Type narrowing fails with isinstance(self, Protocol) when result is stored in a variable #19684
Closed as duplicate of#11979
Description
Bug Report
When using a @runtime_checkable protocol and checking isinstance(self, MyProtocol), mypy does not correctly narrow the type of self if the isinstance result is first stored in a variable. This results in false positives when accessing protocol members, even though the code is valid at runtime.
To Reproduce
from abc import ABC
from typing import Protocol, runtime_checkable
@runtime_checkable
class MyProtocol(Protocol):
def my_method(self, arg: int) -> str: ...
class Base(ABC):
def foo(self) -> None:
has_my_method = isinstance(self, MyProtocol)
if has_my_method:
# ❌ Mypy error:
# test.py:17: error: "Base" has no attribute "my_method" [attr-defined]
self.my_method(42)
if isinstance(self, MyProtocol):
# ✅ Works fine here
self.my_method(42)
class Extends(Base, MyProtocol):
def my_method(self, arg: int) -> str:
return f"Called with {arg}"Expected Behavior
Both forms should refine self to MyProtocol inside the if block, since semantically the two checks are equivalent at runtime. Maybe I'm missing something here?
For example, the same pattern works fine if there is no self involved:
from typing import Protocol, runtime_checkable
@runtime_checkable
class MyProtocol(Protocol):
def my_method(self, arg: int) -> str: ...
class MyClass:
def my_method(self, arg: int) -> str:
return f"Called with {arg}"
obj = MyClass()
has_my_method = isinstance(obj, MyProtocol)
if has_my_method:
# ✅ No error
obj.my_method(42)Environment:
mypy 1.17.0 (compiled: yes)