Python nested lists

Question

Why does this code:

x = [[]]*3
x[0].append('a')
x[1].append('b')
x[2].append('c')
x[0]=['d']
 
print x

print [['d'], ['a', 'b', 'c'], ['a', 'b', 'c']]?

Answer 1

This is best explained step by step:

>>> x = [[]]*3
>>> x
[[], [], []]
>>> x[0].append('a')
>>> x
[['a'], ['a'], ['a']]
>>> x[1].append('b')
>>> x
[['a', 'b'], ['a', 'b'], ['a', 'b']]
>>> x[2].append('c')
>>> x
[['a', 'b', 'c'], ['a', 'b', 'c'], ['a', 'b', 'c']]
>>> x[0]=['d']
>>> x
[['d'], ['a', 'b', 'c'], ['a', 'b', 'c']]

The first statement creates a list with three references to the same element in it. So when you modify the first element, you're also modifying the second and third element. Hence, the append statements add a number to each of the elements of the list.