On 1/30/2013 10:30 AM, Michael Foord wrote:
On 30 January 2013 15:22, Michael Foord
With a Python 3 metaclass that provides default values for *looked up* entries you could have this:
class Color(Enum): RED, WHITE, BLUE
The lookup would create the member - with the appropriate value.
class values(dict): def __init__(self): self.value = 0 def __getitem__(self, key):
Adding 'print(self.value, key)' here prints 0 __name__ 0 __name__ 1 RED 2 WHITE 3 BLUE (I do not understand why it is the second and not first lookup of __name__ that increments the counter, but...)
try: return dict.__getitem__(self, key) except KeyError: value = self[key] = self.value self.value += 1 return value
class EnumMeta(type):
@classmethod def __prepare__(metacls, name, bases): return values()
def __new__(cls, name, bases, classdict): result = type.__new__(cls, name, bases, dict(classdict)) return result
class Enum(metaclass=EnumMeta): pass class Color(Enum): RED, WHITE, BLUE
So RED, WHITE, BLUE are 1, 2, 3; not 0, 1, 2 as I and many readers might expect. That aside (which can be fixed), this is very nice. -- Terry Jan Reedy