A simple implementation of merge sort in Python.

merge_sort.py

def merge_sort(m, sort_by):
	def merge(left, right):
		result = []
		while left and right:
			# keep on merging/sorting from left/right
			# lists on an element basis 
			if sort_by(left[0], right[0]):
				result.append(left[0])
				left.pop(0)
			else:
				result.append(right[0])
				right.pop(0)
		# there might be elements left in left/right list
		for i in left:
			result.append(i)
		for i in right:
			result.append(i)
		return result

	if len(m) <= 1: # if list less than one element return
		return m
	
	middle = len(m) / 2	# split list in half
	left = m[:middle]; right = m[middle:]

	left = merge_sort(left, max_order)		# sort left
	right = merge_sort(right, max_order)	# sort right
	
	return merge(left, right)	# merge together sorted left/right