Find max distance (highest difference between indexes) in pairs from a Python list

samepairs.py

from collections import defaultdict


def get_distance(l):
    assert len(l) > 0
    return l[-1] - l[0]  # takes O(1) time


def solution(A):
    distances = defaultdict(list)
    for i, item in enumerate(A):
        distances[item].append(i)  # takes O(n) time
    distances = {k:get_distance(v) for k, v in distances.items() if len(v) > 1}
    # dict set item takes O(1) in average
    return max(distances.values()) if distances else 0

print solution([4, 6, 2, 2, 6, 6, 1])  # should return 4
print solution([1, 2, 3, 4, 5])  # should return 0
print solution([1, 2, 2, 4, 5])  # should return 1