inspect.signature reporting "()" for all builtin & extension types

[ncoghlan]

BPO	23934
Nosy	@ncoghlan, @larryhastings, @1st1
Files	issue_23934.patch issue_23934-test.patch: unit test fix for Lib/test/test_inspect.py

^{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 2015-05-30.21:10:04.931>
created_at = <Date 2015-04-13.16:38:13.421>
labels = ['type-bug']
title = 'inspect.signature reporting "()" for all builtin & extension types'
updated_at = <Date 2015-05-30.21:10:04.930>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2015-05-30.21:10:04.930>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2015-05-30.21:10:04.931>
closer = 'yselivanov'
components = []
creation = <Date 2015-04-13.16:38:13.421>
creator = 'ncoghlan'
dependencies = []
files = ['38936', '38949']
hgrepos = []
issue_num = 23934
keywords = ['patch']
message_count = 8.0
messages = ['240649', '240650', '240656', '240669', '240715', '240724', '240756', '244500']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'larry', 'python-dev', 'yselivanov', 'james']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23934'
versions = ['Python 3.4', 'Python 3.5']

[ncoghlan]

inspect.signature isn't currently handling builtin & extension types correctly - these show up as having neither __new__ *nor* __init__ as pure Python callables, so the inspect.signature logic falls through into a currently unhandled case.

_testcapi isn't currently exporting an extension type as docstring introspection fodder, only callables, so test_inspect didn't pick up the problem. The problem can be seen with builtin types like str:

>>> import inspect
>>> inspect.signature(str)
<inspect.Signature object at 0x7fb81d44e518>
>>> print(inspect.signature(str))
()

Expected behaviour would be to throw a ValueError as with builtin callables without a signature:

>>> import _testcapi
>>> import inspect
>>> inspect.signature(_testcapi.docstring_no_signature)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 2830, in signature
    return Signature.from_callable(obj)
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 2586, in from_callable
    return _signature_from_callable(obj, sigcls=cls)
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 2064, in _signature_from_callable
    skip_bound_arg=skip_bound_arg)
  File "/home/ncoghlan/devel/py3k/Lib/inspect.py", line 1984, in _signature_from_builtin
    raise ValueError("no signature found for builtin {!r}".format(func))
ValueError: no signature found for builtin <built-in function docstring_no_signature>

[larryhastings]

Would this qualify as an "easy" bug we could throw to a sprinter?

[ncoghlan]

James Powell is currently looking into this at the PyCon sprints.

The key problem appears to be the check that assumes "obj.__init__ is object.__init__" indicates that there's no user defined init or new method, when in fact a builtin or extension type that only overrides __new__ would also pass that check (since the earlier check for __new__ returns None if the method implementation isn't a pure Python callable)

[james]

Discussed with Nick Coghlan.

See attached patch to return signature(object) only if both __new__ and __init__ are shared with object.

Otherwise, raise TypeError indicating built-in types not supported.

https://docs.python.org/3/library/inspect.html#inspect.signature
"Raises ValueError if no signature can be provided, and TypeError if that type of object is not supported."

[ncoghlan]

The main missing piece now is test fodder for this error. The simplest near-term option would be to use one of the actual builtins, such as 'str'.

In Python 3.5+ we could borrow the matmulType that was added to _testcapi to test the "@" operator, future-proofing the test against eventual conversion of the builtin types to support Argument Clinic.

[james]

See attached patch for unittest.

For 3.4, test that inspect.signature(str) raises TypeError.

For >3.5, this can be improved to use _testcapi.matmulType

[1st1]

The patch looks good.

[python-dev]

New changeset e59966bb6de5 by Yury Selivanov in branch '3.5':
Issue bpo-23934: Fix inspect.signature to fail correctly for builtin types.
https://hg.python.org/cpython/rev/e59966bb6de5

New changeset 19e0ffdd1daa by Yury Selivanov in branch 'default':
Issue bpo-23934: Fix inspect.signature to fail correctly for builtin types.
https://hg.python.org/cpython/rev/19e0ffdd1daa