Creating a list of lists Python

Question

I'm used to C programming where if you wanted to allocate space 2d array you just declare it as

int a[20][20];

I'm not too used to Python programming yet (I'm still in the C mindstate)

I want to declare a list of lists in this section of code and initalize each element to none.

class World:
    def __init__(self):
        grid = [[none]*20]*20

For some reason it doesn't look right to me. Can someone help me out?

While valid code (well, by `none`, I assume you mean `None`), I seriously doubt that that's what you want. Take a look at [this](http://stackoverflow.com/a/12791510/198633) — inspectorG4dget, Dec 08 '13 at 02:11

score 4 · Accepted Answer · edited May 23 '17 at 11:49

4

grid = [[None]*20]*20 will give you a list of 20 sublists, each of which contains 20None`s. However, all the sublists will be references to the same list in memory, so changing one will affect them all.

You're better off doing something like this:

grid = [[None for _ in xrange(20)] for __ in xrange(20)]

This will give you 20 distinct sublists, each of which contains 20 Nones

Further reading

edited May 23 '17 at 11:49

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answered Dec 08 '13 at 02:14

inspectorG4dget

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Creating a list of lists Python

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