List changes unexpectedly after assignment How do I clone or copy it to prevent this

Question

While using new_list = my_list, any modifications to new_list changes my_list everytime. Why is this, and how can I clone or copy the list to prevent it?

Answer 1

With new_list = my_list, you don't actually have two lists. The assignment just copies the reference to the list, not the actual list, so both new_list and my_list refer to the same list after the assignment.

To actually copy the list, you have various possibilities:

• You can use the built-in list.copy() method (available since Python 3.3):

  new_list = old_list.copy()
  

• You can slice it:

  new_list = old_list[:]