MyPy incorrectly tracks attribute type in chained ifs with assignment

[serhiy-storchaka]

$ cat b.py 
from typing import Optional

class A:
    x: Optional[str]

def f(a: A) -> None:
    if not a.x:
        pass
    elif a.x == "x":
        a = A()
    else:
        y: str = a.x

$ mypy b.py 
b.py:12: error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "str")

$ mypy --version
mypy 0.701

The assignment a = A() should not affect the code in the else branch.

[ilevkivskyi]

Thanks for reporting! This looks like a bug in conditional type binder. There is one similar issue #5378.

[pmav99]

A simpler demonstration of the bug:

import typing

def to_int(text: str) -> typing.Union[int, str]:
    """ Cast to integer if value is numeric or return unchanged """
    if text.isnumeric():
        result = int(text)
    else:
        result = text
    return result

$ mypy asdf.py
asdf.py:8: error: Incompatible types in assignment (expression has type "str", variable has type "int")

Tried this in versions:

• 0.701
• 0.670
• 0.590

[ilevkivskyi]

@pmav99 this is totally unrelated, mypy doesn't infer unions from conditional definitions, there is a separate issue about this.