Nested enum-like classes in Python

Question

I am trying to make some constants/enum-like objects/classes with attributes in Python. Something like this.

from abc import ABC

class Entity(ABC):
    # allow *labels as attributes

class Label(ABC):
    @property
    def spellings(self):
        raise NotImplementedError

class PeriodLabel(Label):
    @property
    def months(self):
        raise NotImplementedError

class MONTH(Entity):
    class JANUARY(Label):
        spellings = ['jan.', 'january']
    class FEBRUARY(Label):
        spellings = ['febr.', 'february']
    .
    .
    .

class PERIOD(Entity):
   class QUARTER(PeriodLabel):
       spellings = ['q1', 'q2', 'q3', 'q4']
       months = 3
   class HALFYEAR(PeriodLabel):
       spellings = ['6m']
       months = 6
    .
    .
    .

The goal is to go from MONTH object to "MONTH" as a str. That part is easy since I can just use MONTH.__name__. But I'd also like to go the opposite way from "MONTH" to MONTH:

assert Entity("MONTH") == MONTH

I could achieve that by doing the following but it seems to hacky I need the comparisons to be lightning fast so I think there are better ways.

class Entity(ABC):

    def __new__(cls, arg):
        try:
            print(cls.__name__)
            candidate = eval(arg)
            if issubclass(candidate, cls):
                return candidate
        except:
            pass

I would even accept assert "MONTH" == MONTH but I need to get the class from a string. I also need to go from "MONTH.JANUARY" to MONTH.JANUARY. Now I have tried a bunch of different approaches but this thread is already getting out of hand.

EDIT1

A lot simpler approach could be

from typing import List, Optional


class Label:

    def __init__(self, spellings: List[str]):
        self.spellings = spellings


class Entity:

    def __init__(self, **labels: Label):
        for k, v in labels.items():
            self.__setattr__(k, v)

    def get(self, entity: str, label: Optional[str] = None):
        raise NotImplementedError  # todo: how?


PERIOD = Entity(Q1=Label(['q1', 'q2', 'q3', 'q4']))

assert Entity.get('PERIOD') == PERIOD
assert Entity.get('PERIOD', 'Q1') == PERIOD.Q1

Downside is it is not fullfilling as it is AND code completion wont work to reference PERIOD.Q1 since Q1 attribute is indirectly created through __setattr__

EDIT2

Here are a couple of examples of how it would be used. Performance is important. It is really hard to precisely explain what I want though. I hope it makes somewhat sense.

def some_function(entity_name, label_name, spellings)
    print(f"{entity_name}-{label_name}:{spellings}"

# example 1
for entity in [MONTH, PERIOD, ...]:
    entity_name = entity.__name__
    for label in entity:
        label_name = entity.__name__
        some_function(entity_name, label_name, label.spellings)

# example 2 (given entity_name, label_name as strings)
entity = Entity.get(entity_name)
label = entity.get(label_name)
if entity == PERIODS:
    if label.months == 3:
        # do something

# example 3 (alternative to example 1)
for label in LABELS: # ALL_LABELS is auto collecting all labels
   some_function(label.entity.__name__, label.__name__, label.spellings)

# example 4 (alternative to example 2)
label = LABELS.get(entity_name, label_name)
if label.entity == PERIODS:
    if label.months == 3:
        # do something

Have you checked native `enum.Enum` class? https://docs.python.org/3/library/enum.html — abdusco, Jul 05 '19 at 19:48
Take a look at [metaclasses](https://www.pythoncentral.io/how-metaclasses-work-technically-in-python-2-and-3/). A metaclass constructs the class from attribute declarations; you can rework your attributes however you want, turn them into properties, register them for easy search by name, etc. — 9000, Jul 05 '19 at 20:05
Could you add some examples of how these would actually be used? — Ethan Furman, Jul 05 '19 at 20:30
@9000 seems like a good advice. I think I might be able to do it with that. Didn’t know about metaclasses before — mr.bjerre, Jul 05 '19 at 21:30
It sounds like you need conversion code to go from some constants to strings and vice-a-versa, but your other code just uses the strings and not the constants? — Ethan Furman, Jul 05 '19 at 21:43
"Other code" being the rest of the program that somehow uses the information contained in MONTH, PERIOD, etc. — Ethan Furman, Jul 05 '19 at 22:21
Ah yes but they will be used in the real use cases. But I do use months and spellings in the examples — mr.bjerre, Jul 06 '19 at 05:48

score 1 · Accepted Answer · answered Jul 07 '19 at 09:17

metaclasses to the rescue!

