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Tagged unions don't work with Enums #12432
Description
Bug Report
With the example below, the second if doesn't properly narrow the type, it works fine if the Literal['b'] instead of the enum version.
To Reproduce
from queue import Queue
from typing import Literal, TypedDict, Union
from enum import Enum, unique
@unique
class Tag(Enum):
a = 'a'
b = 'b'
class OpA(TypedDict):
tag: Literal[Tag.a]
a: str
class OpB(TypedDict):
tag: Literal[Tag.b]
b: str
Ops = Union[OpA, OpB]
def do(op: Ops):
if op['tag'] == Tag.a:
reveal_type(op)
if op['tag'] == Tag.b:
reveal_type(op)Expected Behavior
if op['tag'] == Tag.b:
reveal_type(op)Should report:
t.py:24: note: Revealed type is "TypedDict('t.OpB', {'tag': Literal[t.Tag.b], 'b': builtins.str})"
Actual Behavior
Reported type is:
t.py:24: note: Revealed type is "Union[TypedDict('t.OpA', {'tag': Literal[t.Tag.a], 'a': builtins.str}), TypedDict('t.OpB', {'tag': Literal[t.Tag.b], 'b': builtins.str})]"
Your Environment
- Mypy version(s) used:
mypy 0.910,mypy 0.941,mypy 0.950+dev.fd8a15e1abdd2bb4f75c3a6319ebfba233e1fe8e (compiled: no) - Mypy command-line flags:
mypy t.py - Mypy configuration options from
mypy.ini(and other config files):nothing - Python version used:
Python 3.10.2 - Operating system and version:
Fedora Linux 35
Activity
AlexWaygood commentedon Mar 23, 2022
Although narrowing using
==is completely valid, and mypy is indeed incorrect here, the more idiomatic way to compare enum values is usingisrather than==. If you useisinstead of==, mypy understands your code flow as you'd expect (mypy playground link here).You'd just rewrite your
dofunction like so:AlexWaygood commentedon Mar 23, 2022
With regard to mypy not understanding enum narrowing using
==, there's already an open PR by @hauntsaninja to fix that: #11521AlexWaygood commentedon Mar 24, 2022
Closing as a duplicate of #10915