Python中字符串中子字符串的重叠计数

def sliding(a, n):    return (a[i:i+n] for i in xrange(len(a) - n + 1))def substring_count(a, b):    return sum(s == b for s in sliding(a, len(b)))assert list(sliding('abcde', 3)) == ['abc', 'bcd', 'cde']    assert substring_count('ababaa', 'aba') == 2