Python Functions, Param Type Checking + Strict Value Checking

Question

I like to define parameter types when writing Python to give the reader an easy understanding of what should be passed into a function, and what the type of data is going to look like. For example,

def reverse_string(string_to_reverse: str) -> str:
   ...
   return reversed_string

I have a special case however, where the function I am making is not only of the type str, but also should always be passed one of two string values. Anything else SHOULD throw and exception and let the user know something messed up.

In code, this should look like,

>>> def some_func(test_string: str["test_string_1", "test_string_2"]) -> str:
...    return f"Valid string passed \'{test_string}\'"
...
>>> some_func("test_string_1") # ✔️
... "Valid string passed 'test_string_1'"
>>> some_func("test_string_2") # ✔️
... "Valid string passed 'test_string_2'"
>>> some_func("test_string_3") # ❌
... "EXCEPTION: SOME EXCEPTION MESSAGE THROWN HERE"

Is there a way to enforce this in Python, with or without libraries?

Suggestions are appreciated.

Thanks!

If you can change the signature of this method, then enum can be an option to enforce required values. — Azhar Khan, Jul 31 '22 at 13:15
This is a duplicate, I made a recent answer which I think is the best (Same as DeepSpace answered below, I also have a way to enforce it though) https://stackoverflow.com/a/72832981/3936044 — Mandera, Jul 31 '22 at 13:20
My bad, @Mandera. I had not seen that thread before. DeepSpace's solution should work in my case. Thanks everyone! — Saif Ul Islam, Jul 31 '22 at 13:57

DeepSpace · Accepted Answer · 2022-07-31T13:15:26.933

3

Python >= 3.8 supports Literal type, in your case it would be

from typing import Literal

def some_func(test_string: Literal["test_string_1", "test_string_2"]):
    ...

This, however, will only provide static type analysis and will not raise any run-time exceptions (as any other Python type annotations/ hints won't). For these you will have to use assert or explicitly raise an exception.

edited Jul 31 '22 at 13:15

answered Jul 31 '22 at 13:14

DeepSpace

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Python Functions, Param Type Checking + Strict Value Checking

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