mypy looks for modules outside of virtualenv #11274

Bug Report

I have mypy installed inside of a virtualenv. To my surprise, mypy looked for modules outside of the virtualenv. It appears that this is the section of code responsible:

https://github.com/python/mypy/blob/a1863c90/mypy/pyinfo.py#L27-L29

The docs for site.getusersitepackages() say this:

To determine if the user-specific site-packages was added to sys.path ENABLE_USER_SITE should be used.

From my reading, the code section linked to above should check site.ENABLE_USER_SITE before adding site.getusersitepackages() to the returned list of paths.

As a workaround, I found that I can set PYTHONUSERBASE to some non-existent path, so that mypy won't look in a valid directory.

To Reproduce

1. Install mypy in virtualenv

2. Install package outside of virtualenv, e.g. pip install --user types-PyYaml

3. Run mypy foo/

4. Notice that mypy doesn't complain about missing stubs for PyYaml package

5. mypy -v also has this line of output:

   LOG:  Metadata fresh for yaml.dumper: file /home/knkski/.local/lib/python3.9/site-packages/yaml-stubs/dumper.pyi
   

Expected Behavior

mypy inside virtualenv would not look outside of the virtualenv for any packages

Actual Behavior

mypy inside virtualenv looked inside system user site packages folder

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: mypy foo/
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.9.5
• Operating system and version: Ubuntu 21.04