Calculate Digit Sum of a String LeetCode solution in Python

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class Solution:
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    def digitSum(self, s: str, k: int) -> str:
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        while len(s) > k:
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            set_3 = [s[i : i + k] for i in range(0, len(s), k)]
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            s = ""
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            for e in set_3:
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                val = 0
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                for n in e:
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                    val += int(n)
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                s += str(val)
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        return s