Python Enum combination

Question

I would like to create a new Enum (IntEnum) class based on two existing ones. There is a working solution for this, like so:

from enum import unique, IntEnum
from itertools import chain
from collections import OrderedDict

@unique
class FirstEnumClass(IntEnum):
    a = 1
    b = 2

@unique
class SecondEnumClass(IntEnum):
    c = 3
    d = 4

# here a combined class is created:
CombinedEnumClass = unique(IntEnum('CombinedEnumClass', OrderedDict([(i.name, i.value) for i in chain(FirstEnumClass, SecondEnumClass)])))

My question: is there a fancy way to achieve this, so that there is a proper class definition? Like overriding some of the metaclass methods, or so? I would like something like this, so that docstring can also be given:

@unique
class CombinedEnumClass(IntEnum):
    """ docstring """
    # magic needed here

Any idea? Thanks!

You don't need the `OrderedDict`, you can give just the list comprehension. — jdehesa, Sep 06 '17 at 11:06
I know, it works that way, the thing is that I'm using PyCharm, and it gives a warning if just a list is given: "Expected type 'Integral', got 'List[Tuple[Any, Any]]' instead", and OrderedDict solved that. But true, it works without that. Anyways, I still cannot manage to do this like a class definition. — waszil, Sep 06 '17 at 11:09
Possible duplicate of [How to extend Python Enum?](https://stackoverflow.com/questions/33679930/how-to-extend-python-enum) — John Crawford, Mar 13 '19 at 19:04

Ethan Furman · Answer 1 · 2017-09-07T18:18:38.577

5

Python 2

Using a trick with vars() this can be accomplished:

class CombinedEnum(IntEnum):
    """ doc string """
    cls = vars()
    for member in chain(list(FirstEnumClass), list(SecondEnumClass)):
        cls[member.name] = member.value
    del member, cls

print(list(CombinedEnum))

This works because:

vars() returns the current namespace
modifying that namespace modifies the class

We delete member and cls because we don't want them to become members.

Python 3

The above doesn't (yet) work with the new Enum in Python3 3.4 and 3.5 (not sure about 3.6). The work-a-round is to use aenum instead, which allows us to tell Enum to ignore certain names:

from aenum import IntEnum

class CombinedEnum(IntEnum):
    """ doc string """
    _ignore_ = 'member cls'
    cls = vars()
    for member in chain(list(FirstEnumClass), list(SecondEnumClass)):
        cls[member.name] = member.value

¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

edited Sep 07 '17 at 18:18

answered Sep 06 '17 at 17:05

Ethan Furman

63,992
20
159
237

It looks good, but it does not work for me: `File "C:\Anaconda3\lib\enum.py", line 66, in __setitem__ raise TypeError('Attempted to reuse key: %r' % key) TypeError: Attempted to reuse key: 'member'` I tried to skip the `cls[member.name] = member.value` line and write a `pass` instead, but still the same is raised. I use Python 3.5.2 with Anaconda3. – waszil Sep 07 '17 at 10:33
I tried it in Python 2.7, and it works there. Do you know a way for Python 3.x? – waszil Sep 07 '17 at 11:01
1

@waszil: Thanks for finding that. I added a Python 3 work around. – Ethan Furman Sep 07 '17 at 18:19
@EthanFurman, so why did you [restrict subclassing of enumerations?]( https://docs.python.org/3/library/enum.html#restricted-subclassing-of-enumerations) in the first place. And what does `_ignore_` var is doing? Thanks for your libraries. – Kruupös Sep 08 '17 at 07:51
1

@OwlMax: subclassing added complications with equality and identity, and we didn't feel it was a net win to allow it; The `_ignore_` variable is telling the `Enum` metaclass to discard those names from the final enum class. – Ethan Furman Sep 08 '17 at 18:45
@EthanFurman I have posted a question on enum about "Combine two or more enumeration classes in order to have more levels access". If you have some tips, I am infinitely grateful to you. Thanks in advance https://stackoverflow.com/questions/71527981/combine-two-or-more-enumeration-classes-in-order-to-have-more-levels-access – Gianni Spear Mar 18 '22 at 13:47

score 0 · Accepted Answer · answered Sep 06 '17 at 13:42

0

The library prevents explicitly to do that:

Subclassing an enumeration is allowed only if the enumeration does not define any members.

Allowing subclassing of enums that define members would lead to a violation of some important invariants of types and instances. On the other hand, it makes sense to allow sharing some common behavior between a group of enumerations.

Therefore, I found a Stackoverflow answer using almost the same workaround as you do. I think this is the only way.

answered Sep 06 '17 at 13:42

Kruupös

5,097
3
27
43

Yeah, it seems like this is the only way. meanwhile I tried something like in [this post](https://stackoverflow.com/questions/32214614/automatically-setting-an-enum-members-value-to-its-name), to subclass the EnumMeta metaclass itself and play with its `__new__` method, but eventually all try runs into the same problem: `TypeError: Cannot extend enumerations`. – waszil Sep 06 '17 at 13:53

Python Enum combination

2 Answers2

Linked