[Python-ideas] constant/enum type in stdlib (original) (raw)

Terry Reedy tjreedy at udel.edu
Wed Jan 30 22:32:12 CET 2013

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

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