Most Pythonic way to declare an abstract class property

Python >= 3.6 Version

(Scroll down for a version that works for Python <= 3.5).

If you are fortunate enough to only be using Python 3.6 and not have to worry about backwards compatibility, you can use the new __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:    passclass 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     @classmethodNotImplementedError: 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.pydef 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)

Python <= 3.5 Version

If you are not fortunate enough to only be using Python 3.6 and not have to worry about backwards compatibility, you will have to use a metaclass. Even though this is perfectly valid Python, one could debate how pythonic the solution is because metaclasses are hard to wrap your brain around, but I think it hits most of the points of The Zen of Python so I think it's not so bad.

class RequirePatternMeta(type):    """Metaclass that enforces child classes define PATTERN."""    def __init__(cls, name, bases, attrs):        # Skip the check if there are no parent classes,        # which allows base classes to not define PATTERN.        if not bases:            return        if attrs.get('PATTERN', NotImplemented) is NotImplemented:            # Choose your favorite exception.            raise NotImplementedError('You forgot to define PATTERN!!!')class PatternDefiningBase(metaclass=RequirePatternMeta):    # Dear programmer: implement this in a subclass OR YOU'LL BE SORRY!    PATTERN = NotImplemented    @classmethod    def sample(cls):        print(cls.PATTERN)class LegalPatternChild(PatternDefiningBase):    PATTERN = r'foo\s+bar'class IllegalPatternChild(PatternDefiningBase):    pass

This behaves exactly like the Python >= 3.6 __init_subclass__ method shown above (except the traceback will look bit different because it is routed through a different set of methods before failing).

Unlike the __init_subclass__ method, if you want to make a subclass an abstract base class you will have to do just a bit of extra work (you'll have to compose the metaclass with ABCMeta).

from abs import ABCMeta, abstractmethodABCRequirePatternMeta = type('ABCRequirePatternMeta', (ABCMeta, RequirePatternMeta), {})class PatternDefiningBase(metaclass=ABCRequirePatternMeta):    # Dear programmer: implement this in a subclass OR YOU'LL BE SORRY!    PATTERN = NotImplemented    @classmethod    def sample(cls):        print(cls.PATTERN)    @abstractmethod    def abstract(self):        return 6class LegalPatternChild(PatternDefiningBase):    PATTERN = r'foo\s+bar'    def abstract(self):        return 5class IllegalPatternChild1(PatternDefiningBase):    PATTERN = r'foo\s+bar'print(LegalPatternChild().abstract())print(IllegalPatternChild1().abstract())class IllegalPatternChild2(PatternDefiningBase):    pass

Outputs just as you'd expect.

5TypeError: Can't instantiate abstract class IllegalPatternChild1 with abstract methods abstract# Then the NotImplementedError if it kept on going.

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.