Enums and Python

[Fred Gansevles]

If you are still interested in using enum (or the nearest Python
equivalent I can think of) after the discussion about dictionaries and
tuple unpacking, I've home-brewn a module caled 'Enum' which gives you
something you might use.
Here it is:

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# Enum.py - Enumerated types in python
# Author: Fred Gansevles <gansevle at cs.utwente.nl>

__version__ = 0, 0, 1

""" Enumerations in Python

    From 'The C++ Programming Language by Bjarne Stroustrup':

        An enumeration is a distinct integral type with named constants.

        For example,

            enum color { red, yellow, green=20, blue };

        makes 'color' an integral type describing various colors.

        The possible values of an object op type 'color' are 'red',
        'yellow', 'green', 'blue'; these values can be converted to the
        'int' values 0, 1, 20 and 21.

    Since names only can spring-into-existance by assigning it a value,
    either as a variable or a keyword, the above syntax is not legal in
    Python.

    The "closest match" would be something like:

        color = enum('red', 'yellow', green=20, 'blue')

    but, since keyword parameters *must always come last* this isn't
    legal either.

    The "next closest match" would be something like:

        color = enum('red', 'yellow', 'green', 'blue', green=20)

    which would be the equivalent of the C++ code above.

    If 'enum' would be an instance of a class, you could not only
    reference the individual values (i.e: "color.blue") but jou can
    also loop-over all the values of color (i.e.: "for col in color:")

    This is what I attempt to make here.

"""
import string

class enum:
    __init = 1
    def __init__(self, *args, **kw):
        value = 0
        self.__names = args
        self.__dict = {}
        for name in args:
            if kw.has_key(name):
                value = kw[name]
            self.__dict[name] = value
            value = value + 1
        self.__init = 0

    def __getitem__(self, item):
        name = self.__names[item]
        return getattr(self, name)

    def __getattr__(self, name):
        return self.__dict[name]

    def __setattr__(self, name, value):
        if self.__init:
            self.__dict__[name] = value
        else:
            raise AttributeError, "enum is ReadOnly"

    def __call__(self, name_or_value):
        if type(name_or_value) == type(0):
            for name in self.__names:
                if getattr(self, name) == name_or_value:
                    return name
            else:
                raise TypeError, "no enum for %d" % name_or_value
        else:
            return getattr(self, name_or_value)

    def __repr__(self):
        result = ['<enum']
        for name in self.__names:
            result.append("%s=%d" % (name, getattr(self, name)))
        return string.join(result)+'>'

def test():
    color = enum('red', 'yellow', 'green', 'blue', green=20)
    print color
    print color.yellow
    print color(21)
    for col in color:
        print color(col), '=', col

if __name__ == '__main__':
    test()

--
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----          Linux/Windows-NT/IntraNetware Administrator          ----
-- Whenever it sounds simple, I've probably missed something! ... Me --
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