Why do these two append methods produce different results?

Question

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

I'm trying to understand the difference between the two methods below:

def first_append(new_item, a_list=[]):
    a_list.append(new_item)
    return a_list

def second_append(new_item, a_list=None):
    if a_list is None:
        a_list = []
    a_list.append(new_item)
    return a_list

first_append keeps adding to a_list when called multiple times, causing it to grow. However, second_append always returns a list of length 1. What is the difference here?

Examples:

>>> first_append('one')
['one']
>>> first_append('two')
['one', 'two']
>>> second_append('one') 
['one']
>>> second_append('two')
['two']

Your names aren't matching .. `bad_append` etc ... – Levon Jul 17 '12 at 19:57 — Levon, Jul 17 '12 at 19:57

score 0 · Accepted Answer · answered Jul 17 '12 at 20:00

0

Function second_append always creates a new local list for you each time it is called.

answered Jul 17 '12 at 20:00

Levon

138,105
33
200
191

Why do these two append methods produce different results?

1 Answers1