human
I found an interesting problem on https://leetcode.com/problems/longest-arithmetic-subsequence/:
Given an array A of integers, return the length of the longest arithmetic subsequence in A.
Recall that a subsequence of A is a list A[i_1], A[i_2], ..., A[i_k] with 0 <= i_1 < i_2 < ... < i_k <= A.length - 1, and that a sequence B is arithmetic if B[i+1] - B[i] are all the same value (for 0 <= i < B.length - 1).
 
Example 1:
Input: A = [3,6,9,12]
Output: 4
Explanation: 
The whole array is an arithmetic sequence with steps of length = 3.

Example 2:
Input: A = [9,4,7,2,10]
Output: 3
Explanation: 
The longest arithmetic subsequence is [4,7,10].

Example 3:
Input: A = [20,1,15,3,10,5,8]
Output: 4
Explanation: 
The longest arithmetic subsequence is [20,15,10,5].

 
Constraints:

2 <= A.length <= 1000
0 <= A[i] <= 500

I tried it in Python, but could not do it. Can you solve it?

What is a the Python program you requested?
assistant
#Here is the Python program you requested.

class Solution:
    def longestArithSeqLength(self, A: List[int]) -> int:
        dp = {}
        for i in range(len(A)):
            for j in range(i+1,len(A)):
                diff = A[j]-A[i]
                dp[j,diff] = dp.get((i,diff),1)+1
        return max(dp.values())