Grouping constants in python

Question

This is mainly a "good python style" question.

I have a module which is using a number of constants that feels should be grouped.

Lets say we have Dogs and cat and each of them have number of legs and favorite food.

Note that

1. we want to model nothing but those constants about Dogs and Cats
2. quite probably we'll have more animals in the future.
3. those constants wont be used outside of the current module.

I thought about the following solutions:

---

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 really grouped, but I think it is the solution I prefer.

---

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 solution as we'll have class that we wont use and they can be actually instantiated.

---

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 dont really want to expose those internal constants

---

what is the most pythonic way to do it / 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