Right now it may return `None` if no signature can be returned for the given builtin. If we decide to implement #17373 in 3.5, I'd like all three methods -- from_builtin, from_function, from_callable -- to either return a signature or to raise an exception.

A patch is attached, please review.

This proposal and patch look good to me. The current behaviour definitely isn't desirable:

>>> import inspect
>>> inspect.Signature.from_function(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 1888, in from_function
    raise TypeError('{!r} is not a Python function'.format(func))
TypeError: 1 is not a Python function
>>> inspect.Signature.from_builtin(1)
>>> 

Signature.from_function() and Signature.from_builtin() should both also be documented, but we may want to wait for PEP 457 and #17373 in Python 3.5 before sorting all that out.

New changeset 9433b380ad33 by Yury Selivanov in branch 'default':
inspect.Signature: Make from_builtin to raise an exception if no signature can
http://hg.python.org/cpython/rev/9433b380ad33

Nick, thanks for the review. Committed.

> Signature.from_function() and Signature.from_builtin() should both also be documented, but we may want to wait for PEP 457 and #17373 in Python 3.5 before sorting all that out.

I would like to wait till 3.5 too. Right now both of them are sort of "private" API, so we still have the liberty to adjust their behaviour in 3.5 if needed.