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Better support for SomeEnum.item == some_literal #19594

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5 changes: 4 additions & 1 deletion mypy/meet.py
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Original file line number Diff line number Diff line change
Expand Up @@ -547,9 +547,12 @@ def _type_object_overlap(left: Type, right: Type) -> bool:
right = right.fallback

if isinstance(left, LiteralType) and isinstance(right, LiteralType):
if left.value == right.value:
if left.value == right.value or (left.fallback.type.is_enum ^ right.fallback.type.is_enum):
# If values are the same, we still need to check if fallbacks are overlapping,
# this is done below.
# Enums are more interesting:
# * if both sides are enums, they should have same values
# * if exactly one of them is a enum, fallback compatibibility is enough
left = left.fallback
right = right.fallback
else:
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47 changes: 47 additions & 0 deletions test-data/unit/check-enum.test
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Expand Up @@ -2681,3 +2681,50 @@ reveal_type(Wrapper.Nested.FOO) # N: Revealed type is "Literal[__main__.Wrapper
reveal_type(Wrapper.Nested.FOO.value) # N: Revealed type is "builtins.ellipsis"
reveal_type(Wrapper.Nested.FOO._value_) # N: Revealed type is "builtins.ellipsis"
[builtins fixtures/enum.pyi]

[case testEnumItemsEqualityToLiterals]
# flags: --python-version=3.11 --strict-equality
from enum import Enum, StrEnum, IntEnum

class A(str, Enum):
a = "b"
b = "a"

A.a == "a"
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It's not clear to me why A.a == "a" should be allowed

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Yep, that's basically what I want to gather feedback on: at runtime it's true, so there is some merit in allowing such comparisons (see also #17162 for exactly such example).

I'll need to update all added tests to check reachability - if the comparison is not considered non-overlapping, corresponding if bodies must be reachable.

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Did I misunderstand something?

>>> import enum
>>> class X(enum.Enum):
...   a = "b"
... 
>>> X.a == "a"
False

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@sterliakov sterliakov Aug 5, 2025 •
edited
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Plain enum.Enum doesn't compare equal, of course.

In the test case, however, I'm checking StrEnum and str, Enum subclasses, and they do compare equal to strings matching the values:

import enum

class A(str, enum.Enum):
    A = 'a'

A.A == 'a'  # True

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(so in #17162 both cases must be reachable, and IMO shouldn't show comparison-overlap errors)

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I think we might be talking about different things. I'm referring to an X.a which is a "b", not a "a". I somehow missed that it doesn't return True for non-string-enums though. For reference:

>>> import enum
>>> class X(str, enum.Enum):
...   a = "b"
... 
>>> X.a == "a"  # this is currently allowed, but returns False
False
>>> X.a == "b"  # this is currently disallowed, but returns True
True

While this PR handles the second case, I think the first case matters too. (Though I haven't cared enough to look at mypy internals and see why it happens ATM)

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Ough, sorry, I must return to this tomorrow with more focus. Yep, that's the question I asked in the PR description: should we reject such comparisons with an incompatible by value literal? I don't have any strong preference here, except that this implementation complexity may not be worth the benefits: how often do people compare two literals for equality? I don't see any obvious use case for that, so just saying "ok, this looks good enough, such comparison is fine at type level" might be a better strategy. Implementing value-based checks would be slightly less trivial.

A.a == "b"

A.a == A.a
A.a == A.b # E: Non-overlapping equality check (left operand type: "Literal[A.a]", right operand type: "Literal[A.b]")

class B(StrEnum):
a = "b"
b = "a"

B.a == "a"
B.a == "b"

B.a == B.a
B.a == B.b # E: Non-overlapping equality check (left operand type: "Literal[B.a]", right operand type: "Literal[B.b]")

B.a == A.a # E: Non-overlapping equality check (left operand type: "Literal[B.a]", right operand type: "Literal[A.a]")

class C(IntEnum):
a = 0

C.a == "a" # E: Non-overlapping equality check (left operand type: "Literal[C.a]", right operand type: "Literal['a']")
C.a == "b" # E: Non-overlapping equality check (left operand type: "Literal[C.a]", right operand type: "Literal['b']")

C.a == C.a
C.a == C.b

class D(int, Enum):
a = 0

D.a == "a" # E: Non-overlapping equality check (left operand type: "Literal[D.a]", right operand type: "Literal['a']")
D.a == "b" # E: Non-overlapping equality check (left operand type: "Literal[D.a]", right operand type: "Literal['b']")

D.a == D.a
D.a == D.b

D.a == C.a # E: Non-overlapping equality check (left operand type: "Literal[D.a]", right operand type: "Literal[C.a]")
[builtins fixtures/dict.pyi]
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