Implement strStr() LeetCode solution in Python

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class Solution(object):
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    def strStr(self, haystack, needle):
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        if needle == "":
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            return 0
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        else:
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            return self.search_substring(haystack, needle)
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    def search_substring(self, haystack, needle):
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        len_substring = len(needle)
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        for i in range(len(haystack)):
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            if haystack[i : i + len_substring] == needle:
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                return i
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        return -1