Most Pythonic way to declare an abstract class property

Question

Assume you're writing an abstract class and one or more of its non-abstract class methods require the concrete class to have a specific class attribute; e.g., if instances of each concrete class can be constructed by matching against a different regular expression, you might want to give your ABC the following:

@classmethod
def parse(cls, s):
    m = re.fullmatch(cls.PATTERN, s)
    if not m:
        raise ValueError(s)
    return cls(**m.groupdict())

(Maybe this could be better implemented with a custom metaclass, but try to ignore that for the sake of the example.)

Now, because overriding of abstract methods & properties is checked at instance creation time, not subclass creation time, trying to use abc.abstractmethod to ensure concrete classes have PATTERN attributes won't work — but surely there should be something there to tell anyone looking at your code "I didn't forget to define PATTERN on the ABC; the concrete classes are supposed to define their own." The question is: Which something is the most Pythonic?

1. Pile of decorators

   @property
   @abc.abstractmethod
   def PATTERN(self):
       pass
   

   (Assume Python 3.4 or higher, by the way.) This can be very misleading to readers, as it implies that PATTERN should be an instance property instead of a class attribute.

2. Tower of decorators

   @property
   @classmethod
   @abc.abstractmethod
   def PATTERN(cls):
       pass
   

   This can be very confusing to readers, as @property and @classmethod normally can't be combined; they only work together here (for a given value of "work") because the method is ignored once it's overridden.

3. Dummy value

   PATTERN = ''
   

   If a concrete class fails to define its own PATTERN, parse will only accept empty input. This option isn't widely applicable, as not all use cases will have an appropriate dummy value.

4. Error-inducing dummy value

   PATTERN = None
   

   If a concrete class fails to define its own PATTERN, parse will raise an error, and the programmer gets what they deserve.

5. Do nothing. Basically a more hardcore variant of #4. There can be a note in the ABC's docstring somewhere, but the ABC itself shouldn't have anything in the way of a PATTERN attribute.

6. Other???

Answer 1

You can use the __init_subclass__ method which was introduced in Python 3.6 to make customizing class creation easier without resorting to metaclasses. When defining a new class, it is called as the last step before the class object is created.

In my opinion, the most pythonic way to use this would be to make a class decorator that accepts the attributes to make abstract, thus making it explicit to the user what they need to define.

from custom_decorators import abstract_class_attributes

@abstract_class_attributes('PATTERN')
class PatternDefiningBase:
    pass

class LegalPatternChild(PatternDefiningBase):
    PATTERN = r'foo\s+bar'

class IllegalPatternChild(PatternDefiningBase):
    pass

The traceback might be as follows, and occurs at subclass creation time, not instantiation time.

NotImplementedError                       Traceback (most recent call last)
...
     18     PATTERN = r'foo\s+bar'
     19 
---> 20 class IllegalPatternChild(PatternDefiningBase):
     21     pass

...

<ipython-input-11-44089d753ec1> in __init_subclass__(cls, **kwargs)
      9         if cls.PATTERN is NotImplemented:
     10             # Choose your favorite exception.
---> 11             raise NotImplementedError('You forgot to define PATTERN!!!')
     12 
     13     @classmethod

NotImplementedError: You forgot to define PATTERN!!!

Before showing how the decorator is implemented, it is instructive to show how you could implement this without the decorator. The nice thing here is that if needed you could make your base class an abstract base class without having to do any work (just inherit from abc.ABC or make the metaclass abc.ABCMeta).

class PatternDefiningBase:
    # Dear programmer: implement this in a subclass OR YOU'LL BE SORRY!
    PATTERN = NotImplemented
    
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)

        # If the new class did not redefine PATTERN, fail *hard*.
        if cls.PATTERN is NotImplemented:
            # Choose your favorite exception.
            raise NotImplementedError('You forgot to define PATTERN!!!')

    @classmethod
    def sample(cls):
        print(cls.PATTERN)

class LegalPatternChild(PatternDefiningBase):
    PATTERN = r'foo\s+bar'

Here is how the decorator could be implemented.

