Fraction to Recurring Decimal LeetCode solution in Python

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from collections import defaultdict
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class Solution:
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    def fractionToDecimal(self, numerator: int, denominator: int) -> str:
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        sign = "" if numerator * denominator >= 0 else "-"
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        numerator, denominator = abs(numerator), abs(denominator)
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        a = numerator // denominator
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        numerator %= denominator
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        if not numerator:
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            return sign + str(a)
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        fractions = []
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        index = defaultdict(int)
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        while 10 * numerator not in index:
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            numerator *= 10
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            index[numerator] = len(fractions)
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            fractions.append(str(numerator // denominator))
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            numerator %= denominator
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        i = index[10 * numerator]
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        return (
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            sign
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            + str(a)
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            + "."
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            + "".join(fractions[:i])
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            + "("
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            + "".join(fractions[i:])
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            + ")"
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            if numerator
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            else sign + str(a) + "." + "".join(fractions[:i])
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        )