from __future__ import annotations
from typing import List, Dict, Tuple, Optional


class EntMeta(type):
    _instances = {}

    def __new__(mcs, classname: str, base_classes: Tuple[type], attrs: Dict) -> EntMeta:
        qualname = attrs.get('__qualname__')
        if qualname not in EntMeta._instances:
            EntMeta._instances[qualname] = super().__new__(mcs, classname, base_classes, attrs)

        return EntMeta._instances[qualname]

    def __call__(cls, entity: str, label: Optional[str] = None) -> EntMeta:
        if label is None:
            qualname = entity
        else:
            qualname = '.'.join([entity, label])
        try:
            return cls._instances[qualname]
        except KeyError:
            raise ValueError(f"{qualname} is not a recognized entity")


class Entity(metaclass=EntMeta):
    pass


class Label(metaclass=EntMeta):

    @property
    def spellings(self) -> List[str]:
        raise NotImplementedError

class PeriodLabel(Label):

    @property
    def months(self) -> int:
        raise NotImplementedError


class PERIOD(Entity):
    class QUARTER(PeriodLabel):
        spellings = ['q1', 'q2', 'q3', 'a4']
        months = 3
    class HALFYEAR(PeriodLabel):
        spellings = ['q1', 'q2', 'q3', 'a4']
        months = 6


class MONTH(Entity):
    class JANUARY(Label):
        spellings = ['jan.', 'january']


assert PERIOD == Entity('PERIOD')
assert MONTH == Entity('MONTH')
assert PERIOD.QUARTER == Entity('PERIOD', 'QUARTER')
assert PERIOD.HALFYEAR == Entity('PERIOD', 'HALFYEAR')
assert MONTH.JANUARY == Entity('MONTH', 'JANUARY')

Mr_and_Mrs_D · Answer 2 · 2019-07-07T20:02:56.460

One can define enums with attributes as shown for instance here. I would avoid defining my own metaclasses if the built in enum could cover my needs - here is a very rough Poc:

"""Enums Poc"""

import enum
_ALL_LABELS = set() # TODO find a way to encapsulate into Label
class Label(enum.Enum):

    def __new__(cls, *args, **kwds):
        value = len(cls.__members__) + 1
        obj = object.__new__(cls)
        obj._value_ = value
        return obj

    def __init__(self, *spellings):
        _ALL_LABELS.add(self)
        self.spellings = spellings

class PeriodLabel(Label):

    def __init__(self, months, *spellings):
        super().__init__(*spellings)
        self.months = months

class Entity(enum.Enum):

    class MONTH(Label): # better use 'Month' here

        JANUARY = ['jan.', 'january']
        FEBRUARY = ['febr.', 'february']
        ...

    class PERIOD(PeriodLabel):

        QUARTER = 3, ['q1', 'q2', 'q3', 'a4']
        HALFYEAR = 6, ['q1', 'q2', 'q3', 'a4']


assert Entity.PERIOD == Entity['PERIOD']
assert Entity.MONTH == Entity['MONTH']

def some_function(entity_name, label_name, spellings):
    print(f"{entity_name}-{label_name}:{spellings}")

# example 1
for entity in Entity:
    entity_name = entity.name
    for label in entity.value: # TODO: directly iterate (not in .value)
        label_name = label.name
        some_function(entity_name, label_name, label.spellings)

# example 2 (given entity_name, label_name as strings)
entity_name = 'PERIOD'
entity = Entity[entity_name]
label = entity.value['QUARTER']
if entity is Entity.PERIOD:
    if label.months == 3:
        print('True')

Nested enum-like classes in Python

EDIT1

EDIT2

2 Answers2