# Is it possible to define a class constant inside an Enum?

## Question or problem about Python programming:

Python 3.4 introduces a new module enum, which adds an enumerated type to the language. The documentation for enum.Enum provides an example to demonstrate how it can be extended:

```>>> class Planet(Enum):
...     MERCURY = (3.303e+23, 2.4397e6)
...     VENUS   = (4.869e+24, 6.0518e6)
...     EARTH   = (5.976e+24, 6.37814e6)
...     MARS    = (6.421e+23, 3.3972e6)
...     JUPITER = (1.9e+27,   7.1492e7)
...     SATURN  = (5.688e+26, 6.0268e7)
...     URANUS  = (8.686e+25, 2.5559e7)
...     NEPTUNE = (1.024e+26, 2.4746e7)
...         self.mass = mass       # in kilograms
...     @property
...     def surface_gravity(self):
...         # universal gravitational constant  (m3 kg-1 s-2)
...         G = 6.67300E-11
...
>>> Planet.EARTH.value
(5.976e+24, 6378140.0)
>>> Planet.EARTH.surface_gravity
9.802652743337129
```

This example also demonstrates a problem with Enum: in the surface_gravity() property method, a constant G is defined which would normally be defined at class level – but attempting to do so inside an Enum would simply add it as one of the members of the enum, so instead it’s been defined inside the method.

If the class wanted to use this constant in other methods, it’d have to be defined there as well, which obviously isn’t ideal.

Is there any way to define a class constant inside an Enum, or some workaround to achieve the same effect?

## How to solve the problem:

### Solution 1:

This is advanced behavior which will not be needed in 90+% of the enumerations created.

According to the docs:

The rules for what is allowed are as follows: _sunder_ names (starting and ending with a single underscore) are reserved by enum and cannot be used; all other attributes defined within an enumeration will become members of this enumeration, with the exception of __dunder__ names and descriptors (methods are also descriptors).

So if you want a class constant you have several choices:

• create it in `__init__`
• add it after the class has been created
• use a mixin
• create your own `descriptor`

Creating the constant in `__init__` and adding it after the class has been created both suffer from not having all the class info gathered in one place.

Mixins can certainly be used when appropriate (see dnozay’s answer for a good example), but that case can also be simplified by having a base `Enum` class with the actual constants built in.

First, the constant that will be used in the examples below:

`class Constant: # use Constant(object) if in Python 2 def __init__(self, value): self.value = value def __get__(self, *args): return self.value def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.value) `

And the single-use Enum example:

`from enum import Enum class Planet(Enum): MERCURY = (3.303e+23, 2.4397e6) VENUS = (4.869e+24, 6.0518e6) EARTH = (5.976e+24, 6.37814e6) MARS = (6.421e+23, 3.3972e6) JUPITER = (1.9e+27, 7.1492e7) SATURN = (5.688e+26, 6.0268e7) URANUS = (8.686e+25, 2.5559e7) NEPTUNE = (1.024e+26, 2.4746e7) # universal gravitational constant G = Constant(6.67300E-11) def __init__(self, mass, radius): self.mass = mass # in kilograms self.radius = radius # in meters @property def surface_gravity(self): return self.G * self.mass / (self.radius * self.radius) print(Planet.__dict__['G']) # Constant(6.673e-11) print(Planet.G) # 6.673e-11 print(Planet.NEPTUNE.G) # 6.673e-11 print(Planet.SATURN.surface_gravity) # 10.44978014597121 `

And, finally, the multi-use Enum example:

`from enum import Enum class AstronomicalObject(Enum): # universal gravitational constant G = Constant(6.67300E-11) def __init__(self, mass, radius): self.mass = mass self.radius = radius @property def surface_gravity(self): return self.G * self.mass / (self.radius * self.radius) class Planet(AstronomicalObject): MERCURY = (3.303e+23, 2.4397e6) VENUS = (4.869e+24, 6.0518e6) EARTH = (5.976e+24, 6.37814e6) MARS = (6.421e+23, 3.3972e6) JUPITER = (1.9e+27, 7.1492e7) SATURN = (5.688e+26, 6.0268e7) URANUS = (8.686e+25, 2.5559e7) NEPTUNE = (1.024e+26, 2.4746e7) class Asteroid(AstronomicalObject): CERES = (9.4e+20 , 4.75e+5) PALLAS = (2.068e+20, 2.72e+5) JUNOS = (2.82e+19, 2.29e+5) VESTA = (2.632e+20 ,2.62e+5 Planet.MERCURY.surface_gravity # 3.7030267229659395 Asteroid.CERES.surface_gravity # 0.27801085872576176 `

Note:

The `Constant` `G` really isn’t. One could rebind `G` to something else:

`Planet.G = 1 `

If you really need it to be constant (aka not rebindable), then use the new aenum library  which will block attempts to reassign `constant`s as well as `Enum` members.

1 Disclosure: I am the author of the Python stdlib `Enum`, the `enum34` backport, and the Advanced Enumeration (`aenum`) library.

