Presence of __init__ in parent class triggering false positive error: Access to generic class variables is ambiguous

[jamesbraza]

Bug Report

I have found what is a very "in the weeds" false positive error. Basically, if a parent class has:

1. Base class inherits from package with missing imports ignored in mypy config file
2. Base class uses a ClassVar with a TypeVar inside
3. Child class populates the TypeVar

If this happens, everything works as expected. However, if you override the __init__ method in the base class, it exposes a false positive error "Access to generic class variables is ambiguous" in mypy.

To Reproduce

from typing import ClassVar, Generic, TypeVar

from Phidget22.Phidget import Phidget as Phidget22PhidgetAPI  # Ignored in config

TKeys = TypeVar("TKeys")

class Base(Phidget22PhidgetAPI, Generic[TKeys]):

    foo: ClassVar[TKeys]

    # If this is not commented out, the bug surfaces
    def __init__(self):
        super().__init__()

reveal_type(Base.foo)

class Child(Base[str]):

    foo: ClassVar[str] = "bar"
    reveal_type(foo)

reveal_type(Child.foo)

Expected Behavior

With the __init__ commented out in Base, there is good behavior:

path/to/classvar_ambiguous_access.py:15:13: error: Access to generic class variables is ambiguous  [misc]
path/to/classvar_ambiguous_access.py:15:13: note: Revealed type is 'Any'
path/to/classvar_ambiguous_access.py: note: In class "Child":
path/to/classvar_ambiguous_access.py:20:17: note: Revealed type is 'builtins.str'
path/to/classvar_ambiguous_access.py: note: At top level:
path/to/classvar_ambiguous_access.py:22:13: note: Revealed type is 'builtins.str'

It makes sense that mypy warns about ambiguous access here in line 15, per #2878.

Actual Behavior

With the __init__ uncommented in Base, the false positive bug surfaces (see 2nd to last line):

path/to/classvar_ambiguous_access.py:15:13: error: Access to generic class variables is ambiguous  [misc]
path/to/classvar_ambiguous_access.py:15:13: note: Revealed type is 'Any'
path/to/classvar_ambiguous_access.py: note: In class "Child":
path/to/classvar_ambiguous_access.py:20:17: note: Revealed type is 'builtins.str'
path/to/classvar_ambiguous_access.py: note: At top level:
path/to/classvar_ambiguous_access.py:22:13: error: Access to generic class variables is ambiguous  [misc]
path/to/classvar_ambiguous_access.py:22:13: note: Revealed type is 'Any'

Here we see:

• In the body of Child, mypy properly parses the type of Child.foo
• Outside of the body at the module-level:
  • mypy now has a false positive error "Access to generic class variables is ambiguous"
  • mypy also now doesn't know the type of Child.foo

---

Your Environment

• Mypy version used: mypy==0.800+dev.990b6a6f03ad82c10cb8edbe70508b943336f7f3 (current master at 990b6a6)
• Mypy command-line flags: none notable
• Mypy configuration options from mypy.ini (and other config files): setup.cfg

[mypy-Phidget22.*]
ignore_missing_imports = True

• Python version used: Python==3.8.6
• Operating system and version: macOS Catalina Version 10.15.7