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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