python - calculating catalan numbers using memoization

Question

I am tring to use memoization in order to calculate catalan numbers, but it just does not seem to work, what do I need to change?

def catalan_mem(n, memo = None):
    if n==0:
        return 1
    if memo == None:
        memo = {}
    b=0
    if n not in memo:
       for i in range (n):
           b+=((catalan_mem(i),memo)[0])*((catalan_mem(n-1-i),memo)[0])
    memo[n]=b
    return memo[n]    

thank you!

Answer 1

If you call catalan_mem on an n in memo then you replace memo[n] with 0. This is probably not what you intend.

Without parsing the logic further, this suggests memo[n]=b should be inside of the if block.