Many-to-one mapping (creating equivalence classes)

>>> monty={ ('parrot','spam','cheese_shop'): 'sketch', 
        ('Cleese', 'Gilliam', 'Palin') : 'actors'}

>>> item=lambda x:[z for y,z in monty.items() if x in y][0]
>>>
>>> item("parrot")
'sketch'
>>> item("Cleese")
'actors'

But let me tell you, It will be slow than normal one to one dictionary.

You could override dict's indexer, but perhaps the following simpler solution would be better:

>>> assoc_list = ( (('parrot','spam','cheese_shop'), 'sketch'), (('Cleese', 'Gilliam', 'Palin'), 'actors') )
>>> equiv_dict = dict()
>>> for keys, value in assoc_list:
    for key in keys:
        equiv_dict[key] = value


>>> equiv_dict['parrot']
'sketch'
>>> equiv_dict['spam']
'sketch'

(Perhaps the nested for loop can be compressed an impressive one-liner, but this works and is readable.)

It seems to me that you have two concerns. First, how do you express your mapping originally, that is, how do you type the mapping into your new_mapping.py file. Second, how does the mapping work during the re-mapping process. There's no reason for these two representations to be the same.

Start with the mapping you like:

monty = { 
    ('parrot','spam','cheese_shop'): 'sketch', 
    ('Cleese', 'Gilliam', 'Palin') : 'actors',
}

then convert it into the mapping you need:

working_monty = {}
for k, v in monty.items():
    for key in k:
        working_monty[key] = v

producing:

{'Gilliam': 'actors', 'Cleese': 'actors', 'parrot': 'sketch', 'spam': 'sketch', 'Palin': 'actors', 'cheese_shop': 'sketch'}

then use working_monty to do the work.