Grouping constants in Python

Question

My module uses constants that I feel should be grouped. Dog and cat have a number of legs and favorite foods.

1. I want to model nothing but those constants about dogs and cats.
2. Likely I'll have more animals in the future.
3. Those constants won't be used outside of the module.

I thought about:

1. Constants at module level:

   DOG_NUMBER_OF_LEGS = 4
   DOG_FAVOURITE_FOOD = ["Socks", "Meat"]
   CAT_NUMBER_OF_LEGS = 4
   CAT_FAVOURITE_FOOD = ["Lasagna", "Fish"]
   

   They seem not grouped, but it is the solution I prefer.

2. Classes as namespaces:

   class Dog(object):
     NUMBER_OF_LEGS = 4
     DOG_FAVOURITE_FOOD = ["Socks", "Meat"]
   class Cat(object):
     NUMBER_OF_LEGS = 4
     FAVOURITE_FOOD = ["Lasagna", "Fish"]
   

   I don't like this as they're classes I won't use but can be actually instantiated.

3. Dictionary of constants:

   ANIMALS_CONFIG = {
      "DOG" : {
        "NUMBER_OF_LEGS" : 4,
        "FAVOURITE_FOOD" : ["Socks", "Meat"]
      },
      "CAT" : {
        "NUMBER_OF_LEGS" : 4,
        "FAVOURITE_FOOD" : ["Lasagna", "Fish"]
      }
   }
   

I also thought about adding submodules but I don't want to expose those internal constants. What is the most pythonic way or how would you do it?

Answer 1

I would go for a fourth option, preferring a collections.namedtuple:

Animal = namedtuple('Animal', 'number_of_legs favourite_food')

You then create instances like:

DOG = Animal(4, ['Socks', 'Meat'])
CAT = Animal(4, ['Lasagna', 'Fish'])

and access the values externally as:

from animals import CAT

print CAT.number_of_legs

There's really no point having classes if you don't need to create any methods, and I think the form of access above is neater than e.g.:

from animals import animals

print animals['CAT']['number_of_legs']

namedtuples, like vanilla tuples, are immutable, too, so you can't accidentally reassign e.g. CAT.number_of_legs = 2somewhere.

Finally, the namedtuple is a lightweight data structure, which may be important if you're creating lots of animals:

>>> import sys
>>> sys.getsizeof({'number_of_legs': 4, 'favourite_food': ['Lasagna', 'Fish']})
140
>>> from collections import namedtuple
>>> Animal = namedtuple('Animal', 'number_of_legs favourite_food')
>>> sys.getsizeof(Animal(4, ['Lasagna', 'Fish']))
36