Python unexpected (to me) behaviour when deleting lists

Question

Bumped into a programming issue that puzzles me a bit. Im parsing data and:

• add each relevant chunk to list1, append list1 to final_list. (for easy and neat future presentation)
• For next relevant piece of information I'm deleting the "old" information in list1 with *list1[:] = []
• append the new fresh information to list1, append list1 to final_list and voilà... I thought :)

All help appreciated!

Example:

final_list=[]
list1 = [1,2,3,4]
list2 = [5,6,7,8]

final_list.append(list1)
final_list.append(list2)
print final_list

list1[:] = []
print final_list

Example output

[[1, 2, 3, 4], [5, 6, 7, 8]]
[[], [5, 6, 7, 8]]

Sys.version

Python: 2.7.9 (default, Dec 10 2014, 12:24:55) [MSC v.1500 32 bit (Intel)]

Answer 1

By calling list1[:] = [], you change the previous value of all list1 everywhere, even in final_list. To preserve the list1 value in final_list, you should make a copy and append the copy instead of the original list. Some explanations and nice methods for list copying can be found here.