Difficulty in implementing Strassen's algorithm in Python

Question

I don't understand how to call my code recursively. Here is my code so far:

import numpy

B = [[5,5,5,5,5,5,5,5],[6,6,6,6,6,6,6,6],[7,7,7,7,7,7,7,7],[8,8,8,8,8,8,8,8],
 [9,9,9,9,9,9,9,9], [10,10,10,10,10,10,10,10],[11,11,11,11,11,11,11,11],       [12,12,12,12,12,12,12,12]]

A = [[5,5,5,5,5,5,5,5],[6,6,6,6,6,6,6,6],[7,7,7,7,7,7,7,7],[8,8,8,8,8,8,8,8],
 [1,1,1,1,1,1,1,1], [2,2,2,2,2,2,2,2],[3,3,3,3,3,3,3,3],[4,4,4,4,4,4,4,4]]

def main():
   strassen(A,B)

def strassen(A, B):
    A = numpy.asarray(A)
    B = numpy.asarray(B)
    lengthA = len(A)
    lengthB = len(B)
    if lengthA == 2:
        print "will calculate"
    else:       
        a, b = strassen(A[:lengthA//2, :lengthA//2], B[:lengthB//2, :lengthB//2])

        lengthA = lengthA//2
        lengthB = lengthB//2
        print a
        print b
        return a, b

I'm trying to reduce a to [[5,5],[6,6]] and b to [[5,5],[6,6]] but I'm getting an error:

a, b = strassen(A[:lengthA//2, :lengthA//2], B[:lengthB//2, :lengthB//2])
TypeError: 'NoneType' object is not iterable. 

a and b are the 1st 2x2 matrices that will be formed after the 2nd whole matrix division for A and B. Please can someone explain this to me. Thanks

Answer 1

You have no return value in your recursion terminating condition. When I run your code, it prints "will calculate" before giving the error. The error happens after that because there is no return value from the strassen function on the last call (when lengthA == 2 ).