typing: Dynamically Create Literal Alias from List of Valid Values

Question

I have a function which validates its argument to accept only values from a given list of valid options. Typing-wise, I reflect this behavior using a Literal type alias, like so:

from typing import Literal


VALID_ARGUMENTS = ['foo', 'bar']

Argument = Literal['foo', 'bar']


def func(argument: 'Argument') -> None:
    if argument not in VALID_ARGUMENTS:
        raise ValueError(
            f'argument must be one of {VALID_ARGUMENTS}'
        )
    # ...

This is a violation of the DRY principle, because I have to rewrite the list of valid arguments in the definition of my Literal type, even if it is already stored in the variable VALID_ARGUMENTS. How can I create the Argument Literal type dynamically, given the VALID_ARGUMENTS variable?

The following things do not work:

from typing import Literal, Union, NewType


Argument = Literal[*VALID_ARGUMENTS]  # SyntaxError: invalid syntax

Argument = Literal[VALID_ARGUMENTS]  # Parameters to generic types must be types

Argument = Literal[Union[VALID_ARGUMENTS]]  # TypeError: Union[arg, ...]: each arg must be a type. Got ['foo', 'bar'].

Argument = NewType(
    'Argument',
    Union[
        Literal[valid_argument]
        for valid_argument in VALID_ARGUMENTS
    ]
)  # Expected type 'Type[_T]', got 'list' instead

So, how can it be done? Or can't it be done at all?

Answer 1

Go the other way around, and build VALID_ARGUMENTS from Argument:

Argument = typing.Literal['foo', 'bar']
VALID_ARGUMENTS: typing.Tuple[Argument, ...] = typing.get_args(Argument)

It's possible at runtime to build Argument from VALID_ARGUMENTS, but doing so is incompatible with static analysis, which is the primary use case of type annotations. Building VALID_ARGUMENTS from Argument is the way to go.

I've used a tuple for VALID_ARGUMENTS here, but if for some reason you really prefer a list, you can get one:

VALID_ARGUMENTS: typing.List[Argument] = list(typing.get_args(Argument))