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fixed Incorrect type of enum in 'if' clause #20234#20248

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fixed Incorrect type of enum in 'if' clause #20234#20248

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sherlock2215 commentedNov 17, 2025

Fixes#20234 --- Correct Enum Literal Narrowing for`in` Operator with Tuples

Problem

When using thein operator with tuples containing enum members, mypy
failed to narrow the left-hand side to the correct enum literal type.

Before this fix:

ifpizzain (Pizza.MARGHERITA,):reveal_type(pizza)# Revealed: Pizza# Expected: Literal[Pizza.MARGHERITA]

Mypy treated the tuple contents as regularInstance types and did not
extract the literal enum values, so narrowing never happened.

Solution

Enhancedcomparison_type_narrowing_helper to detect tuples containing
enum literals and correctly narrow the type based on the enum values
inside the tuple.

Key Points:

Enum values inside tuples appear asInstance types, not
LiteralType.
Their literal identity is available vialast_known_value.
The fix extracts these literal values, builds aUnion of enum
literals, and applies narrowing on the true branch of thein
expression.

Implementation Details

File:mypy/checker.py
Function:comparison_type_narrowing_helper

Added enum-specific logic immediately after the existingNone-removal
narrowing for thein operator:

Extract literal enum values from the tuple's items\
Construct aUnion[Literal[Enum.X], Literal[Enum.Y], ...]\
Use that union as the narrowed type for the LHS\
Supports bothin andnot in

Tests & Behavior

Correctly handled:

Single enum in tuple
python if op in (Op.A,): reveal_type(op) # Literal[Op.A]
Multiple enums
python if op in (Op.A, Op.B): reveal_type(op) # Literal[Op.A] | Literal[Op.B]
Works withnot in
Preserves existing None-removal behavior

Example

fromenumimportEnumclassOp(Enum):A="a"B="b"defprocess(op:Op)->None:ifopin (Op.A,):reveal_type(op)# Literal[Op.A]returnifopisOp.B:reveal_type(op)# Literal[Op.B]return

sherlock2215and others added2 commits

November 17, 2025 22:05

fixed Incorrect type of enum in 'if' clausepython#20234

b778ff0

[pre-commit.ci] auto fixes from pre-commit.com hooks

2ea2c81

for more information, seehttps://pre-commit.ci

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github-actionsbot commentedNov 17, 2025

Diff frommypy_primer, showing the effect of this PR on open source code:

psycopg (https://github.com/psycopg/psycopg)+ tests/pq/test_pgconn.py:508: error: Non-overlapping equality check (left operand type: "Literal[ConnStatus.STARTED, ConnStatus.MADE]", right operand type: "Literal[ConnStatus.OK]")  [comparison-overlap]prefect (https://github.com/PrefectHQ/prefect)+ src/prefect/flows.py:3103: error: Redundant cast to "str"  [redundant-cast]+ src/prefect/flows.py:3116: error: Redundant cast to "str"  [redundant-cast]core (https://github.com/home-assistant/core)- homeassistant/components/shelly/event.py:305: error: Argument 1 to "_trigger_event" of "EventEntity" has incompatible type "Any | None"; expected "str"  [arg-type]pytest (https://github.com/pytest-dev/pytest)+ src/_pytest/config/argparsing.py:247: error: Statement is unreachable  [unreachable]steam.py (https://github.com/Gobot1234/steam.py)- steam/ext/csgo/state.py:180: error: Argument "slot" to "Sticker" has incompatible type "int | None"; expected "Literal[0, 1, 2, 3, 4, 5] | None"  [arg-type]+ steam/ext/csgo/state.py:180: error: Argument "slot" to "Sticker" has incompatible type "int"; expected "Literal[0, 1, 2, 3, 4, 5] | None"  [arg-type]discord.py (https://github.com/Rapptz/discord.py)+ discord/app_commands/models.py:900: error: Unused "type: ignore" comment  [unused-ignore]+ discord/threads.py:286: error: Unused "type: ignore" comment  [unused-ignore]xarray (https://github.com/pydata/xarray)+ xarray/backends/writers.py:841: error: Unused "type: ignore" comment  [unused-ignore]

grayjk reviewed

Nov 17, 2025

View reviewed changes

mypy/checker.py

		ifisinstance(get_proper_type(iterable_type),TupleType):
		# Check if this is an enum instance that can be narrowed
		tuple_type=get_proper_type(iterable_type)
		fori,item_typeinenumerate(tuple_type.items):

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Suggested change

	fori,item_typeinenumerate(tuple_type.items):
	foritem_typeintuple_type.items:

mypy/checker.py

		fori,item_typeinenumerate(tuple_type.items):
		ifisinstance(item_type,Instance):

		ifitem_type.type.is_enum:

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combine with the previous if statement (line 6601)

mypy/checker.py

Comment on lines +6596 to +6597

		literal_types= []
		ifisinstance(get_proper_type(iterable_type),TupleType):

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Suggested change

	literal_types= []
	ifisinstance(get_proper_type(iterable_type),TupleType):
	ifisinstance(get_proper_type(iterable_type),TupleType):
	literal_types= []

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Please don't use an LLM to generate the description or responses! We're fine with extremely short or curt text (I've made so many PRs where the description was just a "fixes [link]"...), and making it longer just wastes time.

Additionally, could you add test cases to the tests? Mypy has an extensive unit test suite. (I know you mentioned them in the PR description, but really they belong as a test case)