Why b[0] = 1 and not 3?

Question

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

Answer 1

when you copy like this [:] if creates another reference in the memory. if you do a = [1,2] , b = a then a == b and a is b. but when you create it like this b = a[:] then a == b and a is not b

and b is [1, [2,5]] because of the shallow copy. which means that when you use this technique like a = b, it copies only first level of list of memory to a new reference. if you have list in list, the second list is not copied but based in the reference of the copied element

if you do this:

a=[1, [2,3]]
b=a
a [0]=3
a [1][1]=5

then b = [3, [2,5]]

there is also another trick to it which helps you avoid shallow copy by doing this:

from copy import deepcopy
a=[1, [2,3]]
b=deepcopy(a)
a [0]=3
a [1][1]=5

then b = [1, [2,3]]

Answer 2

Gleboffsky, the tags are meant to help other users find your question if they have a similar problem.

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