Mergesort in Python, autogenerated test data with hypothesis

mergesort.py

from hypothesis import given
from hypothesis.strategies import lists, integers


def mergesort(lst):
    def merge(l1, l2):
        if not l1:
            return l2
        if not l2:
            return l1
        return ([l1[0]] + merge(l1[1:], l2)
                if l1[0] < l2[0]
                else [l2[0]] + merge(l1, l2[1:]))
    length = len(lst)
    if length < 2:
        return lst
    return merge(
        mergesort(lst[:length/2]),
        mergesort(lst[length/2:]))


@given(lists(integers()))
def test_mergesort(lst):
    assert sorted(lst) == mergesort(lst)