Types of inheritanced class attributes aren't detected

[abhiabhi94]

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.

[Timmmm]

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)

[Timmmm]

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

[erictraut]

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

[hauntsaninja]

Fixed in #13494