Python multiply sequence trick

Question

the trick is:

IPython prompt:

In [1]: A = [ [] ] * 2

In [2]: A
Out[2]: [[], []]

In [3]: A[0].append(1)

In [4]: A
Out[4]: [[1], [1]]

Obvious, it's not my expected result, what's I want is [[1], []]. So why? I found no ref in the docs about python multiply sequence.

And is there any elegant(use no any explicit loop) ways to do that?

Answer 1

A = [ [] ] * 2 creates a list with two references to the same list:

>>> A = [ [] ] * 2
>>> id(A[0])
24956880
>>> id(A[1])
24956880
>>> id(A[0]) == id(A[1])
True
>>>

Instead, you need to use a list comprehension:

>>> A = [[] for _ in xrange(2)]
>>> A
[[], []]
>>> A[0].append(1)
>>> A
[[1], []]
>>>

Note that, if you are on Python 3.x, you will need to replace xrange with range.