[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library

Larry Hastings larry at hastings.org
Fri Apr 26 19:30:40 CEST 2013

On 04/26/2013 09:27 AM, Serhiy Storchaka wrote:
> 26.04.13 18:50, Larry Hastings написав(ла):
>> The standard Java documentation on enums:
>>     http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
> This example requires more than features discussed here. It requires 
> an enum constructor.
> class Planet(Enum):
>     MERCURY = Planet(3.303e+23, 2.4397e6)
> [...]
> This can't work because the name Planet in the class definition is not 
> defined.

It can't work because you inserted the word "Planet" there.  If you omit 
the word "Planet", this would work fine with something like the 
metaclass instantiate-all-data-members behavior in flufl.enum 4.

Here is the hack, demonstrated in Python 3:

    class Metaclass(type):
         def __new__(cls, name, bases, namespace):
             result = type.__new__(cls, name, bases, dict(namespace))
             for name, value in namespace.items():
                 if not (callable(value) or name.startswith("__")):
                     value = result(name, value)
                     setattr(result, name, value)
             return result

    class Planet(metaclass=Metaclass):
         MERCURY = (3.303e+23, 2.4397e6)

         def __init__(self, name, value):
             self.mass, self.radius = value

         def surfaceGravity(self):
             return 6.67300E-11 * self.mass / (self.radius ** 2)

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

    print("If you weigh 175 pounds, on Mercury you'd weigh",

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