Best practice for setting the default value of a parameter that's supposed to be a list in Python?

Question

I have a Python function that takes a list as a parameter. If I set the parameter\'s default value to an empty list like this:

def func(items=[]):
    print          


        

Answer 1

I just encountered this for the first time, and my immediate thought is "well, I don't want to mutate the list anyway, so what I really want is to default to an immutable list so Python will give me an error if I accidentally mutate it." An immutable list is just a tuple. So:

  def func(items=()):
      print items

Sure, if you pass it to something that really does want a list (eg isinstance(items, list)), then this'll get you in trouble. But that's a code smell anyway.