I have a problem when I try to work with a list and a copy of it. I know that there are certain ways to copy a list in Python and I think that I follow them. Here is my problem.
a = []
for i in range(10):
a.append([0]*10)
b = a[:]
for j in a:
j[0] = 1
print a
print b
I create a blank list. Then I add 10 sublists of zeros ([0,0,0,0,0....]). Then I copy the list and finally I iterate through the first one. But when I print them, both lists have been changed.
You only created a shallow copy. This creates a new list with references to the contained values. The lists contained in a are still being shared between the lists a and b.
You can add a level of copying:
b = [elem[:] for elem in a]
This creates a new list object, containing shallow copies of the nested lists from a. Since the nested lists themselves contain immutable objects only (integers) this is enough here.
Or you can use copy.deepcopy() to have a library function do this for you, creating a copy by recursing over your structure and creating copies of anything mutable.
To make a proper copy use copy module
b = copy.deepcopy(a)
3 options:
b = list(list(item) for item in a)
b = [item[:] for item in a]
import copy
b = copy.deepcopy(a)
