The language reference for Python 3.2 (and the development version, as
well), says this in section 3.2:
A built-in function object is a wrapper around a C function. Examples of
built-in functions are len() and math.sin() (math is a standard built-in
module). <...> Special read-only attributes: <...> __self__ is set to None
(but see the next item) <...>.
Now I don't know whether it's a bug in Python or in the docs, but that
doesn't seem to be the case:
ActivePython 184.108.40.206 (ActiveState Software Inc.) based on
Python 3.2.2 (default, Sep 8 2011, 10:55:13) [MSC v.1500 64 bit (AMD64)]
Type "help", "copyright", "credits" or "license" for more information.
<module 'builtins' (built-in)>
<module 'io' (built-in)>
<module 'math' (built-in)>