Assigning a list / dict does not make a copy

• copy
• list
• dict
• mutable

The asker does new_list = old_list or new_dict = old_dict and is surprised when changes to the one reference are reflected in the other. The fix is to copy the list or dict in one of many ways.

• List changes unexpectedly after assignment. How do I clone or copy it to prevent this?
• How to copy a dictionary and only edit the copy