CODEWITHSUNDEEP
pythonlist
deadfire19
deadfire19

Reputation: 329

How does editing lists in function work

I am having trouble understanding lists:

mList = []
def func1(mList):
    mList.append("1")
    return
func1()
print mList

As I understand it because a list is mutable, if I edit it in a function, the main list will also be edited. In a program I am working on this is happening with one list which I am using as a "save file", however, a second list which I am using as a "value_blacklist" is not being saved after it is edited/added to.

I included the problem part of the actual code I am having issues with if it's of any help.

def func04(feedback, test_list, value_blacklist, upper_limit=6):
    if value_blacklist == []:
            value_blacklist = blacklist_gen(length)
       import random
       new_list = []
        for index in list(range(0, len(feedback))):
        if feedback[index] == 0: #This value is correct, leave it
            new_list.append(test_list[index])
        elif feedback[index] == 2:
            value_blacklist = full_blacklist(test_list[index], value_blacklist)
            new_list.append(0)
        elif feedback[index] == 1:
            value_blacklist[index].append(test_list[index])
            new_list.append(0)
    for index in list(range(0, len(feedback))):
        if new_list[index] == 0:
            new_list[index] = pick_new(index, value_blacklist, upper_limit)
    return new_list

next_guess = lambda: func04(feedback(), save_game[-1], value_blacklist, save_game[0])

Thanks for any help, I'm really confused by this.

Upvotes: 1

Views: 51

Answers (2)

John La Rooy
John La Rooy

Reputation: 304393

Whereever you are saying

value_blacklist = ...

You are rebinding value_blacklist to a new (list) object. If you instead say

value_blacklist[:] = ...

You will replace the contents of the list without rebinding it.

Ask lots of questions until you really really understand this. It's very important to "get" it.

Upvotes: 2

thefourtheye
thefourtheye

Reputation: 239623

When you use = operator in a function, you are not modifying the existing object. Right hand side of the expression creates a new list and returns a reference to it and that reference is assigned to value_blacklist.

value_blacklist = blacklist_gen(length)
...
value_blacklist = full_blacklist(test_list[index], value_blacklist)

These are the places where you create new local lists and referring them with value_blacklist. That's why value_blacklist doesn't reflect the changes. You can confirm this by printing the id of value_blacklist after the assignment statements and at the beginning of the function.

Upvotes: 1

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