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June, 2015:

Python matrix initialization gotcha

If you want to spin up a list of lists — a poor man’s matrix — in Python, you may want to initialize it first. That way you can use indices to point directly (random access) into the matrix, with something like:

matrix[i][j][k]

without having to worry whether you’ve managed to make the matrix “big enough” through appending , looping, whatever.

If you are an idiot like me, you will skim StackOverflow and come away with the naive use of the “*” operator to create lists.

In [1]: lol = [[[None]*1]*3]*2

In [2]: lol
Out[2]: [[[None], [None], [None]], [[None], [None], [None]]]

That seems to work fine for our case — a small 3-D matrix (trivial in the third dimension I admit) initialized to None, the pseudo-undefined object of Python. Sounds good. Wait…

In [3]: lol[0][0][0] = 'asdf'

In [4]: lol
Out[4]: [[['asdf'], ['asdf'], ['asdf']], [['asdf'], ['asdf'], ['asdf']]]

Um. Since the same singleton None object was assigned to each of the slots in the matrix, changing it in one place changes it everywhere.

Facepalm.

To do what you actually want to do, use the list comprehension syntax and leave the monstrosity of the * operator alone:


In [21]: lolfixed = [[[None for k in range(1)] for j in range(3)] for i in range(2)]

In [22]: lolfixed = [[[None for k in range(1)] for j in range(3)] for i in range(2)]
KeyboardInterrupt

In [22]: lolfixed
Out[22]: [[[None], [None], [None]], [[None], [None], [None]]]

In [23]: lolfixed[0][0][0] = 'asdf'

In [24]: lolfixed
Out[24]: [[['asdf'], [None], [None]], [[None], [None], [None]]]