Type hinting a list of specific strings in Python

Question

I have a function as shown below:

from typing import List, Literal, Union


def foo(possible_values: List[Union[Literal['abcd', 'efgh', 'ijkl']]]):
    return {}

Now this is how I want the code to behave:

whenever the possible_values parameter gets values other than ["abcd", "efgh","ijkl"]

Eg:

res = foo(possible_values=["abc", "efgh"])

It should throw an error as abc is not defined in the function signature.

However,

res = foo(possible_values=["abcd", "efgh"])

should work fine as they are subset of what is defined.

Currently, with the above code, it just accepts any arbitrary list of strings.

Answer 1

If you want to constrain values to a predefined set, you might want to use Enum . Like mentioned by others, type hinting won't enforce check and error natively in python, you'll have to either implement it in your function's code, or use a library allowing annotation-based control. Here is an example.

from typing import List
from enum import Enum

# Let's define your possible values as an enumeration. Note that it also inherits from 
# str, which will allow to use its members in comparisons as if they were strings
class PossibleValues(str, Enum):
    abcd = 'abcd'
    efgh = 'efgh'
    ijkl = 'ijkl'

Now your function. Note the type-hinting.

def foo(possible_values: List[PossibleValues]):
    # We unroll the enum as a set, and check that possible_values is a subset of it
    if not set(PossibleValues).issuperset(possible_values):
        raise ValueError(f'Only {[v.value for v in PossibleValues]} are allowed.')
    # Do whatever you need to do
    return {}

Now when you use it:

foo(['abcd', 'efgh'])
# output: {}
foo(['abc', 'efgh'])
# ValueError: Only ['abcd', 'efgh', 'ijkl'] are allowed.