Question

I do not understand why my 3 from y is being deleted in the following code. I assume it is some aliasing problem, but is there any way to let y retain the number when x has it removed?

  x = []
  y = []

  for i in range (10):
      if i == 5:
          y.append(x)
          del x[3]
       x.append(i)

print (x)
print (y)

Output:

x = [0, 1, 2, 4, 5, 6, 7, 8, 9]
y = [[0, 1, 2, 4, 5, 6, 7, 8, 9]]

Answer 1

You’ll have to append a copy of x to y:

y.append(x[:])

y.append(x) appends the list that is referenced by x, so y essentially is [x]. Because it is x that is contained by y, and not any other list, you are changing the value of y when you change x.

Also, if x is a nested list, you might want to consider taking a deepcopy of x like so:

import copy
y.append(copy.deepcopy(x))

So that each of the nested lists are copies, and not the same lists in x.