my_phonebook = {}
backwards_phonebook = {}

def add_telephone_number(name,number):
    my_phonebook[name] = number

def interactive_add_telephone_number():
    name = 'nothing'
    while name !='':
        name = input('Enter name or hit return if finished: ')
        if name != '':
            number = input('Enter the number: ')
            add_telephone_number(name,number)

def create_inverse_phonebook(phonebook):
    global backwards_phonebook
    backwards_phonebook={}
    for name in phonebook:
        number = phonebook[name]
        print(name, number)
        if number in backwards_phonebook:
            backwards_phonebook[number] = backwards_phonebook[number]+','+name
        else:
            backwards_phonebook[number] = name

def create_inverse_phonebook2(phonebook):
    global backwards_phonebook
    backwards_phonebook={}
    for name in phonebook:
        number = phonebook[name]
        print(name, number)
        if number in backwards_phonebook:
            backwards_phonebook[number].append(name)
        else:
            backwards_phonebook[number] = [name]