Python type hinting with exceptions

Yuval Pruss picture Yuval Pruss · May 31, 2017 · Viewed 15.1k times · Source

I have a function that looks like this:

def check_for_errors(result):
    if 'success' in result:
        return True

    if 'error' in result:
        raise TypeError

    return False

In successful run of this function, I should get a bool, but if there is an error I should get a TypeError- which is OK because I deal with it in another function.

My function first line looks like this:

def check_for_errors(result: str) -> bool:

My question is: Should I mention the error in my type hinting?

Answer

Martijn Pieters picture Martijn Pieters · May 31, 2017

Type hinting can't say anything about exceptions. They are entirely out of scope for the feature. You can still document the exception in the docstring however.

From PEP 484 -- Type Hints:

Exceptions

No syntax for listing explicitly raised exceptions is proposed. Currently the only known use case for this feature is documentational, in which case the recommendation is to put this information in a docstring.

Guido van Rossum has strongly opposed adding exceptions to the type hinting spec, as he doesn't want to end up in a situation where exceptions need to be checked (handled in calling code) or declared explicitly at each level.