TIL: Passing exceptions as arguments in Python

Mypy needs an extra identifier to not choke on an exception passed as an argument.

2024-06-07T11:13:05.553336

Mypy needs an extra identifier to not choke on an exception passed as an argument.

This will throw a mypy error:

# code.py
class MyException(Exception):
    pass


def myfunc(custom_exception: Exception) -> None:
    try:
        print('Test')
    except custom_exception:
        print('error)

myfunc(MyException)

The error mypy will throw looks something like this:

$ mypy code.py

code.py:6: error: Exception type must be derived from BaseException (or be a tuple of exception classes)  [misc]
code.py:9: error: Argument 1 to "custom_exception" has incompatible type "type[MyException]"; expected "Exception"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

The solution is to use typing.Type:

# code.py
from typing import Type


class MyException(Exception):
    pass


def myfunc(custom_exception: Type[Exception]) -> None:
    try:
        print('Test')
    except custom_exception:
        print('error)

myfunc(MyException)

Tags: TIL

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