How not to create 2D arrays in Python

Python has many idioms that make writing the language a joy. One of them is, to quickly initialize an array of n elements you can do this

>>> a = [None] * 4 
>>> a
[None, None, None, None]

Then what about creating a 2D 4x4 array, should it be this:

>>> a = [[None] * 4] * 4

The answer is no. Doing that gives you an array where each row has the same underlying memory.

>>> a[0][1] = 1
>>> a
[[None, 1, None, None], [None, 1, None, None], [None, 1, None, None], [None, 1, None, None]]

Then what is the proper way to initialize a 2D array with each element an independent memory location:

>>> b = [[None] * 4 for i in range(0, 4)]
>>> b[0][1] = 1
>>> b
[[None, 1, None, None], [None, None, None, None], [None, None, None, None], [None, None, None, None]]
>>> 

Voila!