Strange behavior of default function parameters in Python

Sometimes things don’t work in the way we want them to. Today I was asked why this piece of code is not working properly (OK, given problem was a bit different and much more “real-life-applicable”, but it’s just an example):

1
2
3
4
5
6
7
8
9
def mypush(val, mylist=[]):
	mylist.append(val)
	print mylist, ': ', id(mylist)
 
lst = []
mypush(1, lst)
mypush(2, lst)
mypush(1)
mypush(2)

The output is:

1
2
3
4
[1]    :  139750946213184
[1, 2] :  139750946213184
[1]    :  139750946218496
[1, 2] :  139750946218496

What’s wrong with it? NOTHING – that’s the way Python should and will behave in such case.

Do you agree? If yes – stop reading, because you won’t learn anything new. Go to XKCD instead. If no – here’s a brief explaination:

Continue reading