the possibility that isinstance(some_var, (A, B)) causes mypy to forget about a some_var's type

[finite-state-machine]

Bug Report

Certain patterns of if isinstance(...) cause mypy to forget that a variable has a particular generic type.

To Reproduce

Run the following Python code through mypy with --strict:

from typing import TypeVar

# context:
T = TypeVar('T')
class A: pass
class B: pass


def function1(obj: T) -> T:  # expected behaviour:
    if isinstance(obj, A):
        pass
    elif isinstance(obj, B):
        pass
    return obj  # no problem here

def function2(obj: T) -> T:  # unexpected behaviour:
    if isinstance(obj, (A, B)):
        pass
    return obj  # mypy has forgotten that obj is of type T

The issue also reproduces with this alternate definition of function2:

def function2(obj: T) -> T:  # unexpected behaviour:
    if isinstance(obj, A) or isinstance(obj, B):
        pass
    return obj  # mypy has forgotten that obj is of type T

Expected Behavior

mypy should issue no errors.

Actual Behavior

<last line of function2>: error: Returning Any from function declared to return "T" 
[no-any-return]
        return obj  # mypy has forgotten that 'obj' is of type 'T'
        ^
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.910
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): no additional flags are necessary to reproduce the issue
• Python version used: CPython 3.8.3 (from pyenv)
• Operating system and version: macOS 10.14

[erictraut]

This appears to be the same issue as #10817.

[finite-state-machine]

It could very well be the same root cause; given my lack of knowledge of mypy internals, I can't say. If there is a distinction, it's this: in #10817, it appears that the type of the variable is being positively inferred, overriding the already-known generic type. In this issue, no inference is possible, but the type somehow changes from the generic T to Any.

[finite-state-machine]

I also note that in this issue, testing against one class (at a time) is not sufficient to trigger the problem; testing against two classes is required. (See function1, which behaves as expected.)

[finite-state-machine]

This appears to be resolved in 0.991.
https://mypy-play.net/?mypy=latest&python=3.10&gist=c04e5bbd46caaf91e8549f7552b2c074