Python enums revisited

[Will Ware]

Third time's a charm, one hopes.
=======================================================================
"""I once tried to give Python something like C's enums, as described
here: http://groups.google.com/groups?selm=G6qzLy.6Fo%40world.std.com
That approach had the flaw of trying to assign to a dictionary
returned by the locals() function, intending that such assignments
would become class attributes, but that isn't guaranteed to work as
versions of Python change."""

import types, string, pprint, exceptions

class EnumException(exceptions.Exception):
    pass

class Enumeration:
    def __init__(self, name, enumList):
        self.__doc__ = name
        lookup = { }
        reverseLookup = { }
        i = 0
        uniqueNames = [ ]
        uniqueValues = [ ]
        for x in enumList:
            if type(x) == types.TupleType:
                x, i = x
            if type(x) != types.StringType:
                raise EnumException, "enum name is not a string: " + x
            if type(i) != types.IntType:
                raise EnumException, "enum value is not an integer: " + i
            if x in uniqueNames:
                raise EnumException, "enum name is not unique: " + x
            if i in uniqueValues:
                raise EnumException, "enum value is not unique for " + x
            uniqueNames.append(x)
            uniqueValues.append(i)
            lookup[x] = i
            reverseLookup[i] = x
            i = i + 1
        self.lookup = lookup
        self.reverseLookup = reverseLookup
    def __getattr__(self, attr):
        if not self.lookup.has_key(attr):
            raise AttributeError
        return self.lookup[attr]
    def whatis(self, value):
        return self.reverseLookup[value]

Volkswagen = Enumeration("Volkswagen",
    ["JETTA",
     "RABBIT",
     "BEETLE",
     ("THING", 400),
     "PASSAT",
     "GOLF",
     ("CABRIO", 700),
     "EURO_VAN",
     "CLASSIC_BEETLE",
     "CLASSIC_VAN"
     ])

Insect = Enumeration("Insect",
    ["ANT",
     "APHID",
     "BEE",
     "BEETLE",
     "BUTTERFLY",
     "MOTH",
     "HOUSEFLY",
     "WASP",
     "CICADA",
     "GRASSHOPPER",
     "COCKROACH",
     "DRAGONFLY"
     ])

def demo(lines):
    previousLineEmpty = 0
    for x in string.split(lines, "\n"):
        if x:
            if x[0] != '#':
                print ">>>", x; exec x; print
                previousLineEmpty = 1
            else:
                print x
                previousLineEmpty = 0
        elif not previousLineEmpty:
            print x
            previousLineEmpty = 1

def whatkind(value, enum):
    return enum.__doc__ + "." + enum.whatis(value)

class ThingWithType:
    def __init__(self, type):
        self.type = type

demo("""
car = ThingWithType(Volkswagen.BEETLE)
print whatkind(car.type, Volkswagen)
bug = ThingWithType(Insect.BEETLE)
print whatkind(bug.type, Insect)

# Notice that car's and bug's attributes don't include any of the
# enum machinery, because that machinery is all CLASS attributes and
# not INSTANCE attributes. So you can generate thousands of cars and
# bugs with reckless abandon, never worrying that time or memory will
# be wasted on redundant copies of the enum stuff.

print car.__dict__
print bug.__dict__
pprint.pprint(Volkswagen.__dict__)
pprint.pprint(Insect.__dict__)
""")

-- 
-----------------------------------+---------------------
 22nd century: Esperanto, geodesic | Will Ware
 domes, hovercrafts, metric system | wware at world.std.com