如何处理可变数量的嵌套for循环？

Question

I have a dict for which I would like to find all combinations of sums of values, multiplied by an increasing factor. A possible code for the case where the size of the dict is 2:

# data and n come from elsewhere
data = {'a': 1, 'b': 2}
n = 3

for x in xrange(0, n):
    for y in xrange(0, n):
        print("{0} * {1} + {2} * {3} = {4}".format(x, data['a'], y, data['b'], x * data['a'] + y * data['b']))

which gives

0 * 1 + 0 * 2 = 0
0 * 1 + 1 * 2 = 2
0 * 1 + 2 * 2 = 4
1 * 1 + 0 * 2 = 1
(...)
2 * 1 + 2 * 2 = 6

The problem I have is that the number of elements in the dict will vary, so the number of nested for should be changing as well. Is there a better way to code such a problem to accommodate such a variable dict?

Answer 1

You can replace your nested loop with a single loop over the cartesian product

from itertools import product
for x, y in product(range(n), repeat=2):
    ...

This isn't too useful in itself as you still hardcode 2 variables in there. But it leads us on the the next point - itertools.product yields tuples as you iterate over it

from itertools import product
num_loops = 5 # len(data) in your case
for item in product(range(n), repeat=num_loops):
    ... # item is a 5-tuple