[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library (original) (raw)

Ethan Furman ethan at stoneleaf.us
Fri Apr 26 18:54:07 CEST 2013

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On 04/26/2013 09:27 AM, Serhiy Storchaka wrote:

26.04.13 18:50, Larry Hastings написав(ла):

On 04/26/2013 12:34 AM, Greg Ewing wrote:

Or if, as Guido says, the only sensible things to use as enum values are ints and strings, just leave anything alone that isn't one of those.

The standard Java documentation on enums: http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html has an example enum of a "Planet", a small record type containing mass and radius--each of which are floats. I don't know whether or not it constitutes good programming, but I'd be crestfallen if Java enums were more expressive than Python enums ;-) This example requires more than features discussed here. It requires an enum constructor. This can't work because the name Planet in the class definition is not defined.

The metaclass can define it easily:

• have generic helper class (I call mine attrs)
• prepare creates an instance of the custom dict
• prepare then inserts the helper class into the custom dict with the same name as the (to be created) custom Enum type
• return the custom dict to Python

class is processed using custom dict

• new gets the custom dict back from Python
• new replaces all instances of the helper class with actual Enum instances (which it creates on the spot)
• any other housekeeping necessary
• new returns the new Enum type, complete with all Enum instances

Here's an example run:

8<----------planets.py------------------------------------------------ from aenum import Enum

class Planet(Enum): MERCURY = Planet(3.303e+23, 2.4397e6) VENUS = Planet(4.869e+24, 6.0518e6) EARTH = Planet(5.976e+24, 6.37814e6) MARS = Planet(6.421e+23, 3.3972e6) JUPITER = Planet(1.9e+27, 7.1492e7) SATURN = Planet(5.688e+26, 6.0268e7) URANUS = Planet(8.686e+25, 2.5559e7) NEPTUNE = Planet(1.024e+26, 2.4746e7)

 def __init__(self, mass, radius):
     self.mass = mass     # in kilograms
     self.radius = radius # in meters

 @property
 def surfaceGravity(self):
     # universal gravitational constant  (m3 kg-1 s-2)
     G = 6.67300E-11
     return G * self.mass / (self.radius * self.radius)

 def surfaceWeight(self, otherMass):
     return otherMass * self.surfaceGravity

print(int(Planet.VENUS)) print(repr(Planet.VENUS)) print(Planet.VENUS.surfaceGravity) 8<----------planets.py------------------------------------------------

8<----------actual run------------------------------------------------ 2 Planet('VENUS', 4.869e+24, 6051800.0, integer=2) 8.871391908774457 8<----------actual run------------------------------------------------

-- Ethan

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