Description</b></td>
</tr>
<tr>
<td width="17%">_com_interfaces_</td>
<td width="83%">The interface IDs (IIDs) supported by the component.&nbsp;
For simplicity, this may be either a single IID, or a list of IIDs.&nbsp;
There is no need to specify base interfaces, as all parent interfaces are
automatically supported. Thus, it is never necessary to nominate <i>
nsISupports</i> in the list of interfaces.
<p>This attribute is required. Objects without such an attribute are
deemed unsuitable for use as a XPCOM object.</td>
</tr>
<tr>
<td width="17%">_reg_contractid_</td>
<td width="83%">The contract ID of the component.&nbsp; Required if the
component requires registration (i.e., exists in the components directory).</td>
</tr>
<tr>
<td width="17%">_reg_clsid_</td>
<td width="83%">The Class ID (CLSID) of the component, as a string in the
standard &quot;{XXX-XXX-XXX-XXX}&quot; format. Required if the
component requires registration (i.e., exists in the components directory).</td>
</tr>
<tr>
<td width="17%">_reg_registrar_</td>
<td width="83%">Nominates a function that is called at registration
time. The default is for no extra function to be called. This can
be useful if a component has special registration requirements and needs
to hook into the registration process.</td>
</tr>
<tr>
<td width="17%">_reg_desc_</td>
<td width="83%">The description of the XPCOM object. This may be used by
browsers or other such objects.&nbsp; If not specified, the contract ID
is used.</td>
</tr>
</table>
<h3><a name="Properties">Properties</a></h3>
<p>A Python class can support XPCOM properties in one of two ways.&nbsp; Either
a standard Python property of the same name can exist - our sample
component demonstrates this with the <i>boolean_value</i> property.&nbsp;
Alternatively, the class can provide the <i>get_propertyName(self)</i> and <i>set_propertyName(self,
value)</i> functions (with <i>propertyName</i> changed to the appropriate value for the
property), and these functions will be called instead.</p>
<h4>Example:&nbsp; The Python XPCOM Test Component</h4>
<p>As an example, examine the Python XPCOM Test Component.&nbsp; This
code can be found in <i>py_test_component.py</i>.</p>
<pre>from xpcom import components
class PythonTestComponent:
_com_interfaces_ = components.interfaces.nsIPythonTestInterface
_reg_clsid_ = &quot;{7EE4BDC6-CB53-42c1-A9E4-616B8E012ABA}&quot;
_reg_contractid_ = &quot;Python.TestComponent&quot;
def __init__(self):
self.boolean_value = 1
...
def do_boolean(self, p1, p2):
ret = p1 ^ p2
return ret, not ret, ret
...</pre>
<p><b>Note:</b> This component only specifies the mandatory attributes - <i>_com_interfaces</i>,
<i>_reg_clsid_</i> and <i>_reg_contractid_</i>.</p>
<p>This sample code demonstrates supporting the <i>boolean_value</i> attribute,
supported implicitly, as it is defined in the IDL and exists as a real Python
attribute of that name, and a method called <i>do_boolean</i>.</p>
<h4>Tip: The xpcom/xpt.py Script</h4>
<p> The xpcom/xpt.py script is a useful script that can generate the skeleton of a class for
any XPCOM interface.&nbsp; Just specify the interface name on the command-line,
and paste the output into your source file.</p>
<p>This is the output of running this program over the <i>nsISample</i>
interface (i.e., assuming we wanted to implement a component that supported this
interface):</p>
<pre>class nsISample:
_com_interfaces_ = xpcom.components.interfaces.nsISample
# If this object needs to be registered, the following 2 are also needed.
# _reg_clsid_ = {a new clsid generated for this object}
# _reg_contractid_ = &quot;The.Object.Name&quot;
def get_value( self ):
# Result: string
pass
def set_value( self, param0 ):
# Result: void - None
# In: param0: string
pass
def writeValue( self, param0 ):
# Result: void - None
# In: param0: string
pass
def poke( self, param0 ):
# Result: void - None
# In: param0: string
pass</pre>
<p><b>Note:</b> The types of the parameters and the function itself are included in
the comments.&nbsp;You need to implement the functions
themselves.&nbsp; Another advantage of this script is that the <a href="#HiddenParams">hidden
parameters</a> are handled for you; the comments indicate when parameters
have been hidden.</p>
<h2><a name="Parameters">Parameters and Types</a></h2>
<p>This section briefly describes the XPCOM type support in
Python.</p>
<p>All XPCOM interfaces define parameters of a specific type.&nbsp; There is
currently no concept of a variant, or union of all types. Thus, the
conversion rules are very straightforward, and generally surprise free: for
any given XPCOM method, there is only one possible type for a given parameter.</p>
<h3>Type Conversion Rules:</h3>
<ul>
<li>All numeric types will attempt to be coerced to the correct type.&nbsp;
Thus, you can pass a Python float to an XPCOM method expecting an integer,
or vice-versa. Specifically, when an integer is required, you can pass
any Python object for which <i>int()</i> would succeed; for a Python float,
any object for which <i>float()</i> would succeed is acceptable.&nbsp; This
means that you can pass a Python string object as an integer, as long as the
string was holding a valid integer.</li>
<li>Strings and Unicode objects are interchangeable, but no other automatic
string conversions are performed.&nbsp; Thus, you can not pass an integer
where a string is expected, even though the reverse is true.</li>
<li>Any sequence object can be passed as an array.&nbsp; List objects are
always returned for arrays.</li>
<li>Any Python instance suitable for use as a XPCOM object (i.e., with the
<a href="#Implementing">necessary annotations</a>) can be
passed as a XPCOM object. No special wrapping step is needed to turn a
Python instance into a XPCOM object.&nbsp; Note you must pass a class <i>instance</i>,
not the class itself.</li>
<li><a name="HiddenParams">Many XPCOM <b> method signatures</b> specify
&quot;count&quot; or &quot;size&quot; parameters.&nbsp; For example, every
time an array is passed via XPCOM, the method signature will always specify
an integer that holds the count of the array.&nbsp; These parameters are
always hidden in Python.&nbsp; As the size param can be implied from the
length of the Python sequence passed, the Python programmer need never pass
these parameters;&nbsp;in contrast, JavaScript requires these redundant parameters.</a></li>
</ul>
<h2>Interface Flattening</h2>
<p>Most people can ignore this information - Python XPCOM objects just
work.&nbsp; However, if you are familiar with xpcom from C++ and the concept of <i>QueryInterface</i>,
you may like to read this.</p>
<p>Most components support the concept of &quot;interface
flattening&quot;.&nbsp; Such objects can report the interfaces they support,
allowing languages such as Python and Javascript avoid using <i>QueryInterface</i>.&nbsp;
When you are using an XPCOM object from Python, you can just call methods and
reference properties without regard for the interface that implements it.</p>
<p>When multiple interfaces share the same method or property name, you can use
the name of the interface as a differentiator.&nbsp; Thus, <i>ob.nsIFoo.close()</i>
will call close on <i>ob</i>'s <i>nsIFoo</i> interface, while <i>ob.nsIBar.close()</i>
will use the <i>nsIBar</i> interface.&nbsp; <i>ob.close()</i> is not defined.</p>
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