Updating values in a dictionary

[Peter Otten]

Chris Rebert wrote:

> Nested comprehensions may be hard to understand, so you may wish to
> write it using a function instead:
> 
> def make_row():
> return dict([(x,0) for x in range(3)])
> 
> matrix = dict([(x,make_row()) for x in range(-3,4,1)])

Another way to skin the cat:

>>> row = dict.fromkeys(range(3), 0)
>>> matrix = dict((x, row.copy()) for x in range(-3, 4))
>>> matrix[2][1] += 1
>>> matrix[-1][2] += 1
>>> from pprint import pprint
>>> pprint(matrix)
{-3: {0: 0, 1: 0, 2: 0},
 -2: {0: 0, 1: 0, 2: 0},
 -1: {0: 0, 1: 0, 2: 1},
 0: {0: 0, 1: 0, 2: 0},
 1: {0: 0, 1: 0, 2: 0},
 2: {0: 0, 1: 1, 2: 0},
 3: {0: 0, 1: 0, 2: 0}}