Prints all possible combinations of words from input lists; maintains order

set_combination.py

#!/usr/bin/python

from itertools import product

def set_combination(list_of_lists):
    '''
    Prints out all possible combinations of words (each combination only once),
    where one word is taken from each list, in the same order the lists are 
    given. 
    '''
    if type(list_of_lists) == list:
        for item in list_of_lists:
            if type(item) != list:
                raise GeneratorExit
        temp = set()
        for elem in product(*list_of_lists):
            if elem not in temp:
                temp.add(elem)
                yield ' '.join(elem)
    else:
        raise GeneratorExit
            
if __name__ == '__main__':
    LIST_OF_ITEMS = [['apple', 'banana', 'pear'], ['car', 'truck'],
     ['zambia', 'malawi', 'kenya']]
    try:
        for combination_string in set_combination(LIST_OF_ITEMS):
            print combination_string
    except GeneratorExit:
        print "The argument should be a list of lists!"