Mypy forgets type information after a nested if-statement.

[PetterS]

Mypy forgets type information after a nested if-statement. Small example:

class Base:
    pass


class B(Base):
    def __init__(self):
        self.args = []

class C(Base):    
    def __init__(self):
        self.args = []


def check(x: Base):
    if isinstance(x, (B, C)):

        print(x.args)  # No error here.

        if isinstance(x, B):
            print("B")
        elif isinstance(x, C):
            print("C")

        print(x.args)  # error: "Base" has no attribute "args"

Mypy 0.620 correctly infers that x has a member args for the first print statement, but after the nested if statement, it has forgotten about that and throws an error.

[gvanrossum]

Yeah, this does look like a bug. I somehow thought that the "binder" (the part of mypy that handles this) uses a stack, but here it appears it has popped too much off the stack.

Does this also occur with older mypy versions?

[PetterS]

When I encountered this, I had 0.610.

[PetterS]

I wonder how easy this would be to fix? I would be happy if I could fix it, but I haven't had the time to study the mypy code yet.

[gvanrossum]

The binder is a piece of work, so this may not be an easy project for a beginner. Sorry.

[PetterS]

This bug still happens in 1.7.1.

[hauntsaninja]

This passes after #18138 from @sterliakov :-)