Merge sort.

mergesort.py

def mergesort(a):
   if len(a) <= 1:
      return

   mid = len(a) // 2
   b = a[:mid]
   mergesort(b)
   c = a[mid:]
   mergesort(c)

   i, j, k = 0, 0, 0
   while j < len(b) and k < len(c):
      if b[j] < c[k]:
         a[i] = b[j]
         j += 1
      else:
         a[i] = c[k]
         k += 1
      i += 1
   while j < len(b):
      a[i] = b[j]
      i += 1
      j += 1
   while k < len(c):
      a[i] = c[k]
      i += 1
      k += 1

if __name__ == '__main__':
   import random

   a = [random.randint(0, 1000) for i in range(100)]
   print(a)
   mergesort(a)
   print(a)