Adam Matan Adam Matan - 1 year ago 118
Python Question

Python many-to-one mapping (creating equivalence classes)

I have a project of converting one database to another. One of the original database columns defines the row's category. This column should be mapped to a new category in the new database.

For example, let's assume the original categories are:

parrot, spam, cheese_shop, Cleese, Gilliam, Palin

Now that's a little verbose for me, And I want to have these rows categorized as
sketch, actor
- That is, define all the sketches and all the actors as two equivalence classes.

>>> monty={'parrot':'sketch', 'spam':'sketch', 'cheese_shop':'sketch',
'Cleese':'actor', 'Gilliam':'actor', 'Palin':'actor'}
>>> monty
{'Gilliam': 'actor', 'Cleese': 'actor', 'parrot': 'sketch', 'spam': 'sketch',
'Palin': 'actor', 'cheese_shop': 'sketch'}

That's quite awkward- I would prefer having something like:

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

But this, of course, sets the entire tuple as a key:

>>> monty['parrot']

Traceback (most recent call last):
File "<pyshell#29>", line 1, in <module>
KeyError: 'parrot'

Any ideas how to create an elegant many-to-one dictionary in Python?



Answer Source

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 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


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

then use working_monty to do the work.

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