python list referencing?

[dwang040]

I just wanted to double check something about python referencing. I was trying to create a 2d list using a tempList and a finalList, but noticed that by doing tempList.clear(), it would delete all the contents in finalList as well. However, if I were to just do tempList = [], the contents would be saved in finalList. Does this happen because in the former example, the finalList is still referencing tempList, so all the contents are just linked up? Whereas in the latter example, tempList = [] is creating a new object, thus allowing for the old contents to be kept vs being destroyed? 

 

Code and outputs: 

tempList = []
finalList = []

for i in range(3):
    for j in range (4):
        tempList.append(j)
    finalList.append(tempList)
    tempList.clear()
    
print (finalList)

#Output: [[], [], []]
##############################################

finalList = []

for i in range(3):
    tempList = []
    for j in range (4):
        tempList.append(j)
    finalList.append(tempList)
    
print (finalList)

#Output: [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]

 

[tarfeef101]

Yeah, you're thinking the right stuff. 

By default, this is a copy by reference. This means that finalList is appending tempList to itself, and if tempList changes, then since that is what's inside of finalList, it gets changed too.

Whereas in the second example, each time your outer loop is created, you assign a new tempList instead of modifying the old one. So all of those copies of it inside of finalList are not modified, since each time you're working on a new one.

This is hopefully a good guide on copying/passing by reference vs value (I didn't read it tbh, I just took computer science in university): https://medium.freecodecamp.org/understanding-by-reference-vs-by-value-d49139beb1c4

 

P.S. if you want to do stuff the way in the top for whatever reason, python has a method copy.deepcopy you can use to deep copy an object. If that interests you, you may want to google shallow vs deep copies as well.