`match-case` does not work correctly with Literals vs Enums
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bug
topic-enum
topic-literal-types
topic-match-statement
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
from enum import StrEnum
from typing import Literal, assert_never, reveal_type
class Color(StrEnum):
RED = "r"
GREEN = "g"
BLUE = "b"
def test_literal_as_enum(x: Literal["g"] = "g") -> None:
match x:
case Color.RED:
assert_never(x)
case Color.GREEN:
reveal_type(x)
case Color.BLUE:
assert_never(x)
case _:
assert_never(x)
def test_enum_as_litearl(y: Literal[Color.BLUE] = Color.BLUE) -> None:
match y:
case "r":
assert_never(y)
case "g":
assert_never(y)
case "b":
reveal_type(y)
case _:
assert_never(y)
test_literal_as_enum() # selects green branch at runtim
test_enum_as_litearl() # selects blue branch at runtime
Expected no errors, but mypy-playground reports
main.py:14: error: Statement is unreachable [unreachable]
main.py:18: error: Argument 1 to "assert_never" has incompatible type "Literal['g']"; expected "Never" [arg-type]
main.py:27: error: Statement is unreachable [unreachable]
main.py:29: error: Argument 1 to "assert_never" has incompatible type "Literal[Color.BLUE]"; expected "Never" [arg-type]
Related: https://discuss.python.org/t/amend-pep-586-to-make-enum-values-subtypes-of-literal/59456
Also: #16327, #17162, #19616 (but none of these is specifically about match-case)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the reported errors in the linked mypy-playground example with Python 3.12, master, and warn-unreachable enabled. Trace the match-case narrowing for Literal strings and StrEnum members, then use the shown assert_never and reveal_type checks as the completion criteria: the example should produce no errors.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 42/100