Maximum Common Substring

Given two strings, write a function max_common_substring that finds the longest common substring between the input strings.

A substring is a contiguous sequence of characters within a string. If there is no common substring, your function should return an empty string.
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def max_common_substring(str1, str2):
    res = ''
    start = 0
    for i in range(len(str1)+1):
        cur = str1[start:i]
        if cur in str2:
            if len(cur) > len(res):
                res = cur
        else:
            start+=1
    return res