Best Position for a Service Centre LeetCode solution in Python

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class Solution:
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    def getMinDistSum(self, positions: List[List[int]]) -> float:
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        n = len(positions)
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        if n == 1:
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            return 0
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        def gradient(x, y):
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            ans = [0, 0]
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            for i in range(n):
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                denom = math.sqrt(
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                    pow(x - positions[i][0], 2) + pow(y - positions[i][1], 2)
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                )
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                ans[0] += (x - positions[i][0]) / denom if denom else 0
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                ans[1] += (y - positions[i][1]) / denom if denom else 0
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            return ans
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        def fn(x, y):
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            res = 0
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            for i in range(n):
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                res += math.sqrt(
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                    pow(x - positions[i][0], 2) + pow(y - positions[i][1], 2)
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                )
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            return res
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        x = sum(x for x, _ in positions) / n
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        y = sum(y for _, y in positions) / n
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        lr = 1
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        while lr > 1e-7:
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            dx, dy = gradient(x, y)
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            x -= lr * dx
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            y -= lr * dy
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            lr *= 0.997
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            if not dx and not dy:
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                lr /= 2
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        return fn(x, y)