Class constants in python

Question

In python, I want a class to have some "constants" (practically, variables) which will be common in all subclasses. Is there a way to do it with friendly syntax? Right now I use:

class Animal:
    SIZES=["Huge","Big","Medium","Small"]

class Horse(Animal):
    def printSize(self):
        print(Animal.SIZES[1])

and I'm wondering if there is a better way to do it or a way to do it without then having to write "Animal." before the sizes. Horse inherits from Animal.

Not a full answer but If `SIZES` never changes, definitely use a tuple instead of a list. Like so `("Huge","Big","Medium","Small")` — jamylak, May 20 '12 at 09:53
@jamylak your comment seems to be popular, many people seem to consider it valueable. Could you explain the logic behind it? — dmigo, Apr 29 '21 at 13:11
@dmigo Just that the question was asking about constants and a list can be mutated, so it's better to use a tuple which is immutable. A constant should not change — jamylak, Apr 29 '21 at 13:46

score 191 · Answer 1 · edited Sep 23 '16 at 13:20

191

Since Horse is a subclass of Animal, you can just change

print(Animal.SIZES[1])

with

print(self.SIZES[1])

Still, you need to remember that SIZES[1] means "big", so probably you could improve your code by doing something like:

class Animal:
    SIZE_HUGE="Huge"
    SIZE_BIG="Big"
    SIZE_MEDIUM="Medium"
    SIZE_SMALL="Small"

class Horse(Animal):
    def printSize(self):
        print(self.SIZE_BIG)

Alternatively, you could create intermediate classes: HugeAnimal, BigAnimal, and so on. That would be especially helpful if each animal class will contain different logic.

edited Sep 23 '16 at 13:20

pmaruszczyk

2,157
2
24
49

answered May 20 '12 at 10:46

betabandido

18,946
11
62
76

SIZE_BIG=["BigOne","BigTwo"] shall i use this type of constant. – Lova Chittumuri Aug 13 '19 at 16:56

Fred Foo · Answer 2 · 2012-05-20T10:54:13.263

27

You can get to SIZES by means of self.SIZES (in an instance method) or cls.SIZES (in a class method).

In any case, you will have to be explicit about where to find SIZES. An alternative is to put SIZES in the module containing the classes, but then you need to define all classes in a single module.

edited May 20 '12 at 10:54

answered May 20 '12 at 09:53

Fred Foo

355,277
75
744
836

Horse is indeed a subclass of Animal. Actually, it is perfectly possible to use self.SIZES[1] instead of Animal.SIZES[1]. – betabandido May 20 '12 at 10:47
1

@betabandido: the OP corrected the question. Will update the answer accordingly. – Fred Foo May 20 '12 at 10:53

score 20 · Answer 3 · answered May 20 '12 at 09:53

20

class Animal:
    HUGE = "Huge"
    BIG = "Big"

class Horse:
    def printSize(self):
        print(Animal.HUGE)

answered May 20 '12 at 09:53

amit kumar

20,438
23
90
126

Also you can access via self, ex. "self.HUGE" or "self.BIG". This would only work internally within the class. – cevaris Jan 08 '14 at 17:17

score 6 · Answer 4 · answered May 21 '12 at 07:28

Expanding on betabandido's answer, you could write a function to inject the attributes as constants into the module:

def module_register_class_constants(klass, attr_prefix):
    globals().update(
        (name, getattr(klass, name)) for name in dir(klass) if name.startswith(attr_prefix)
    )

class Animal(object):
    SIZE_HUGE = "Huge"
    SIZE_BIG = "Big"

module_register_class_constants(Animal, "SIZE_")

class Horse(Animal):
    def printSize(self):
        print SIZE_BIG

`print SIZE_BIG` out of class will be the same? Will it be editable `Animal.SIZE_HUGE = 'Small'`? — Crusader, Mar 22 '17 at 13:17

Class constants in python

4 Answers4