Can named arguments be used with Python enums?

Question

Example:

class Planet(Enum):      MERCURY = (mass: 3.303e+23, radius: 2.4397e6)      def __init__(self, mass, radius):         self.mass = mass       # in kilograms         self.radius = radius   # in meters 

Ref: https://docs.python.org/3/library/enum.html#planet

Why do I want to do this? If there are a few primitive types (int, bool) in the constructor list, it would be nice to used named arguments.

Answer 1

While you can't use named arguments the way you describe with enums, you can get a similar effect with a namedtuple mixin:

from collections import namedtuple from enum import Enum  Body = namedtuple("Body", ["mass", "radius"])  class Planet(Body, Enum):      MERCURY = Body(mass=3.303e+23, radius=2.4397e6)     VENUS   = Body(mass=4.869e+24, radius=6.0518e6)     EARTH   = Body(mass=5.976e+24, radius=3.3972e6)     # ... etc. 

... which to my mind is cleaner, since you don't have to write an __init__ method.

Example use:

>>> Planet.MERCURY <Planet.MERCURY: Body(mass=3.303e+23, radius=2439700.0)> >>> Planet.EARTH.mass 5.976e+24 >>> Planet.VENUS.radius 6051800.0 

Note that, as per the docs, "mix-in types must appear before Enum itself in the sequence of bases".

Answer 2

The accepted answer by @zero-piraeus can be slightly extended to allow default arguments as well. This is very handy when you have a large enum with most entries having the same value for an element.

class Body(namedtuple('Body', "mass radius moons")):     def __new__(cls, mass, radius, moons=0):         return super().__new__(cls, mass, radius, moons)     def __getnewargs__(self):         return (self.mass, self.radius, self.moons)  class Planet(Body, Enum):      MERCURY = Body(mass=3.303e+23, radius=2.4397e6)     VENUS   = Body(mass=4.869e+24, radius=6.0518e6)     EARTH   = Body(5.976e+24, 3.3972e6, moons=1) 

Beware pickling will not work without the __getnewargs__.

class Foo:     def __init__(self):         self.planet = Planet.EARTH  # pickle error in deepcopy  from copy import deepcopy  f1 = Foo() f2 = deepcopy(f1)  # pickle error here