Get all numbers that add up to a number

Here is one way to solve this problem:

def sum_to_n(n, size, limit=None):
    """Produce all lists of `size` positive integers in decreasing order
    that add up to `n`."""
    if size == 1:
        yield [n]
        return
    if limit is None:
        limit = n
    start = (n + size - 1) // size
    stop = min(limit, n - size + 1) + 1
    for i in range(start, stop):
        for tail in sum_to_n(n - i, size - 1, i):
            yield [i] + tail

You can use it like this.

for partition in sum_to_n(6, 3):
    print partition

[2, 2, 2]
[3, 2, 1]
[4, 1, 1]