Signature.from_builtin should raise a ValueError when no signature can be provided

[1st1]

BPO	20422
Nosy	@brettcannon, @ncoghlan, @larryhastings, @1st1
Files	from_builtin_errors_01.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-01-29.15:47:10.182>
created_at = <Date 2014-01-28.17:00:19.862>
labels = ['type-bug']
title = 'Signature.from_builtin should raise a ValueError when no signature can be provided'
updated_at = <Date 2014-01-29.15:49:12.815>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2014-01-29.15:49:12.815>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2014-01-29.15:47:10.182>
closer = 'yselivanov'
components = []
creation = <Date 2014-01-28.17:00:19.862>
creator = 'yselivanov'
dependencies = []
files = ['33784']
hgrepos = []
issue_num = 20422
keywords = ['patch', 'needs review']
message_count = 6.0
messages = ['209565', '209586', '209638', '209649', '209650', '209651']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'ncoghlan', 'larry', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20422'
versions = ['Python 3.4']

[1st1]

Right now it may return None if no signature can be returned for the given builtin. If we decide to implement bpo-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.

[1st1]

A patch is attached, please review.

[ncoghlan]

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 bpo-17373 in Python 3.5 before sorting all that out.

[python-dev]

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

[1st1]

Nick, thanks for the review. Committed.

[1st1]

Signature.from_function() and Signature.from_builtin() should both also be documented, but we may want to wait for PEP-457 and bpo-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.