Question

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

I’m using the MailSnake in Python, which is a wrapper for the MailChimp API.

Now I’m getting some curious behaviour for a function I’ve written to pull lists of subscribers we have. This is the code I’m using:

from mailsnake import MailSnake
from mailsnake.exceptions import *

ms = MailSnake('key here')

def return_members (status, list_id, members = [], start = 0, limit = 15000, done = 0):
    temp_list = ms.listMembers(status=status, id=list_id, start=page, limit=limit, since='2000-01-01 01:01:01')
    for item in temp_list['data']:  # Add latest pulled data to our list
        members.append(item)
    done = limit + done
    if done < temp_list['total']:  # Continue if we have yet to 
        start = start + 1
        if limit > (temp_list['total'] - done):  # Restrict how many more results we get out if are on the penultimate page
            limit = temp_list['total'] - done
        print 'Making another API call to get complete list'
        return_members(status, list_id, members, page, limit, done)
    return members

for id in lists:
    unsubs = return_members('subscribed',id)
    for person in unsubs:
        print person['email']

print 'Finished getting information'

So this function runs recursively until we have pulled all members from a given list.

But what I’ve noticed is that the variable unsubs seems to just get bigger and bigger. In that when the function return_members is called with different list ids, I get an amalgamation of the emails of every list I have called so far (rather than just one particular list).

If I call return_members(‘subscribed’, id, []) which explicitly gives it a fresh array then it’s fine. But I don’t see why I need to do this, as if I am calling the function with a different list ID, it’s not running recursively and since I haven’t specificed the members variable, it defaults to []

I think this may be a quirk of python, or I’ve just missed something!

Answer 1

The linked SO infamous question by Martjin would help you understand the underline issue, but to get this sorted out you can write the following loop

for item in temp_list['data']:  # Add latest pulled data to our list
    members.append(item)

to a more pythonic version

members = members + temp_list['data'] # Add latest pulled data to our list

this small change would ensure that you are working with an instance different from the one passed as the parameter to return_members