Count of Range Sum LeetCode solution in Python

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class Solution:
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    def countRangeSum(self, nums: List[int], lower: int, upper: int) -> int:
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        acc = list(accumulate(nums))
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        ans = a = 0
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        for n in nums:
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            a += n
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            ans += sum(1 for x in acc if lower <= x <= upper)
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            acc.pop(0)
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            lower += n
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            upper += n
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        return ans