### Solution 2:

The most elegant solution (IMHO) is to use mixins / base class to provide the correct behavior.

• base class to provide the behavior that’s needed for all implementation that’s common to e.g. `Satellite` and `Planet`.
• mixins are interesting if you decide to provide optional behavior (e.g. `Satellite` and `Planet` may have to provide a different behavior)

Here is an example, where you first define your behavior:

`# # business as usual, define your class, methods, constants... # class AstronomicalObject: # universal gravitational constant G = 6.67300E-11 def __init__(self, mass, radius): self.mass = mass # in kilograms self.radius = radius # in meters class PlanetModel(AstronomicalObject): @property def surface_gravity(self): return self.G * self.mass / (self.radius * self.radius) class SatelliteModel(AstronomicalObject): FUEL_PRICE_PER_KG = 20000 @property def fuel_cost(self): return self.FUEL_PRICE_PER_KG * self.mass def falling_rate(self, destination): return complicated_formula(self.G, self.mass, destination) `

Then create your `Enum` with the correct base classes / mixins.

`# # then create your Enum with the correct model. # class Planet(PlanetModel, Enum): MERCURY = (3.303e+23, 2.4397e6) VENUS = (4.869e+24, 6.0518e6) EARTH = (5.976e+24, 6.37814e6) MARS = (6.421e+23, 3.3972e6) JUPITER = (1.9e+27, 7.1492e7) SATURN = (5.688e+26, 6.0268e7) URANUS = (8.686e+25, 2.5559e7) NEPTUNE = (1.024e+26, 2.4746e7) class Satellite(SatelliteModel, Enum): GPS1 = (12.0, 1.7) GPS2 = (22.0, 1.5) `

### Solution 3:

`from enum import Enum class classproperty(object): """A class property decorator""" def __init__(self, getter): self.getter = getter def __get__(self, instance, owner): return self.getter(owner) class classconstant(object): """A constant property from given value, visible in class and instances""" def __init__(self, value): self.value = value def __get__(self, instance, owner): return self.value class strictclassconstant(classconstant): """A constant property that is callable only from the class """ def __get__(self, instance, owner): if instance: raise AttributeError( "Strict class constants are not available in instances") return self.value class Planet(Enum): MERCURY = (3.303e+23, 2.4397e6) VENUS = (4.869e+24, 6.0518e6) EARTH = (5.976e+24, 6.37814e6) MARS = (6.421e+23, 3.3972e6) JUPITER = (1.9e+27, 7.1492e7) SATURN = (5.688e+26, 6.0268e7) URANUS = (8.686e+25, 2.5559e7) NEPTUNE = (1.024e+26, 2.4746e7) def __init__(self, mass, radius): self.mass = mass # in kilograms self.radius = radius # in meters G = classconstant(6.67300E-11) @property def surface_gravity(self): # universal gravitational constant (m3 kg-1 s-2) return Planet.G * self.mass / (self.radius * self.radius) print(Planet.MERCURY.surface_gravity) print(Planet.G) print(Planet.MERCURY.G) class ConstantExample(Enum): HAM = 1 SPAM = 2 @classproperty def c1(cls): return 1 c2 = classconstant(2) c3 = strictclassconstant(3) print(ConstantExample.c1, ConstantExample.HAM.c1) print(ConstantExample.c2, ConstantExample.SPAM.c2) print(ConstantExample.c3) # This should fail: print(ConstantExample.HAM.c3) `

The reason why @property does NOT work and classconstant DOES work is quite simple, and explained in the answer here

The reason that the actual property object is returned when you access
it via a class Hello.foo lies in how the property implements the
__get__(self, instance, owner) special method. If a descriptor is accessed on an instance, then that instance is passed as the
appropriate argument, and owner is the class of that instance.
On the other hand, if it is accessed through the class, then instance
is None and only owner is passed. The property object recognizes this
and returns self.

Thus, the code in `classproperty` is actually a generalization of `property`, lacking the `if instance is None` part.

### Solution 4:

A `property` can be used to provide most of the behaviour of a class constant:

`class Planet(Enum): # ... @property def G(self): return 6.67300E-11 # ... @property def surface_gravity(self): return self.G * self.mass / (self.radius * self.radius) `

This would be a little unwieldy if you wanted to define a large number of constants, so you could define a helper function outside the class:

`def constant(c): """Return a class property that returns `c`.""" return property(lambda self: c) `

… and use it as follows:

`class Planet(Enum): # ... G = constant(6.67300E-11) `

One limitation of this approach is that it will only work for instances of the class, and not the class itself:

`>>> Planet.EARTH.G 6.673e-11 >>> Planet.G `

### Solution 5:

TLDR; NO, it can not be done inside an Enum class.

This said, as the other answers showed, there are ways to get such class owned values associated to an Enum (i.e. via class inheritance / mixins) but such values are not “defined .. inside an Enum”.