Base 7 LeetCode solution in Python

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class Solution:
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    def convertToBase7(self, num: int) -> str:
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        if not num:
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            return "0"
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        sign = num < 0
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        num = abs(num)
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        stack = []
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        while num:
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            stack.append(str(num % 7))
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            num = num // 7
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        return "-" * sign + "".join(stack[::-1])