Sanitized Submission

class DPState:
    def __init__(self):
        # number which end at this
        self.num_two_length = 0
        self.num_arithmetic = 0

class Solution:
    def numberOfArithmeticSlices(self, nums: List[int]) -> int:
        N = len(nums)
        dp = [defaultdict(DPState) for _ in range(N)]

        ans = 0
        for i in range(N):
            for j in range(i + 1, N):
                delta = nums[j] - nums[i]

                dp[j][delta].num_two_length += 1
                if delta not in dp[i]:
                    continue

                to_add = dp[i][delta].num_two_length + dp[i][delta].num_arithmetic
                dp[j][delta].num_arithmetic += to_add
                ans += to_add

        return ans