def invert(lst):
	lst = list(lst) 				# Makes sure arugment is a list
	
	#list_range = range(0, len(lst))  # Code I might have used if I used a dictionary comprehension
	
	
	ret_dict = dict() 				# Create empty dict
	counter = 0 					# Create counter
	
	for i in lst:
		ret_dict[i] = counter 		# Assign each element of list to 
									# its postion in list
		
		counter = counter + 1 		# Increment counter

	if (len(lst) > len(ret_dict)): 	# Check if the length of new dict is less than 
		return None					# input list, if so, there is duplicates so return none
	
	return ret_dict					# Return created dictionary