default function parameter values in python

Posted by on March 24, 2011

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:

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:

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 :roll: , but I know I’ve made this mistake before and there’s a non-zero chance something like this running in production. Quite embarrassing.

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