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

[sherlock2215]

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

Problem

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

Before this fix:

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

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

---

Solution

Enhanced comparison_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 as Instance types, not LiteralType.
• Their literal identity is available via last_known_value.
• The fix extracts these literal values, builds a Union of enum literals, and applies narrowing on the true branch of the in expression.

---

Implementation Details

File: mypy/checker.py
Function: comparison_type_narrowing_helper

Added enum-specific logic immediately after the existing None-removal narrowing for the in operator:

• Extract literal enum values from the tuple's items\
• Construct a Union[Literal[Enum.X], Literal[Enum.Y], ...]\
• Use that union as the narrowed type for the LHS\
• Supports both in and not 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 with not in
• Preserves existing None-removal behavior

---

Example

from enum import Enum

class Op(Enum):
    A = "a"
    B = "b"

def process(op: Op) -> None:
    if op in (Op.A,):
        reveal_type(op)  # Literal[Op.A]
        return

    if op is Op.B:
        reveal_type(op)  # Literal[Op.B]
        return

[github-actions[bot]]

Diff from mypy_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]

[A5rocks]

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)

[sherlock2215]

Thanks for the info, i thought the LLM prompt was so detailed, i'll add a shorter PR next time and fix the tests:)