Question

I would like to create instances of a class containing a list that’s empty by default; instead of later setting this list to the final full list I would like to successively add items to it. Here’s a piece of sample code illustrating this:

#!/usr/bin/python

class test:
    def __init__(self, lst=[], intg=0):
        self.lista   = lst
        self.integer = intg

name_dict = {}
counter   = 0

for name in ('Anne', 'Leo', 'Suzy'):
    counter += 1

    name_dict[name] = test()
    name_dict[name].integer += 1
    name_dict[name].lista.append(counter)

    print name, name_dict[name].integer, name_dict[name].lista

When I ran the above program I expected to get

Anne 1 [1]
Leo 1 [2]
Suzy 1 [3]

as I assumed lista to always be initialised to an empty list.

What I got instead was this:

Anne 1 [1]
Leo 1 [1, 2]
Suzy 1 [1, 2, 3]

If I replace self.lista = lst by self.lista = [] it works fine, just like when I add the line name_dict[name].lista = [] to the for loop.

Why is it that the contents of the previous objects’ lists are retained, yet their values of integer aren’t? I am rather new to Python, so it would be great if someone could point out to me where my thoughts/assumptions have gone astray.

Thanks a lot in advance for your replies.

Answer 1

It is a very bad idea to use a mutable object as a default value, as you do here:

def __init__(self, lst=[], intg=0):
     # ...

Change it to this:

def __init__(self, lst=None, intg=0):
     if lst is None:
         lst = []
     # ...

The reason that your version doesn’t work is that the empty list is created just once when the function is defined, not every time the function is called.

In some Python implementations you can see the value of the default values of the function by inspecting the value of func_defaults:

print test.__init__.func_defaults
name_dict[name] = test()
# ...

Output:

([],)
Anne 1 [1]
([1],)
Leo 1 [1, 2]
([1, 2],)
Suzy 1 [1, 2, 3]