nullcontext treated as having no __enter__ or __exit__ method

[l0b0]

Bug Report

Using nullcontext to randomly get a specific context manager results in mypy errors.

To Reproduce

Run mypy on the following code:

$ cat test.py 
from contextlib import nullcontext
from random import getrandbits

with (open("path") if bool(getrandbits(1)) else nullcontext()):
    pass

Expected Behavior

As far as I can tell from looking at contextlib.py it does have __enter__ and __exit__, so this error should not happen.

Actual Behavior

$ mypy test.py 
test.py:4: error: "object" has no attribute "__enter__"  [attr-defined]
test.py:4: error: "object" has no attribute "__exit__"  [attr-defined]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.800

• Mypy command-line flags: None

• Mypy configuration options from mypy.ini (and other config files):

  [mypy]
  check_untyped_defs = true
  disallow_any_generics = true
  disallow_untyped_defs = true
  no_implicit_optional = true
  show_error_codes = true
  warn_redundant_casts = true
  warn_return_any = true
  warn_unused_ignores = true
  

• Python version used: 3.8.6

• Operating system and version: Arch Linux

[JelleZijlstra]

The issue isn't with nullcontext, it's that mypy infers (open("path") if bool(getrandbits(1)) else nullcontext()) to be of type object. Variants of this have been reported many times. A workaround is to first declare a variable like cm: ContextManager[None] = (open("path") if bool(getrandbits(1)) else nullcontext()).

[l0b0]

Thank you @JelleZijlstra, is there an overarching issue I can follow? That way we can also close this as a duplicate.

[OddBloke]

The workaround described doesn't seem to work for me. With:

from contextlib import nullcontext
from random import getrandbits
from typing import ContextManager

example: ContextManager[None] = (open("path") if bool(getrandbits(1)) else nullcontext())
with example:
    pass

I still get:

reproducer2.py:5: error: Incompatible types in assignment (expression has type "object", variable has type "ContextManager[None]")
Found 1 error in 1 file (checked 1 source file)

[graingert]

I have to use a TextIO | to make this work

from contextlib import nullcontext
from random import getrandbits
from typing import ContextManager, TextIO


example: TextIO | ContextManager[None] = (open("path") if bool(getrandbits(1)) else nullcontext())

with example:
    pass

https://mypy-play.net/?mypy=latest&python=3.10&flags=strict&gist=d3c744b47fe06aba0782a8b0672d0240

[OddBloke]

Thanks Tom, that works for me also!

[jamesderlin]

Seems like a duplicate of #5512.

[JelleZijlstra]

Yes, one of many in the #join-v-union genre.