Initialise array of empty arrays

Question

This is my code:

a = [[]] * 10
a[0].append(1)
print a # Outputs [[1], [1], [1], [1], [1], [1], [1], [1], [1], [1]]

How can I get a to output

[[1], [], [], [], [], [], [], [], [], []]

?

• Praveen Gollakota: docs.python.org/faq/…
• Paolo Moretti: Initialise a list to a specific length in Python
• Wooble: @PaoloMoretti: That's not the same question, although there are countless duplicates on SO.
• Paolo Moretti: @Wooble No, it's not, but the answer from Alex Martelli also answers this question.

Answer 1

Try

a=[[] for i in xrange(10)]

In your code you're adding the same list 10 times. The following output should clarify this:

>>> a=[[]] * 5
>>> for i in a: print id(i)
... 
155302636
155302636
155302636
155302636
155302636
>>> a=[[] for i in xrange(5)]
>>> for i in a: print id(i)
... 
155302668
155302732
155302924
155303020
155303052

As you can see, in the first example a contains 5 times a reference to the same array object, in the second example it contains references to 5 different array objects.

Answer 2

Your current code creates an array containing the same array ten times.

Use [[] for i in xrange(10)] so you actually create separate arrays.

Answer 3

Try this:

>>> a = [[] for i in range(10)]
>>> a[0].append(1)
>>> a
[[1], [], [], [], [], [], [], [], [], []]