Python seems to be automatically matching two lists, preventing me from separating them

Question

# arranges a lists indexes from which ones have the maximum 
# value to which ones have the minimum value
def sortIndex(L):
  I = []
  original = L
  new = L
  for i in range(len(original)):
    I.append(original.index(max(new)))
    del new[I[len(I)-1]]
    print("I=",I)
    print("O=",original)
    print("N=",new)
  return I

A = [1,6,3,8,4]

print(sortIndex(A))

That's the code. When I run it, I get the output (note I print I, O and N after every iteration):

  I= [3]
  O= [1, 6, 3, 4]
  N= [1, 6, 3, 4]
  I= [3, 1]
  O= [1, 3, 4]
  N= [1, 3, 4]
  I= [3, 1, 2]
  O= [1, 3]
  N= [1, 3]
  I= [3, 1, 2, 1]
  O= [1]
  N= [1]
  I= [3, 1, 2, 1, 0]
  O= []
  N= []
  [3, 1, 2, 1, 0]

I don't understand why this is happening. Del seems to be deleting values from both new and original even though they are clearly separate lists. Does anyone know what is going on here?

They're absolutely not separate lists, they're references to the same list `L`. — sco1, Feb 12 '18 at 18:00
*they are clearly separate lists* - `original` and `new` are clearly names for the same list — Peter Wood, Feb 12 '18 at 18:00

score 2 · Accepted Answer · answered Feb 12 '18 at 17:59

2

Because they aren't actually separate lists. Both variables, new and original are pointing to the same list in memory. You can create a copy like you want with copy.deepcopy

from copy import deepcopy
original = L
new=deepcopy(original)

answered Feb 12 '18 at 17:59

SuperStew

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Or, you can just do new = list(L) – Matt_G Feb 12 '18 at 18:02
1

This is true, but that doesn't work for lists of lists. – SuperStew Feb 12 '18 at 18:05
Thank you! I did not know about pointers before now. – Shahb M Feb 12 '18 at 18:06
@ShahbM No problem, it was a painful lesson for me to learn the first time. – SuperStew Feb 12 '18 at 18:08
@SuperStew did op specify a list of lists? – Matt_G Feb 12 '18 at 20:10

Python seems to be automatically matching two lists, preventing me from separating them

1 Answers1