mypy does not show error in excluded folder when PYTHONPATH set

excluded_folder/excluded.py と included_folder/included.py を含む再現用のパッケージレイアウトから始め、次に PYTHONPATH=. の有無による 2 つの mypy コマンドを比較します。--exclude とインポートされたモジュールがどのように検出されるかを追跡します。PYTHONPATH=. を指定した呼び出しで excluded.py 内の互換性のない戻り値が報告され、両方のコマンドをカバーするリグレッションテストがあれば完了です。

Bug Report

As far as I understand, when I use the exclude setting for some folder mypy should still show errors in these packages when imported, because of the follow imports logic. However, if I set the PYTHONPATH to the current working directory, it does not show these errors anymore.

To Reproduce
Here is the folder structure:

src/
|--excluded_folder/
|           |-- excluded.py
|           |-- __init__.py
|--included_folder/
            |-- included.py
            |-- __init__.py

The contents of the file excluded.py are as follows:

def function() -> str:  # obviously wrong return type
        return 5

File included.py looks like so:

from excluded_folder.excluded import function

print(function)

Now, if i run mypy like so, with the setting to exclude the excluded folder, it reports this:

$ mypy --strict --exclude excluded_folder src                          
src/excluded_folder/excluded.py:3: error: Incompatible return value type (got "int", expected "str")  [return-value]
Found 1 error in 1 file (checked 2 source files)

This is what i would expect. It still shows the errors in the file, although excluded.

But if i run mypy with PYTHONPATH set to the current working directory, it does not show any error anymore:

% PYTHONPATH=. mypy --strict --exclude excluded_folder src
Success: no issues found in 2 source files

Expected Behavior
Mypy should also show the errors when the PYTHONPATH is set.

Actual Behavior
Mypy does not show the errors when the PYTHONPATH is set.

Your Environment

• Mypy version used: mypy 1.8.0 (compiled: yes)
• Mypy command-line flags: see commands above
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.11.3