Recursive Mergesort in Python

mergesort.py

# Alex Walczak, 2017
# Simple recursive merge sort implementation.

def merge(left, right):                  
  if not (len(left) and len(right)):     
    return left or right                 
  result = []                            
  i = j = 0                              
  while len(result) < len(left) + len(right):                                      
    if i == len(left) or j == len(right):                                          
      result.extend(left[i:] or right[j:])                                         
    elif left[i] < right[j]:             
      result.append(left[i])             
      i += 1                             
    else:                                
      result.append(right[j])            
      j += 1                             
  return result                          

def mergesort(A):                        
  if len(A) <= 1:                        
    return A                             
  return merge(mergesort(A[:len(A) // 2]), mergesort(A[len(A) // 2:]))             

if __name__ == '__main__':
  A = [5, 1, 9, 3, 2, 4, 6, 8, 7]          
  print(mergesort(A))