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Python 3.3.0 (default, Mar 7 2013, 00:24:38) 
[GCC 4.6.3] on linux
q = open('/dev/null')
help(q.close)
the entire output is:
-------------------------------
Help on built-in function close:
close(...)
(END)
-------------------------------
But close() is NOT a built-in, it's a method.
(In Python 2.7.* the output is:
-------------------------------
Help on built-in function close:
close(...)
 close() -> None or (perhaps) an integer. Close the file.
 
 Sets data attribute .closed to True. A closed file cannot be used for
 further I/O operations. close() may be called more than once without
 error. Some kinds of file objects (for example, opened by popen())
 may return an exit status upon closing.
(END)
-------------------------------
which is only partially wrong.

In fact it is a built-in method without a doc string:
>>> f = open("/dev/null")
>>> f.close
<built-in method close of _io.TextIOWrapper object at 0x7feefaab42f0>
>>> f.close.__doc__
All functions and methods that are implemented in C are referred to as built-in functions. It's an implementation detail.

Closing with Victor Stinner's approval.