Does [:] slice only make shallow copy of a list?

Question

I have experienced peculiar bugs from this [:] copy.

The docs say [:] makes only a shallow copy but seems:

a = [1,2,3]
id(         


        

Answer 1

It is a shallow copy, but changing b does not affect a in this case because the elements are just numbers. If they were references then a would be updated:

a = [1, 2, 3]
b = a[:]

b[1] = 5
print "a: ", a
print "b: ", b
# a: [1, 2, 3]
# b: [1, 5, 3]

vs

a = [[1], [2], [3]]
b = a[:]

b[1][0] = 5
print "a: ", a
print "b: ", b
# a:  [[1], [5], [3]]
# b:  [[1], [5], [3]]