call-overload error on `environ.pop("…", default="…")`

[l0b0]

Bug Report

It's all in the title.

As far as I can tell it's not a duplicate of #10152, because I'm not dealing with TypeVars, or #3750, because I'm not doing any overloading myself.

To Reproduce

$ mypy --strict <(echo 'from os import environ; environ.pop("…", default="…")')
/dev/fd/63:1: error: No overload variant of "pop" of "MutableMapping" matches argument types "str", "str"  [call-overload]
/dev/fd/63:1: note: Possible overload variants:
/dev/fd/63:1: note:     def pop(self, str) -> str
/dev/fd/63:1: note:     def [_T] pop(self, str, Union[str, _T] = ...) -> Union[str, _T]
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

This should succeed. This is a regression since 0.910.

Your Environment

• Mypy version used: 0.930
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.12
• Operating system and version: NixOS 21.11

[jamescooke]

Similar problems happen with get(). A reminder that the help for environ.get() includes the default kwarg in the signature:

In [2]: os.environ.get?
Signature: os.environ.get(key, default=None)
Docstring: D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None.
File:      /usr/lib/python3.8/_collections_abc.py
Type:      method

With the following:

cat > papersize.py 
import os

print(os.environ.get('PAPERSIZE', default='fool'))

Then:

mypy papersize.py
papersize.py:3: error: No overload variant of "get" of "Mapping" matches argument types "str", "str"
papersize.py:3: note: Possible overload variants:
papersize.py:3: note:     def get(self, key: str) -> Optional[str]
papersize.py:3: note:     def [_T] get(self, str, Union[str, _T]) -> Union[str, _T]

I've reproduced with mypy==0.930, strict is not required and on Pythons 3.8.10 and 3.6.15.

I think these problems might be related to this change in typeshed here: python/typeshed#5772 which is an attempt to solve this error: python/typeshed#5771

What I'm wondering is what Python developers are meant to experience with this functionality? Use the os.environ methods as described in help() (with explicit and readable kwargs), but get a type error - so have to remove the helpful kwargs to get it to work correctly?! 🤔 There must be a better way!

[jamescooke]

For anyone looking back on this issue, this is still failing as of 0.942. The connected PR in typeshed is waiting on a sync from typeshed happening in this PR: #12321

[domWalters]

I got this error today in slightly different circumstances.

mypy 1.2.0, python 3.10.10.

The example given in this issue does not error for me:

$ mypy --strict <(echo 'from os import environ; environ.pop("…", default="…")')
Success: no issues found in 1 source file

However, a modification does:

$ mypy --strict <(echo '{}.pop("…", default=None)')
/proc/self/fd/11:1: error: No overload variant of "pop" of "dict" matches argument types "str", "None"  [call-overload]
/proc/self/fd/11:1: note: Possible overload variants:
/proc/self/fd/11:1: note:     def pop(self, <nothing>, /) -> <nothing>
/proc/self/fd/11:1: note:     def [_T] pop(self, <nothing>, _T, /) -> _T
Found 1 error in 1 file (checked 1 source file)

Interestingly, calling pop on a type annotated variable, plus passing None as a positional argument instead of a keyword avoids this issue:

$ mypy --strict <(echo 'a: dict[str, str] = {}; a.pop("…", None)')
Success: no issues found in 1 source file

If you keep default=None, this will error:

$ mypy --strict <(echo 'a: dict[str, str] = {}; a.pop("…", default=None)')
/proc/self/fd/11:1: error: No overload variant of "pop" of "dict" matches argument types "str", "None"  [call-overload]
/proc/self/fd/11:1: note: Possible overload variants:
/proc/self/fd/11:1: note:     def pop(self, str, /) -> str
/proc/self/fd/11:1: note:     def [_T] pop(self, str, Union[str, _T], /) -> Union[str, _T]
Found 1 error in 1 file (checked 1 source file)

[hauntsaninja]

mypy's correct here:

λ python
Python 3.9.9 (main, Aug  7 2022, 02:25:38) 
[Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> {}.pop("...", default=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: dict.pop() takes no keyword arguments