adjust contextlib.nullcontext.__init__ signature to fix divergences in type checkers

[pinterior]

currently contextlib.nullcontext.__init__ has two overloads with ambiguous parameter types, which result in the following:

1. mypy infers nullcontext[int] | nullcontext[None] if enter_result is int | None
2. pyright infers nullcontext[None] when enter_result is Any or Unknown

typeshed/stdlib/contextlib.pyi

Lines 212 to 215 in 0e16ea3

	@overload
	def __init__(self: nullcontext[None], enter_result: None = None) -> None: ...
	@overload
	def __init__(self: nullcontext[_T], enter_result: _T) -> None: ... # pyright: ignore[reportInvalidTypeVarUse] #11780

this PR removes the enter_result parameter from the first overload of nullcontext.__init__, making both mypy and pyright infer the same type.

Check

from contextlib import nullcontext as nc
from typing import Any

def f(a: Any, s: list, o: int | None):
    reveal_type(nc())     # None
    reveal_type(nc(None)) # None

    reveal_type(nc(""))   # str
    reveal_type(nc(123))  # int

    reveal_type(nc(a))    # Any
    reveal_type(nc(s[0])) # Unknown (pyright), Any (mypy)

    # Optional
    reveal_type(nc(o))              # int | None
    _a: nc[int | None] = nc(o)      # mypy: error -> ok, pyright: ok -> ok
    _b: nc[int] | nc[None] = nc(o)  # mypy: ok -> error, pyright: error -> error

Without patch

$ mypy nc.py 
nc.py:5: note: Revealed type is "contextlib.nullcontext[None]"
nc.py:6: note: Revealed type is "contextlib.nullcontext[None]"
nc.py:8: note: Revealed type is "contextlib.nullcontext[builtins.str]"
nc.py:9: note: Revealed type is "contextlib.nullcontext[builtins.int]"
nc.py:11: note: Revealed type is "contextlib.nullcontext[Any]"
nc.py:12: note: Revealed type is "contextlib.nullcontext[Any]"
nc.py:15: note: Revealed type is "contextlib.nullcontext[builtins.int] | contextlib.nullcontext[None]"
nc.py:16: error: Incompatible types in assignment (expression has type "nullcontext[int | None] | nullcontext[None]", variable has type "nullcontext[int | None]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

$ npx pyright nc.py
nc.py
  nc.py:5:17 - information: Type of "nc()" is "nullcontext[None]"
  nc.py:6:17 - information: Type of "nc(None)" is "nullcontext[None]"
  nc.py:8:17 - information: Type of "nc("")" is "nullcontext[str]"
  nc.py:9:17 - information: Type of "nc(123)" is "nullcontext[int]"
  nc.py:11:17 - information: Type of "nc(a)" is "nullcontext[None]"
  nc.py:12:17 - information: Type of "nc(s[0])" is "nullcontext[None]"
  nc.py:15:17 - information: Type of "nc(o)" is "nullcontext[int | None]"
  nc.py:17:30 - error: Type "nullcontext[int | None]" is not assignable to declared type "nullcontext[int] | nullcontext[None]"
    Type "nullcontext[int | None]" is not assignable to type "nullcontext[int] | nullcontext[None]"
      "nullcontext[int | None]" is not assignable to "nullcontext[int]"
        Type parameter "_T@nullcontext" is invariant, but "int | None" is not the same as "int"
      "nullcontext[int | None]" is not assignable to "nullcontext[None]"
        Type parameter "_T@nullcontext" is invariant, but "int | None" is not the same as "None" (reportAssignmentType)
1 error, 0 warnings, 7 informations

With this patch

$ mypy nc.py --custom-typeshed-dir=../typeshed
nc.py:5: note: Revealed type is "contextlib.nullcontext[None]"
nc.py:6: note: Revealed type is "contextlib.nullcontext[None]"
nc.py:8: note: Revealed type is "contextlib.nullcontext[builtins.str]"
nc.py:9: note: Revealed type is "contextlib.nullcontext[builtins.int]"
nc.py:11: note: Revealed type is "contextlib.nullcontext[Any]"
nc.py:12: note: Revealed type is "contextlib.nullcontext[Any]"
nc.py:15: note: Revealed type is "contextlib.nullcontext[builtins.int | None]"
nc.py:17: error: Incompatible types in assignment (expression has type "nullcontext[int | None]", variable has type "nullcontext[int] | nullcontext[None]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

$ npx pyright nc.py -t ../typeshed
nc.py
  nc.py:5:17 - information: Type of "nc()" is "nullcontext[None]"
  nc.py:6:17 - information: Type of "nc(None)" is "nullcontext[None]"
  nc.py:8:17 - information: Type of "nc("")" is "nullcontext[str]"
  nc.py:9:17 - information: Type of "nc(123)" is "nullcontext[int]"
  nc.py:11:17 - information: Type of "nc(a)" is "nullcontext[Any]"
  nc.py:12:17 - information: Type of "nc(s[0])" is "nullcontext[Unknown]"
  nc.py:15:17 - information: Type of "nc(o)" is "nullcontext[int | None]"
  nc.py:17:30 - error: Type "nullcontext[int | None]" is not assignable to declared type "nullcontext[int] | nullcontext[None]"
    Type "nullcontext[int | None]" is not assignable to type "nullcontext[int] | nullcontext[None]"
      "nullcontext[int | None]" is not assignable to "nullcontext[int]"
        Type parameter "_T@nullcontext" is invariant, but "int | None" is not the same as "int"
      "nullcontext[int | None]" is not assignable to "nullcontext[None]"
        Type parameter "_T@nullcontext" is invariant, but "int | None" is not the same as "None" (reportAssignmentType)
1 error, 0 warnings, 7 informations

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[pinterior]

was originally reported on microsoft/pyright#11304

[srittau]

Thanks!