default function parameter values in python

posted on March 24, 2011 - tagged as: 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:

def my_busted_record_keeper(
    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 =
    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.

Comments !