python lcs length

1def lcs(X, Y):
2  
3    n = len(Y)
4    m = len(X)
5    
6    L = [[None]*(n + 1) for i in range(m + 1)]
7    for i in range(m + 1): 
8        for j in range(n + 1): 
9            if i == 0 or j == 0 : 
10                L[i][j] = 0
11            elif X[i-1] == Y[j-1]: 
12                L[i][j] = L[i-1][j-1]+1
13            else: 
14                L[i][j] = max(L[i-1][j], L[i][j-1])
15                
16    return L[m][n]