Keeping an object static in python

Question

I'm sure this is a basic question and is in the help file somewhere, the problem is I'm not a programmer and not sure the right terminology to use to search for it. Anyway, I'm having trouble with one object updating dynamically instead of importing static values (again, probably the wrong technical terms but the closest I can get). For instance:

L=[]
O=[]
i=0
while i<3:
  O.append(i)
  L.append(O)
  i+=1
print(L)

I want it to return [[0],[0,1],[0,1,2]] but instead I get [[0,1,2],[0,1,2],[0,1,2]]. Basically the list L seems to be treating O as a variable and simply returning the ending value [0,1,2] three times instead of listing the three different values of O at the time I appended them to L. How do I convince it to do the latter?

It seems you want to *copy* the list before appending. You can use `I.copy()` or `I[:]` for that. — Arminius, Oct 17 '20 at 23:27
Sorry, meant L.append(O) not I. I had originally used "I" as the object name in my code but then changed it to "O" here to try to avoid sentences like "I changed the value of I." Guess I forgot to swap one instance. — khammyke, Oct 17 '20 at 23:30

score 0 · Answer 1 · answered Oct 17 '20 at 23:28

The reason for what you're seeing is that what you are appending onto L each time is a reference to the same single list that you created and assigned to O in the second line of the program. L.append(O) does not copy what's in O. It just puts a reference to it on the list L.

If you copy O when you append it onto L, you'll get what you expected:

  L.append(O[:])

Keeping an object static in python

1 Answers1