SF 582071 clarified the .split() method's docstring to note that sep=None

will trigger splitting on any whitespace.

Objects/stringobject.c

@@ -1126,8 +1126,8 @@ PyDoc_STRVAR(split__doc__,
 \n\
 Return a list of the words in the string S, using sep as the\n\
 delimiter string.  If maxsplit is given, at most maxsplit\n\
-splits are done. If sep is not specified, any whitespace string\n\
-is a separator.");
+splits are done. If sep is not specified or is None, any\n\
+whitespace string is a separator.");
 
 static PyObject *
 string_split(PyStringObject *self, PyObject *args)