Appending to nested list inside a list of lists

Question

Consider this example:

>>> result = [[]] * 8
>>> result
[[], [], [], [], [], [], [], []]
>>> result[0]
[]
>>> result[0].append("foo")
>>> result  # wtf? expected result: [['foo'], [], [], [], [], [], [], []]
[['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo'], ['foo']]

I'm terribly confused by this. Maybe I don't understand how append is expected to be used. How would I append to the ith nested listed inside a list?

See [this question](http://stackoverflow.com/questions/19427735/where-goes-wrong-in-this-list-manipulation/19427770#19427770) from earlier today. — mdml, Oct 17 '13 at 15:14
You are duplicating references to the same list. `result[0]` is the same object as `result[1]`, `result[2]`, etc. — Frédéric Hamidi, Oct 17 '13 at 15:14
Ah... I see. Pretty subtle. Thanks, voted to close my question as duplicate. — David Chouinard, Oct 17 '13 at 15:15

score 5 · Answer 1 · answered Oct 17 '13 at 15:14

That's because, by doing this:

result = [[]] * 8

you make 8 copies of the same list. Your code should be:

>>> result = [[] for _ in xrange(8)]
>>> result
[[], [], [], [], [], [], [], []]
>>> result[0]
[]
>>> result[0].append("foo")
>>> result
[['foo'], [], [], [], [], [], [], []]
>>>

As a proof, consider this:

>>> lst = [[]] * 2
>>> lst
[[], []]
>>> id(lst[0])
28406048
>>> id(lst[1])
28406048
>>>

Notice that the id's of the lists are the same where as here:

>>> lst = [[] for _ in xrange(2)]
>>> lst
[[], []]
>>> id(lst[0])
28408408
>>> id(lst[1])
28418096
>>>

they are different.

Appending to nested list inside a list of lists

1 Answers1