Python Cookbook

Simulating Enumerations in Python

Credit: Will Ware

Problem

You want to define an enumeration in the spirit of C’s enum type.

Solution

Python’s introspection facilities let you add a version of enum, even though Python, as a language, does not support this construct:

import types, string, pprint, exceptions

class EnumException(exceptions.Exception):
    pass

class Enumeration:
    def _ _init_ _(self, name, enumList, valuesAreUnique=1):
        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 uniqueNames.has_key(x):
                raise EnumException, "enum name is not unique: " + x
            if valuesAreUnique and uniqueValues.has_key(i):
                raise EnumException, "enum value is not unique for " + x
            uniqueNames[x] = 1
            uniqueValues[i] = 1
            lookup[x] = i
            reverseLookup[i] = x
            i = i + 1
        self.lookup = lookup
        self.reverseLookup = reverseLookup
    def _ _getattr_ _(self, attr):
        try: return self.lookup[attr]
        except KeyError: raise AttributeError
    def whatis(self, value):
        return self.reverseLookup[value]

Discussion

In C, enum lets you declare several constants, typically with unique values (although you can also explicitly arrange for a value to be duplicated under two different names), without necessarily specifying the actual values (except when you want it to). ...