[Python] Why does mypy report different errors when run directly versus through pre-commit?

I'm experiencing an inconsistent behavior with mypy type checking in my Python project. When I run mypy directly on a file, I get different results compared to running it through pre-commit hooks.

What I've tried:


Running mypy directly:

$ mypy django_model_file.py
django_model_file.py:5: error: "ForeignKey" expects 1 type argument, but 2 given [type-arg]


Running through pre-commit:

$ pre-commit run mypy --files idjango_model_file
# Different errors or no errors at all


I've confirmed that both are supposed to use the same config file:

• My pre-commit hook specifies entry: mypy --config-file=mypy.ini
• Direct mypy also finds mypy.ini (confirmed with --verbose flag)

Environment details:

• macOS 24.4.0
• Python 3.12
• mypy 1.15.0
• django-stubs 5.1.3
• pre-commit 4.2.0

Key configuration:


# .pre-commit-config.yaml
- id: mypy
name: mypy type checker
entry: mypy --config-file=mypy.ini
language: system # <-- This might be relevant
types: [python]


# mypy.ini
[mypy]
python_version = 3.12
exclude = (?x)(
venv|
build|
dist|
(^|.*/)test_.*\.py$|
(^|.*/)(tests?|__tests__)/.*\.py$)

What could be causing this discrepancy?

1. Is it because pre-commit uses language: system while I'm in a virtualenv?
2. Could the exclude patterns be interpreted differently?
3. Is there some caching mechanism causing stale results?
4. Does pre-commit modify the environment or path in a way that affects mypy?

I need to ensure consistent type checking between CI and local development. Any help identifying why these two approaches produce different results would be greatly appreciated!

Continue reading...