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Title: inspect.signature reporting "()" for all builtin & extension types
Type: behavior Stage: resolved
Components: Versions: Python 3.4, Python 3.5
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: Nosy List: james, larry, ncoghlan, python-dev, yselivanov
Priority: normal Keywords: patch

Created on 2015-04-13 16:38 by ncoghlan, last changed 2022-04-11 14:58 by admin. This issue is now closed.

File name Uploaded Description Edit
issue_23934.patch james, 2015-04-13 17:20 review
issue_23934-test.patch james, 2015-04-13 20:04 unit test fix for Lib/test/ review
Messages (8)
msg240649 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2015-04-13 16:38
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/", line 2830, in signature
    return Signature.from_callable(obj)
  File "/home/ncoghlan/devel/py3k/Lib/", line 2586, in from_callable
    return _signature_from_callable(obj, sigcls=cls)
  File "/home/ncoghlan/devel/py3k/Lib/", line 2064, in _signature_from_callable
  File "/home/ncoghlan/devel/py3k/Lib/", 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>
msg240650 - (view) Author: Larry Hastings (larry) * (Python committer) Date: 2015-04-13 16:41
Would this qualify as an "easy" bug we could throw to a sprinter?
msg240656 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2015-04-13 16:49
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)
msg240669 - (view) Author: James Powell (james) Date: 2015-04-13 17:20
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.
"Raises ValueError if no signature can be provided, and TypeError if that type of object is not supported."
msg240715 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2015-04-13 19:43
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.
msg240724 - (view) Author: James Powell (james) Date: 2015-04-13 20:04
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
msg240756 - (view) Author: Yury Selivanov (yselivanov) * (Python committer) Date: 2015-04-13 21:13
The patch looks good.
msg244500 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2015-05-30 21:09
New changeset e59966bb6de5 by Yury Selivanov in branch '3.5':
Issue #23934: Fix inspect.signature to fail correctly for builtin types.

New changeset 19e0ffdd1daa by Yury Selivanov in branch 'default':
Issue #23934: Fix inspect.signature to fail correctly for builtin types.
Date User Action Args
2022-04-11 14:58:15adminsetgithub: 68122
2015-05-30 21:10:04yselivanovsetstatus: open -> closed
resolution: fixed
stage: test needed -> resolved
2015-05-30 21:09:33python-devsetnosy: + python-dev
messages: + msg244500
2015-04-13 21:13:34yselivanovsetmessages: + msg240756
2015-04-13 20:04:07jamessetfiles: + issue_23934-test.patch

messages: + msg240724
2015-04-13 19:43:31ncoghlansetmessages: + msg240715
2015-04-13 17:20:35jamessetfiles: + issue_23934.patch
keywords: + patch
messages: + msg240669
2015-04-13 16:49:23ncoghlansetmessages: + msg240656
2015-04-13 16:41:07larrysetmessages: + msg240650
2015-04-13 16:38:13ncoghlancreate