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Newsgroups: comp.lang.python
From: Demian Brecht <demianbre...@gmail.com>
Date: Sun, 4 Nov 2012 22:27:52 -0800
Local: Mon, Nov 5 2012 1:27 am
Subject: Multi-dimensional list initialization
So, here I was thinking "oh, this is a nice, easy way to initialize a 4D matrix" (running 2.7.3, non-core libs not allowed):
m = [[None] * 4] * 4
The way to get what I was after was:
m = [[None] * 4, [None] * 4, [None] * 4, [None * 4]] (Obviously, I could have just hardcoded the initialization, but I'm too lazy to type all that out ;))
The behaviour I encountered seems a little contradictory to me. [None] * 4 creates four distinct elements in a single array while [[None] * 4] * 4 creates one distinct array of four distinct elements, with three references to it:
>>> a = [None] * 4
['a', None, None, None]
>>> a[0] = 'a' >>> a >>> m = [[None] * 4] * 4
[['m', None, None, None], ['m', None, None, None], ['m', None, None, None], ['m', None, None, None]]
>>> m[0][0] = 'm' >>> m Is this expected behaviour and if so, why? In my mind either result makes sense, but the inconsistency is what throws me off.
Demian Brecht
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