exclude does not work when file contains invalid syntax?

[pawamoy]

Bug Report

I'm having an issue with exclude. Basically, I exclude my tests/fixtures folder because it contains code with syntax that is only supported by Python 3.8, 3.9, etc, while my project supports Python 3.7. So the files in tests/fixtures contain invalid syntax on Python 3.7.

When I run mypy on Python 3.7, it errors with this:

tests/fixtures/validation_38.py:1: error: invalid syntax [syntax]
Found 1 error in 1 file (errors prevented further checking)

I exclude the folder with exclude = tests/fixtures in mypy.ini.
I tried excluding the file name directly, for example with exclude = validation_38\.py but it still fails the same way.

To Reproduce

# tests/fixtures/validation_38.py
def full(ctx, a: int, /, b: int, *c: int, d: int, e: int = 0, **f: int):
    pass

Checking only the parent folder, I'm able to exclude it:

% python3.7 -m mypy --exclude validation_38.py --ignore-missing-imports tests/fixtures/
Success: no issues found in 8 source files

Any other way fails:

% python3.7 -m mypy --exclude validation_38.py --ignore-missing-imports tests          
tests/fixtures/validation_38.py:1: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

% python3.7 -m mypy --exclude fixtures/validation_38.py --ignore-missing-imports tests
tests/fixtures/validation_38.py:1: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

% python3.7 -m mypy --exclude '.*/validation_38\.py' --ignore-missing-imports tests
tests/fixtures/validation_38.py:1: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

% python3.7 -m mypy --exclude tests/fixtures --ignore-missing-imports tests
tests/fixtures/validation_38.py:4: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

% python3.7 -m mypy --exclude fixtures --ignore-missing-imports tests 
tests/fixtures/validation_38.py:4: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)

Expected Behavior

Invalid syntax errors should be ignored if the file in question is excluded.
It seemed to work previously (before 1.0.0?).

Actual Behavior

An excluded file with invalid syntax makes mypy terminate with an error.

Your Environment

• Python version used: 3.7
• Mypy version used: 1.0.0
• Mypy command-line flags: see above
• Mypy configuration options from mypy.ini (not used in the examples above):

[mypy]
ignore_missing_imports = true
exclude = tests/fixtures
warn_unused_ignores = true
show_error_codes = true

Possibly related to #9981 and #6897.

[adam-grant-hendry]

This would be useful as well for cookiecutter template files that use jinja syntax in import statements, like:

import {{ cookiecutter.project_slug }}

which is, of course, syntactical invalid python.