mypy v0.812 incompatible type "str"; expected "Union[Literal['a'], Union[Literal['b']..]

user_7535684 0
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Is there a way to make mypy happy with this code without changing class Config, at the moment I cannot change that class (using typer for a cli app). I am aware I can use # type: ignore but is there any other solution?

"""
mypy --version
mypy 0.812
"""
from enum import Enum
from pathlib import Path

from typing_extensions import Literal


class Config(str, Enum):
  Debug = 'Debug'
  Release = 'Release'


BuildMode = Literal["ASAN", "UBSAN", "Coverage", "Release", "Debug"]


def run_c_tests(results_dir: Path, mode: BuildMode):
  ...


configuration = Config.Debug

# mypy error: Argument 2 to "run_c_tests" has incompatible type "str"; expected "Union[Literal['ASAN'], Literal['UBSAN'], Literal['Coverage'], Literal['Release'], Literal['Debug']]"
run_c_tests(Path('apath'), configuration.value)

Mypy v0.782 does not complain on the code above but 0.812 does.

typer mypy python-3.x
3 Answers

As of today the best I could do was to simply ignore the problem

# type: ignore[arg-type]

I also tried adding an assert by duplicating the same tuple already available in Literal, this assert did not help mypy.

assert configuration.value in ["ASAN", "UBSAN", "Coverage", "Release", "Debug"]
run_c_tests(Path('apath'), configuration.value)
user_75356840

I had a similar problem as you and fixed it by using the Final annotation. eg

from typing import Final

STRICT: Final = "strict"

I found the hint here: https://stackoverflow.com/a/63892386/3787959

Annotating a variable as Final indicates that its value will not be substituted for a value of similar type. This makes it correct to infer the type as the specific Literal value, instead of just the general type.

user_37879590

Just to make Tim's answer clearer with minor changes on official example:

def validate_simple(data: Any) -> Literal[True]:  # always returns True
    ...

MODE = Literal['r', 'rb', 'w', 'wb']
def open_helper(file: str, mode: MODE) -> str:
    ...

from typing import Final

# open_mode = 'r' # this gets error with mypy
open_mode: Final = 'r'

open_helper('/some/path', open_mode)  # Passes type check
user_93994020
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