def correct_word(word_string):
    '''
    Finds all valid one and two letter corrections for word_string, returning the word
    with the highest relative probability as type str.
    '''
    if word_string is None:
        return ""
    elif isinstance(word_string, str):
        return max(find_candidates(word_string), key=find_word_prob)
    else:
        raise InputError("string or none type variable not passed as argument to correct_word")
Finds all valid one and two letter corrections for word_string, returning the word
    with the highest relative probability as type str.