weird behaviour with list of dictionaries in python

Question

Here is a simple code that performs operations on lists:

>>> a = [0] * 5
>>> a
[0, 0, 0, 0, 0]
>>> a[0] = 5
>>> a
[5, 0, 0, 0         


        

Answer 1

This is not weird.

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Workaround:

a = [{} for i in xrange(5)]

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[…] * 5 creates one … and a list of five pointers to this ….

0 is an immutable integer. You cannot modify it, you can just replace it with another integer (such as a[0] = 5). Then it is a different integer.

{} is a mutable dictionary. You are modifying it: a[0]['b'] = 4. It is always the same dictionary.

Answer 2

Try this,

a = map([].append, {} for i in xrange(3))