Mypy failing to understand ProgressBar and contextlib.nullcontext() are both context managers

[jamesbraza]

Bug Report

Mypy 1.8 seems to be failing to combine prompt-toolkit's ProgressBar with contextlib.nullcontext() into a context manager object.

To Reproduce

import contextlib

from prompt_toolkit.shortcuts import ProgressBar

debug_print: bool = True
cm = ProgressBar() if debug_print else contextlib.nullcontext()
reveal_type(cm)  # Revealed type is "builtins.object"
with cm:
    pass

Expected Behavior

No type errors

Actual Behavior

a.py:7:13: note: Revealed type is "builtins.object"
a.py:8:6: error: "object" has no attribute "__enter__"  [attr-defined]
a.py:8:6: error: "object" has no attribute "__exit__"  [attr-defined]

Your Environment

• Mypy version used: 1.8
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.12.1

[AlexWaygood]

Duplicate of #5512 and #10109.

You can work around the mypy limitation here by using two branches rather than the ternary; for unfortunate historical reasons, mypy has different inference rules for the two ways of writing the same thing:

import contextlib

from prompt_toolkit.shortcuts import ProgressBar

debug_print: bool = True
cm: contextlib.AbstractContextManager[object]
if debug_print:
    cm = ProgressBar()
else:
    cm = contextlib.nullcontext()
reveal_type(cm)  # Revealed type is "contextlib.AbstractContextManager[object]"
with cm:
    pass