[Python-Dev] PEP 435 -- Adding an Enum type to the Python standard library

Barry Warsaw barry at python.org
Thu Apr 25 21:15:12 CEST 2013

On Apr 23, 2013, at 04:33 PM, Antoine Pitrou wrote:

>That said, I don't see why it wouldn't make sense for an enum value to be an
>instance of that class. It can be useful to write `isinstance(value,
>MyEnumClass)`. Also, any debug facility which has a preference for writing
>out class names would produce a better output than the generic "EnumValue".

These semantics seem very weird to me, but at least we have a principled way
to lie about it in Python 3.  We could add this to the metaclass:

    def __instancecheck__(cls, instance):
        return instance.enum is cls or cls in instance.enum.__bases__


    >>> X = Enum('X', 'a b c')
    >>> Y = Enum('Y', 'z y x')
    >>> class Z(Y):
    ...   d = 4
    ...   e = 5
    >>> isinstance(Z.d, Y)
    >>> isinstance(Z.d, Z)
    >>> isinstance(Z.d, X)
    >>> isinstance(Y.z, Y)
    >>> isinstance(Y.z, Z)


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