inheritable_enum

The inheritable_enum() class decorator is used to make an Enum and all its members inheritable.

Decorating a Class with inheritable_enum

The inheritable_enum class decorator can only be applied to classes with the EnumMeta metaclass. Any other class will raise a TypeError.

try:
    @inheritable_enum
    class MyClass:
        pass
except TypeError:
    print("Can't decorate a non-enum class with inheritable_enum")

>>> Can't decorate a non-enum class with inheritable_enum

The decorator applies the following modifications to the class before returning it:

A new list attribute, __inheritable_members__, is added to the class, and all defined members are appended to it in the order they are defined.
The _member_names_ list attribute is cleared, removing all member names from the list. This is required to bypass the checks within EnumMeta for existing members, which raises TypeError when attempting to subclass an Enum.
The restore() is added to the class as a bound classmethod.

from enum import Enum
from extendableenum import inheritable_enum

@inheritable_enum
class Fruit(Enum):
    APPLE = 1
    BANANA = 2
    PEAR = 3

Fruit.__inheritable__members__
Fruit._mebers_names_
Fruit.restore
Fruit.__members__

>>> ['APPLE', 'BANANA', 'PEAR']
>>> []
>>> <bound method _restore of <enum 'Fruit'>>
>>> {'APPLE': <Fruit.APPLE: 1>, 'BANANA': <Fruit.BANANA: 2>, 'PEAR': <Fruits.Pear: 3>}

The Decorated Class

Member Access

Once decorated, the Enum members can still be accessed as normal:

Fruit['APPLE']      # Access by name
Fruit(2)            # Access by value
Fruit.PEAR          # Attribute access

>>> Fruit.APPLE
>>> Fruit.BANANA
>>> Fruit.PEAR

Modified Enum Behaviour

Removing the member from _member_names_ attribute impacts the following normal behaviour of the Enum:

`dir`

The __dir__ function appends the _member_names_ to the returned list, so the actual members will be excluded:

dir(Fruit)

>>> ['__class__', '__doc__', '__members__', '__module__']

`len`

The __len__ function returns the number of members in _member_names_, thus the decorated class will always return 0:

len(Fruit)

>>> 0

`reversed`

The __reversed__ function returns the _member_names_ in reversed order, thus the decorated class will not return any of the members:

[member for member in reversed(Fruit)]

>>> []

`iter`

Iteration of Enum s relies on the _member_names_, thus direct iteration of the Enum is not possible. Iteration is still possible through the __members__ attribute:

[member for member in Fruit]
[member for member in Fruit.__members__]

>>> []
>>> ['APPLE', 'BANANA', 'PEAR']

Restoring the Enum

At any time, simply call the restore classmethod of the decorated Enum, which will undo all changes to it:

__inheritable_members__ are added back to _member_names_ in definition order.
__inheritable_members__ attribute is deleted from the class.
restore function is removed from the class.

Fruit.restore()
hasattr(Fruit, '__inheritable_members__')
hasattr(Fruit, 'restore')
[member for member in Fruit]

>>> False
>>> False
>>> [<Fruit.APPLE: 1>, <Fruit.BANANA: 2>, <Fruit.PEAR: 3>]

Subclasses of the Decorated Class

An inheritable Enum, unlike a normal Enum, can be subclassed:

try:
    class BadFruit(Fruit):
        MANGO = 4
        DRAGONFRUIT = 5
except TypeError:
    print("This fails because Fruit was restored previously, and normal enums can't be extended")

inheritable_enum(Fruit)    # Make Fruit inheritable again.

class MoreFruit(Fruit):
    MANGO = 4
    DRAGONFRUIT = 5



>>> This fails because Fruit was restored previously, and normal enums can't be extended

As shown above, the subclass can add additional members. Only these new members will appear in the _member_names_ attribute of the new class.

MoreFruit._member_names_

>>> ['MANGO', 'DRAGONFRUIT']

Side Effects

The super class members being excluded from the subclass _member_names_ attribute has the same side effects as for the decorated class:

dir(MoreFruit)
len(MoreFruit)
[member for member in reversed(MoreFruit)]
[member for member in MoreFruit]

>>> ['MANGO', 'DRAGONFRUIT', '__class__', '__doc__', '__members__', '__module__']
>>> 2
>>> [<MoreFruit.DRAGONFRUIT: 5> , <MoreFruit.MANGO: 4>]
>>> [<MoreFruit.MANGO: 4>, <MoreFruit.DRAGONFRUIT: 5>]

Attribute Access

The members defined in the subclass be accessed as usual for an Enum. The super class members may not be accessed by either name or value:

try:
    MoreFruit['APPLE']
except KeyError:
    print('Access to super members by name not supported')

try:
    MoreFruit(1)
except ValueError:
    print('Access to super members by value not supported')

>>> Access to super members by name not supported
>>> Access to super members by value not supported

However, super members can be accessed directly as attributes from the subclass.

Note

The members returned by the subclass are the super class Enum members:

MoreFruit.APPLE
MoreFruit.APPLE is Fruit.APPLE

>>> Fruit.APPLE
>>> True