pythonic way of splitting a list of lists into a length-keyed dictionary?

xs = [
        [1,2,3,4],
        [5,6,7,8],
        [9,0,0,1],
        [2,3],
        [0],
        [5,8,3,2,5,1],
        [6,4],
        [1,6,9,9,2,9]
]

""" expected output:

    xs_dict = {
        1: [[0]]
        2: [[2,3],[6,4]]
        4: [[1,2,3,4],[5,6,7,8],[9,0,0,1]]
        6: [[5,8,3,2,5,1],[1,6,9,9,2,9]]
    }
"""

I can do this, for example, by

xs_dict = {}
for x in xs:
    aux = xs_dict.get(len(x),[])
    aux.append(x)
    xs_dict[len(x)] = aux

print(xs_dict)

But I can't help feeling there should be a more pythonic way to achieve this.

What is it?