[issue43355] __future__.annotations breaks inspect.signature()

[Jelle Zijlstra]

Jelle Zijlstra  added the comment:

Python 3.10 will add an eval_str= argument to inspect.signature() that lets you 
evaluate string annotations. I don't think it makes sense to change anything in 
the bugfix branches, so I propose that this issue be closed.

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nosy: +Jelle Zijlstra

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[issue43355] __future__.annotations breaks inspect.signature()

[Florian Bruhin]

Change by Florian Bruhin :


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nosy: +The Compiler

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[issue43355] __future__.annotations breaks inspect.signature()

[Terry J. Reedy]

Terry J. Reedy  added the comment:

3.7 only gets security fixes

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versions:  -Python 3.7

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric V. Smith]

Eric V. Smith  added the comment:

I think this is all about: should inspect.signature() resolve string 
annotations into actual types (via get_type_hints, or whatever)? I don't use 
expect much, so I can't offer an opinion there.

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric Engestrom]

Eric Engestrom  added the comment:

(... and I had the window open for too long and didn't notice Ken's reply 
before mine ^^')

@kj: thanks for the workaround! it's good to be able to change our code and not 
have to depend on a yet-to-be-released patch on each python version :)

That said, typing.get_type_hints() only returns the type hint, whereas our 
current test checks for other things (like the default value) which need to 
match (I think) for cython's use of `__signature__`? (I inherited all this 
code, so I'm not familiar with it and its history, and I'm beginning to 
question this test's necessity.)

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric Engestrom]

Eric Engestrom  added the comment:

Sorry, I just realized you asked for the "success" value as well.

For python 3.7-3.9 without the `import annotations`, and for python 3.10, the 
type annotation is a class, ie. ``.

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[issue43355] __future__.annotations breaks inspect.signature()

[Ken Jin]

Ken Jin  added the comment:

@eric.smith, here's a summarized version. I hope it helps:

def foo(x: str) -> str: ...

In Python 3.7 - 3.9 with from __future__ import annotations, inspect.signature 
sees foo's parameter as:



Without the future import (and also in Python 3.10):



The reason for the difference in 3.10 (IIRC) is that inspect.signature auto 
converts all strings to typing.ForwardRef internally and then resolves them. 
This is a result of PEP 563 becoming default (also mentioned here 
https://docs.python.org/3.10/whatsnew/3.10.html#pep-563-postponed-evaluation-of-annotations-becomes-default
 )

Prior to 3.10, the future import treats function annotations as strings and 
inspect.signature _doesn't_ convert them. I don't know of this is a bug or 
intentional. Especially considering what PEP 563 has to say: 
https://www.python.org/dev/peps/pep-0563/#resolving-type-hints-at-runtime

@1ace, a possible workaround if you want full compatibility regardless of the 
future import is to use typing.get_type_hints:

(The result doesn't change even with from __future__ import annotations. This 
also works from 3.7-3.10)
>>> from typing import get_type_hints
>>> get_type_hints(foo)
{'x': , 'return': }

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric Engestrom]

Eric Engestrom  added the comment:

Sure thing.

Putting the reproducing code in `test_foo.py` and running `docker run --rm -it 
-v $PWD:/code python sh -c 'pip install pytest && pytest -vvv 
/code/test_foo.py'` yields:

```
Collecting pytest
  Downloading pytest-6.2.2-py3-none-any.whl (280 kB)
 || 280 kB 516 kB/s
Collecting toml
  Downloading toml-0.10.2-py2.py3-none-any.whl (16 kB)
Collecting py>=1.8.2
  Downloading py-1.10.0-py2.py3-none-any.whl (97 kB)
 || 97 kB 1.6 MB/s
Collecting attrs>=19.2.0
  Downloading attrs-20.3.0-py2.py3-none-any.whl (49 kB)
 || 49 kB 1.8 MB/s
Collecting iniconfig
  Downloading iniconfig-1.1.1-py2.py3-none-any.whl (5.0 kB)
Collecting packaging
  Downloading packaging-20.9-py2.py3-none-any.whl (40 kB)
 || 40 kB 833 kB/s
Collecting pluggy<1.0.0a1,>=0.12
  Downloading pluggy-0.13.1-py2.py3-none-any.whl (18 kB)
Collecting pyparsing>=2.0.2
  Downloading pyparsing-2.4.7-py2.py3-none-any.whl (67 kB)
 || 67 kB 1.5 MB/s
Installing collected packages: pyparsing, toml, py, pluggy, packaging, 
iniconfig, attrs, pytest
Successfully installed attrs-20.3.0 iniconfig-1.1.1 packaging-20.9 
pluggy-0.13.1 py-1.10.0 pyparsing-2.4.7 pytest-6.2.2 toml-0.10.2
= test session starts ==
platform linux -- Python 3.9.2, pytest-6.2.2, py-1.10.0, pluggy-0.13.1 -- 
/usr/local/bin/python
cachedir: .pytest_cache
rootdir: /code
collected 1 item

code/test_foo.py::test_foo FAILED[100%]

=== FAILURES ===
___ test_foo ___

def test_foo():
expected = (
Parameter("x", Parameter.POSITIONAL_OR_KEYWORD, annotation=str),
)

actual = tuple(signature(foo).parameters.values())

>   assert expected == actual
E   assert (,) == (,)
E At index 0 diff:  != 
E Full diff:
E - (,)
E ? -   -
E + (,)

code/test_foo.py:16: AssertionError
=== short test summary info 
FAILED code/test_foo.py::test_foo - assert (,) == (

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric V. Smith]

Eric V. Smith  added the comment:

Can you show the values of “expected” and “actual” for both the failures and 
successes?

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nosy: +eric.smith

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[issue43355] __future__.annotations breaks inspect.signature()

[Eric Engestrom]

New submission from Eric Engestrom :

We have a pytest that boils down to the following:

```
#from __future__ import annotations
from inspect import Parameter, signature


def foo(x: str) -> str:
return x + x


def test_foo():
expected = (
Parameter("x", Parameter.POSITIONAL_OR_KEYWORD, annotation=str),
)

actual = tuple(signature(foo).parameters.values())

assert expected == actual
```

(execute with `pip install pytest && pytest -vv test_foo.py`)

I tried importing 3.10 annotations (so that we can get rid of quotes around the 
class containing `foo()`, which is omitted here because it isn't necessary to 
reproduce the bug), but doing so changes the output of `inspect.signature()` 
but not the output `inspect.Parameter()`, causing a mismatch between the two 
that breaks the test.

The above passes on 3.7.9, 3.8.7 & 3.9.1, and if I uncomment the first line, it 
fails on those same versions.
As can be expected, the annotations import is a no-op on 3.10.0a5 and the test 
passes either way.

I expect `inspect` might have not been correctly updated to support postponed 
annotations, but I haven't looked at the implementation (I'm not familiar with 
the CPython codebase at all) so it's just a guess.

--
components: Library (Lib)
messages: 387875
nosy: 1ace
priority: normal
severity: normal
status: open
title: __future__.annotations breaks inspect.signature()
type: behavior
versions: Python 3.7, Python 3.8, Python 3.9

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