Python challenge question

Question

I got this question in a Python Challenge. Answer is [1,[2,5]]

a =[1,[2,3]]
b=a[:]
a[0]=3
a[1][1]=5
print(b)

So what does [:] do, and why is 5 added into b but 3 is not added?

Answer 1

Hi! Lists are mutable objects. When you use b=a[:], it’s the same as use b=a.copy(); you create a copy of a.

Answer 2

# Hi! You can try test these two:

import copy

a = [1, [2, 3]]
b = copy.deepcopy(a)
a[0] = 3
a[1][1] = 5
print("deepcopy:", b)
del a, b

a =[1, [2, 3]]
b = copy.copy(a)
a[0] = 3
a[1][1] = 5
print("copy:", b)

# When you use a[:] or a.copy() there will be only a shallow copy of a. But in a = [1, [2, 3]] there is a list [2, 3]. This will not be copied with a[:]. To copy even ’on the deep’, you import copy and use b = copy.deepcopy(a). Then the objects in b no longer will be change, when you change the values in a.

Answer 3

Ah thanks Per! I understand now

Answer 4

Hi Per! So in that case, why does a[1][1] replaces the number with 5 in b as well?