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Waylon Walker
Waylon Walker

Posted on • Originally published at waylonwalker.com

Don't inherit from python BaseException, Here's why.

I ran into a PR this week where the author was inheriting what BaseException rather than exception. I made this example to illustrate the unintended side effects that it can have.

Try running these examples in a .py file for yourself and try to kill them with control-c.

You cannot Keybard interrupt

Since things such as KeyboardInterrupt are created as an exception that inherits from BaseException, if you except BaseException you can no longer
KeyboardInterrupt.

from time import sleep

while True:
    try:
        sleep(30)
    except BaseException: # ❌
        pass
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except from Exception or higher

If you except from exception or something than inherits from it you will be better off, and avoid unintended side effects.

from time import sleep

while True:
    try:
        sleep(30)
    except Exception: # ✅
        pass
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This goes with Custom Exceptions as well

When you make custom exceptions expect that users, or your team members will want to catch them and try to handle them if they can. If you inherit from
BaseException you will put them in a similar situation when they use your
custom Exception.

class MyFancyException(BaseException): # ❌
    ...

class MyFancyException(Exception): # ✅
    ...
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