Decoded String at Index LeetCode solution in Python

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class Solution:
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    def decodeAtIndex(self, S: str, K: int) -> str:
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        idx = {}
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        acclens = [0]
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        prevd = 1
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        j = 0
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        for i, c in enumerate(S + "1"):
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            if c.isalpha():
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                idx[acclens[-1] * prevd + j] = i
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                j += 1
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            else:
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                acclens.append(acclens[-1] * prevd + j)
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                prevd = int(c)
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                j = 0
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        k = K - 1
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        for al in reversed(acclens[1:]):
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            k %= al
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            if k in idx:
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                return S[idx[k]]
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        return None  # should never reach this