def list_difference(list1, list2):
    """
    Probably the most useful "list/set" operation for my use
    Takes list1 and returns the items from list1 that AREN'T in list2.
    so 
    a = {1,2,3}
    b = {2,3,4}
    a - b == {1}

    Useful if you have, for example, a bunch of urls to save. list1 could be the urls to save,
    and list2 could be a list of urls that are already saved. Using the function would return a 
    list of only new urls that need to be saved
    """

    return list(set(list1) - set(list2))