Initializing 2D array in Python

Question

I have a problem with initialzing a 2D array in python. I want a 6x6 array, I did

arr = [[None]*6]*6

But when I do:

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Answer 1

Using list comprehensions, you can say:

arr = [[None for x in range(6)] for y in range(6)]

Then you will have arr[1][2] = 10 working as expected. This is not a very normal thing to do, however. What are you going to use the nested lists for? There may be a better way. For example, working with arrays is made much easier with the numpy package.

Answer 2

@Cameron is correct in suggesting that you use NumPy to deal with arrays of numerical data. And for the second part of your question, ~Niklas B. is spot on with his suggestion to use defaultdict.

What hasn't been covered is why [[None]*6]*6 behaves strangely.

The answer is that [None]*6 creates a list with six Nones in it (like you expect), but [list]*6 does not make six independent copies of list - it makes six copies of a reference to the same list.

Idiomatic Python has a section that may explain this better: "Other languages have variables - Python has names".

Answer 3

arr = [[None for i in range(6)] for j in range(6)]

arr[1][2] = 10

arr