Most Pythonic way to declare an abstract class property

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)