Enum function implementation in python fails

Question

I have the following piece of code that was already written by someone else which implements the "enumerated" type like in C, but the code is failing in __cmp__ function with the error:

assert self.EType is other.EType, 
AttributeError: 'str' object has no attribute 'EType'

Could you please let me have some suggestions how to make the comparison between a str and an EType objects, to fix the error?

def enum(*parameters):
    assert parameters, "Empty enums are not supported"

    class EClass(object):
        """ Internal class """
        __slots__ = parameters

        def __iter__(self):
            return iter(constants)

        def __len__(self):
            return len(constants)

        def __getitem__(self, i):
            return constants[i]

        def __repr__(self):
            return 'Enum' + str(parameters)

        def __str__(self):
            return 'enum ' + str(constants)

    class EValue(object):
        """ Internal class """
        __slots__ = '__value'

        def __init__(self, value):
            self.__value = value

        Value = property(lambda self: self.__value)
        EType = property(lambda self: EType)

        def __hash__(self):
            return hash(self.__value)

        def __cmp__(self, other):
            assert self.EType is other.EType, \
                "Only values from the same enum are comparable"
            return cmp(self.__value, other.__value)

        def __invert__(self):
            return constants[maximum - self.__value]

        def __nonzero__(self):
            return bool(self.__value)

        def __repr__(self):
            return str(parameters[self.__value])

    maximum = len(parameters) - 1
    print ("maximum %d") % maximum
    constants = [None] * len(parameters)

    for i, each in enumerate(parameters):
        val = EValue(i)
        setattr(EClass, each, val)
        constants[i] = val
    constants = tuple(constants)
    EType = EClass()
    return EType

if __name__ == "__main__":
    call_type = "test1"
    if call_type not in enum('test1', 'test2'):
        print 1

Can you fix your indentation? It's hard to evaluate code that is not runnable. See http://sscce.org — Blckknght, Jan 29 '14 at 10:54
There's a ` in the line `call_type = "test1"` - is that an error from posting here or is it in your actual code? — kaspermoerch, Jan 29 '14 at 11:24
and, i'm not really aware what is the meaning of "Value = property(lambda self: self.__value)" "EType = property(lambda self: EType)" ...could you please explain what is exactly the mechanism of those ? — user3248290, Jan 29 '14 at 11:25
@user3248290 have a look at http://stackoverflow.com/questions/17330160/python-how-does-decorator-property-work — zmo, Jan 29 '14 at 11:27
"how to make the comparison between a str and an EType" Well, how _would_ you compare them? The code pretty clearly states that you should not. How would you compare an int and a string, or a double and a list? — tobias_k, Jan 29 '14 at 11:35
and btw, if you want to use enums, switch to python 3.4! http://stackoverflow.com/questions/36932/how-can-i-represent-an-enum-in-python — zmo, Jan 29 '14 at 11:38
@zmo, you will be surprised in how many places you just can't switch to Python 3.! — volcano, Jan 29 '14 at 11:41
@tobias_k, indeed is stated clearly that the objects should be both enum, so in this case, it is possbile to cast variable call_type to enum? how can I do this. — user3248290, Jan 29 '14 at 12:58
@zmo, for the moment is not possbile to switch to Python 3. I have to fix this error in Python 2.6 — user3248290, Jan 29 '14 at 12:59
so actually I would like to check it value of call type is in the enum('test1', 'test2') — user3248290, Jan 29 '14 at 13:01
and why don't you just use a tuple of strings, if what you want to do is to check a string against the tuple of strings? — zmo, Jan 29 '14 at 13:20

score 0 · Accepted Answer · answered Jan 29 '14 at 13:35

The problem is not with the __cmp__ function but with the way you are using the enum:

call_type = "test1"
if call_type not in enum('test1', 'test2'): ...

The objects in the enum are not strings, but instances of EValue, and the __cmp__ function of EValue is not meant to compare with anything else than another EValue (and it would not make much sense to do so, either).

To work around this, you could either check whether the string is in the string representations of the values of the enum, or add a function to the enum class returning the values (stored in __slots__).

the_enum = enum('test1', 'test2')
print "test1" in map(repr, the_enum)
print "test1" in the_enum.values()

The values() method could look like this:

def values(self):
    return tuple(self.__slots__)

Thanks a lot tobias, your solution is working fine. You've save my day :) — user3248290, Jan 29 '14 at 14:04
@RickyA Thanks for pointing this out. And just to anticipate any misunderstandings, "flagging as solved" is done with the "accept"-checkmark to the left, not by actually "flagging" this answer as "other -> solved". ;-) — tobias_k, Jan 29 '14 at 15:57

Enum function implementation in python fails

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