# custom_decorators.py

def abstract_class_attributes(*names):
    """Class decorator to add one or more abstract attribute."""

    def _func(cls, *names):
        """ Function that extends the __init_subclass__ method of a class."""

        # Add each attribute to the class with the value of NotImplemented
        for name in names:
            setattr(cls, name, NotImplemented)

        # Save the original __init_subclass__ implementation, then wrap
        # it with our new implementation.
        orig_init_subclass = cls.__init_subclass__

        def new_init_subclass(cls, **kwargs):
            """
            New definition of __init_subclass__ that checks that
            attributes are implemented.
            """

            # The default implementation of __init_subclass__ takes no
            # positional arguments, but a custom implementation does.
            # If the user has not reimplemented __init_subclass__ then
            # the first signature will fail and we try the second.
            try:
                orig_init_subclass(cls, **kwargs)
            except TypeError:
                orig_init_subclass(**kwargs)

            # Check that each attribute is defined.
            for name in names:
                if getattr(cls, name, NotImplemented) is NotImplemented:
                    raise NotImplementedError(f'You forgot to define {name}!!!')

        # Bind this new function to the __init_subclass__.
        # For reasons beyond the scope here, it we must manually
        # declare it as a classmethod because it is not done automatically
        # as it would be if declared in the standard way.
        cls.__init_subclass__ = classmethod(new_init_subclass)

        return cls

    return lambda cls: _func(cls, *names)

Answer 2

I've been searching for something like this for quite a while, until yesterday I decided to dive into it. I like @SethMMorton's reply a lot, however 2 things are missing: allow a an abstract class to have a subclass that is abstract itself, and play nice with typehints and static typing tools such as mypy (which makes sense, since back in 2017 these were hardly a thing).

I started to set out to write a reply here with my own solution, however I realised I needed lots of tests and documentation, so I made it a proper python module: abstractcp.

Use (as of version 0.9.5):

class Parser(acp.Abstract):
    PATTERN: str = acp.abstract_class_property(str)

    @classmethod
    def parse(cls, s):
        m = re.fullmatch(cls.PATTERN, s)
        if not m:
            raise ValueError(s)
        return cls(**m.groupdict())

class FooBarParser(Parser):
    PATTERN = r"foo\s+bar"

    def __init__(...): ...

class SpamParser(Parser):
    PATTERN = r"(spam)+eggs"

    def __init__(...): ...

See for full use the page on pypi or github.

Answer 3

Alternative Answer

Using dedicated class to annotate class variables

import abc
from typing import Generic, Set, TypeVar, get_type_hints

T = TypeVar('T')


class AbstractClassVar(Generic[T]):
    pass


class Abstract(abc.ABC):
    def __init_subclass__(cls) -> None:

        def get_abstract_members(cls) -> Set[str]:
            """Gets a class's abstract members"""
            abstract_members = set()
            if cls is Abstract:
                return abstract_members
            for base_cls in cls.__bases__:
                abstract_members.update(get_abstract_members(base_cls))
            for (member_name, annotation) in get_type_hints(cls).items():
                if getattr(annotation, '__origin__', None) is AbstractClassVar:
                    abstract_members.add(member_name)
            return abstract_members

        # Implementation checking for abstract class members
        if Abstract not in cls.__bases__:
            for cls_member in get_abstract_members(cls):
                if not hasattr(cls, cls_member):
                    raise NotImplementedError(f"Wrong class implementation {cls.__name__} " +
                                              f"with abstract class variable {cls_member}")
        return super().__init_subclass__()

Usage

class Foo(Abstract):
    
    foo_member: AbstractClassVar[str]


class UpperFoo(Foo):
    # Everything should be implemented as intended or else...
    ...

Not Implementing the abstract class member foo_member will result in a NotImplementedError.

Answer was taken from my original answer to this question: enforcement for abstract properties in python3