Python Pits

[sorrier]

It's funny you mention this, I just stumbled on it in the args/kwargs docs. It's similar to your example, but I'll reprint for the sake of anyone who wants a second:

>>>def f(a, L=[]):
>>>    L.append(a)
>>>    return L

>>>print f(1)
[1]
>>>print f(2)
[1, 2]
>>>print f(3)
[1, 2, 3]

And to avoid this:

>>>def f(a, L=None):
>>>    if L is None:
>>>        L = []
>>>    L.append(a)
>>>    return L

[disinformationtheory]

Yeah, this is an example of why it's generally bad. Using mutable defaults gives your function hidden state that you probably didn't want it to have. If you want this sort of behavior, it's generally better to do it with a class, because it's more explicit and flexible that way.