default function parameter values in python

Sure, we all know that“default parameter values are evaluated when the function definition is executed” in python. But what may not be clear is a fun way to introduce time-dependent bugs. See, this:

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    def my_busted_record_keeper(last_update=datetime.date.today()):
        ...
        print("last update was %s" % last_update)

Is totally broken.

If the code runs for more than a day, we aren’t going to get the results we expect since last_update will have been evaluated at definition time which is now yesterday. Instead we should do something like:

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    def my_busted_record_keeper(last_update=None):
        if last_update is None:
            last_update = datetime.date.today()
        ...
        print("last update was %s" % last_update)

Usually this isn’t a problem because my code crashes at least once a day, but I know I’ve made this mistake before and there’s a non-zero chance something like this running in production. Quite embarrassing.