Types of inheritanced class attributes aren't detected

Bug Report

mypy doesn't detect type of inherited class variables.

To Reproduce

from typing import List

class A:
    a: List[int]

class B(A):
    a = []  # error: Need type annotation for 'a' (hint: "a: List[<type>] = ...")

Expected Behavior

The type of B.a should ideally be inferred from the parent class A.

Actual Behavior

I get an error on the line where I initialize the attribute inside the class B with the message error: Need type annotation for 'a' (hint: "a: List[<type>] = ...").

This works if I define the attribute as an instance attribute inside the __init__ method.

from typing import List

class A:
    def __init__(self) -> None:
        self.a: List[int]

class B(A):
    def __init__(self) -> None:
        super().__init__()
        self.a = []

Your Environment

Mypy version used: 0.812
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: 3.8.5
Operating system and version: Ubuntu 20.04.2 LTS

For what it's worth, I also tried using the mypy playground and the same issue can be reproduced there as well with python 3.9 and mypy latest. You can take a look at https://mypy-play.net/?mypy=latest&python=3.9&gist=ed3482447c995e3b1acd57706c066633

I will be happy to help if there are any clarifications required or anything else that can help resolve the issue.

I looked into the code and it seems like it just isn't implemented. I had a quick go, in checker.py change the code to the following:

    def check_lvalue(self, lvalue: Lvalue) -> Tuple[Optional[Type],
                                                    Optional[IndexExpr],
                                                    Optional[Var]]:
        lvalue_type = None
        index_lvalue = None
        inferred = None

        if self.is_definition(lvalue) and (
            not isinstance(lvalue, NameExpr) or isinstance(lvalue.node, Var)
        ):
            if isinstance(lvalue, NameExpr):
                # If this is a class variable without a type annotation and
                # there's a base class with the same name then set the type to
                # that variable's type.
                if lvalue.node.type is None:
                    for base in lvalue.node.info.mro[1:]:
                        tnode = base.names.get(lvalue.node.name)
                        if tnode is not None and tnode.type is not None:
                            lvalue_type = tnode.type
                            lvalue.node.type = lvalue_type
                            self.store_type(lvalue, lvalue_type)
                            break

                if lvalue.node.type is None:
                    inferred = cast(Var, lvalue.node)
                    assert isinstance(inferred, Var)
            else:
...

Not sure about the store_type(). Needs one of the developers to say if this is a vaguely correct fix.

Test case:

class Base:
  X: list[int] = []

class Derived0(Base):
  X = []

class Derived1(Base):
  X: list[str] = []

class Derived2(Base):
  X = ["a"]

Before:

example.py:5: error: Need type annotation for "X" (hint: "X: List[<type>] = ...")
example.py:8: error: Incompatible types in assignment (expression has type "List[str]", base class "Base" defined the type as "List[int]")
example.py:11: error: List item 0 has incompatible type "str"; expected "int"
Found 3 errors in 1 file (checked 1 source file)

After:

example.py:8: error: Incompatible types in assignment (expression has type "List[str]", base class "Base" defined the type as "List[int]")
example.py:11: error: List item 0 has incompatible type "str"; expected "int"
Found 2 errors in 1 file (checked 1 source file)

Ok I added my attempt in #12022 but it really needs feedback from an actual MyPy developer. I'd appreciate any help.

Allow class variables in subsubclasses to only match base class type

This bug appears to have been fixed. I can no longer repro it with the latest version of mypy (1.5).

Fixed in #13494

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Types of inheritanced class attributes aren't detected #10375

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Types of inheritanced class attributes aren't detected #10375

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Timmmm commented on Dec 16, 2021

Timmmm commented on Jan 19, 2022

erictraut commented on Aug 12, 2023

hauntsaninja commented on Aug 12, 2023

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