appending nested list - is it supposed to work this way?

Question

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Unexpected feature in a Python list of lists
How to initialize a two-dimensional array in Python?

I wanted to create a nested list of nsize and then append items into it one by one according to a criterion.

For this I wrote it like following ( nsize = 4 )

a = [[]] * 4  # which creates on display a = [ [] , [] , [] , [] ]

Now I do something like this a[1].append(3) which gives me the output [[3], [3], [3], [3]]

What am I doing wrong here ? Should'nt only the 2nd list ( a[1] ) be updated with the value 3 ?

Answer 1

What you are doing creates a list with 4 references to the same sub-list.

You need to do something like a = [[] for _ in xrange(4)] instead.

Answer 2

Yes, it is supposed to work that way, even though it is often surprising. It is equivalent to this

b=[]
a=[b]*4

Then when you say a[1].append(3), that's just like saying b.append(3). And since a contains four references to that same list, you see it appear four times when you print a.