AttributeError: referencing enum from a class function

[ethanfurman]

AttributeError: referencing enum from a class function

[ethanfurman]

A couple corrections:

• Python 2.7 doesn't have an Enum (you must be using a third-party library, possibly my enum34)
• is_aaa is not a class method

Thanks for the report, but this is expected behavior, and will also fail in 3.11+.

History:

In Python 3.4, when Enum was created, it was not possible to reference one member from another (so F.AAA.BBB failed, as did self.AAA). In 3.5, due to a speed boost implementation detail, it became possible, but was strongly discouraged.

Future:

In Python 3.11 it is again possible to disallow member-to-member access, so that is what is happening. Note that member-to-class-to-member access works fine:

    @property
    def is_aaa(self):
        return self is self.__class__.AAA

On a stylistic note, the recommended way to test for an enum member is

some_var = F.BBB
if some_var is F.AAA:
    # stuff

[BrendanSimon]

Thanks @ethanfurman

Yes, enum34 on Python2.7, and yes I prefer to use aenum (as I am using Flags too).

Yes, it didn't feel right accessing other defined enum values via self, but that is allowable with standard classes.

Using self.__class__.AAA works, as does E.AAA. Is one more efficient than the other?

I felt encapsulating the test in a method made the code more robust as I have the test in multiple places, so less chance of getting the test wrong somewhere (e.g. in a refactor). If you had to test for multiple things then that probability increases.

e.g if my_enum in [ X.CCC, X.AAA, X.FFF, X.EEE ] : # do stuff

Having that test repeated in many places is more error prone. One option is to set a module level variable with the list of things to check.

CAFE = [ X.CCC, X.AAA, X.FFF, X.EEE ]

if my_enum in CAFE:
    # do stuff

Not sure what the python purists prefer, but I like the encapsulating in a function approach - but as always can be persuaded ;-)

Cheers :)

[ethanfurman]

E.AAA is only two lookups, while self.__class__.AAA is three.

If testing multiple things at once a function is fine. I made the comment mainly because you were comparing with == instead of is

[BrendanSimon]

Yep. Using timeit I can see that is performs much better than ==.

Presumably it's a simple pointer comparison rather than a call to an equivalence function that does some checking.

Thanks :)