[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Zac Hatfield-Dodds]

Zac Hatfield-Dodds  added the comment:

I think that with PEP-563 reverted, this issue has been fixed.

If we have a similar problem from e.g. PEP-649, I'll open another ticket then!

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resolution:  -> fixed
stage:  -> resolved
status: open -> closed

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[jack1142]

Change by jack1142 :


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nosy: +jack1142

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Guido van Rossum]

Guido van Rossum  added the comment:

Note that this may be solved in a different way *if* PEP 649 is accepted. This 
was discussed extensively on python-dev last week. It would roll back the 
stringification of PEP 563 (unless `from __future__ import annotations` is 
used, which would be deprecated).

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nosy: +larry

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Ken Jin]

Ken Jin  added the comment:

For anyone interested, I went to do some digging on the 3 issues Zac listed:

1. Similar to the first message, this is caused by inspect.getfullarg/signature 
using get_type_hints in Py 3.10. get_type_hints internally converts None to 
NoneType.

2. I don't know what's causing that TypeDict signature to fail.

3. I'm not too sure, but maybe this is intentional according to 
https://www.python.org/dev/peps/pep-0563/#keeping-the-ability-to-use-function-local-state-when-defining-annotations
 ? You can fix it by passing in locals() to get_type_hints (this should work 
all the way back to 3.6)::

import typing

def f():
A = typing.TypeVar("A")
def same_type_args(a: A, b: A):
assert type(a) == type(b)
print(typing.get_type_hints(same_type_args, localns=locals()))

>>> f()
{'a': ~A, 'b': ~A}

The whatsnew should probably be updated to mention this, this is a backwards 
incompatible change after all.

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Zac Hatfield-Dodds]

Zac Hatfield-Dodds  added the comment:

And I promise this is the last one:


import inspect
import typing

def f():
A = typing.TypeVar("A")

def same_type_args(a: A, b: A):
assert type(a) == type(b)

print(inspect.signature(same_type_args))
print(typing.get_type_hints(same_type_args))

f()


$ python3 t.py 
(a: ~A, b: ~A)
{'a': ~A, 'b': ~A}

$ python3.10 t.py 
(a: 'A', b: 'A')
Traceback (most recent call last):
  ...
NameError: name 'A' is not defined


My apologies to everyone involved here; I'm looking forward to postponed 
annotation evaluation, appreciate all your work on this, and regret that I have 
such bizzare bugs to report <3

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Zac Hatfield-Dodds]

Zac Hatfield-Dodds  added the comment:

Aaaand it looks like another problem I'm having is also related:


import inspect

class T(typing.TypedDict):
a: int

print(inspect.signature(T))

was `(*args, **kwargs)` and now raises `ValueError: no signature found for 
builtin type `

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Zac Hatfield-Dodds]

Zac Hatfield-Dodds  added the comment:

A closely related problem, also via 
https://github.com/HypothesisWorks/hypothesis/issues/2767


import inspect

def f(x) -> None:
pass

annotations = inspect.getfullargspec(f).annotations
assert annotations == {"return": None}, annotations


Under Python 3.10, annotations is now {"return": }

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Ken Jin]

Ken Jin  added the comment:

> This is correct for typing.get_type_hints(), but *not* for 
> inspect.signature(). I therefore suspect that this is an accidental 
> side-effect from support for PEP-563 deferred evaluation of annotations.

Spot on about PEP 563 becoming the default being the culprit!

The what's new for Python 3.10 states that inspect.signature will try to 
resolve types 
https://docs.python.org/3.10/whatsnew/3.10.html#pep-563-postponed-evaluation-of-annotations-becomes-default.

For annotations, inspect.signature internally calls get_type_hints: 
https://github.com/python/cpython/blob/master/Lib/inspect.py#L2194

A simple solution I can think of is to add a flag to get_type_hints to disable 
converting to Optional[int] for inspect.signature.

I nosied some people involved in issue38605 so that they can give their 
opinions on this.

--
nosy: +BTaskaya, gvanrossum, kj

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[issue43006] Changed behaviour of inspect.signature() in Python 3.10

[Zac Hatfield-Dodds]

New submission from Zac Hatfield-Dodds :

Consider the following snippet, which passes on Python 3.9 and earlier:


import inspect

def f(x: int = None):
pass

print(inspect.signature(constructor))
assert inspect.signature(constructor).parameters["a"].annotation == int


But under Python 3.10 (alpha 4), the annotation is instead read as 
Optional[int].  This is correct for typing.get_type_hints(), but *not* for 
inspect.signature().

I therefore suspect that this is an accidental side-effect from support for 
PEP-563 deferred evaluation of annotations.

--
components: Library (Lib)
messages: 385524
nosy: Zac Hatfield-Dodds
priority: normal
severity: normal
status: open
title: Changed behaviour of inspect.signature() in Python 3.10
versions: Python 3.10

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