Convert list of class objects to list of lists

Question

My goal is to write a function, which receives a list of class objects and converts these into a list of lists, this is what I currently have:

def convertToListOfLists(toConvert):
    listOfLists = []
    temp = []
    for t in toConvert:
        temp.append(t.datenbestand)
        temp.append(t.aktenzeichen)
        temp.append(t.markendarstellung)
        temp.append(t.aktenzustand)
        listOfLists.append(temp)
        temp.clear()
    print(listOfLists)
    return listOfLists

Output: [[], [], [], [], [], [], [], [], [], [], [], [], [], [], [], [], []] while 'toConvert' holding 17 objects

If I move the print into the loop and print out my 'listOfLists' I can see that the objects are added to my 'listOfLists' correctly but as you can see, if I access 'listOfLists' outside the loop, 'listOfLists' only holds empty lists.

What am I missing?

Mandatory link to [Ned Batchelder](https://nedbatchelder.com/text/names.html) — quamrana, May 11 '22 at 09:53
"temp.clear" would do that, just initialise temp inside of the loop — Sayse, May 11 '22 at 09:53
I think you need to deepcopy the temp list. You are appending only a reference to the list and then deleting it. Deepcopy is from a standard library. — alanturing, May 11 '22 at 09:55
@alanturing Yes, but that would be an overcomplicated workaround… — deceze, May 11 '22 at 09:55
@deceze Ok, there is an answer that is bether than my solution. XD — alanturing, May 11 '22 at 09:57

score 1 · Accepted Answer · answered May 11 '22 at 09:54

You need to make a new temp everytime through the loop:

def convertToListOfLists(toConvert):
    listOfLists = []
    for t in toConvert:
        temp = []
        temp.append(t.datenbestand)
        temp.append(t.aktenzeichen)
        temp.append(t.markendarstellung)
        temp.append(t.aktenzustand)
        listOfLists.append(temp)
    print(listOfLists)
    return listOfLists

Convert list of class objects to list of lists

1 Answers1