Python abstract property() "Can't instantiate abstract class [] with abstract methods", but I did

Question

I'm trying to create a base class with a number of abstract python properties, in python 3.7.

I tried it one way (see 'start' below) using the @property, @abstractmethod, @property.setter annotations. This worked but it doesn't raise an exception if the subclass doesn't implement a setter. That's the point of using @abstract to me, so that's no good.

So I tried doing it another way (see 'end' below) using two @abstractmethod methods and a 'property()', which is not abstract itself but uses those methods. This approach generates an error when instantiating the subclass:

        # {TypeError}Can't instantiate abstract class FirstStep with abstract methods end

I'm clearly implementing the abstract methods, so I don't understand what it means. The 'end' property is not marked @abstract, but if I comment it out, it does run (but I don't get my property). I also added that test non-abstract method 'test_elapsed_time' to demonstrate I have the class structure and abstraction right (it works).

Any chance I'm doing something dumb, or is there some special behavior around property() that's causing this?

class ParentTask(Task):
    def get_first_step(self):
        # {TypeError}Can't instantiate abstract class FirstStep with abstract methods end
        return FirstStep(self)


class Step(ABC):
    #     __metaclass__ = ABCMeta
    def __init__(self, task):
        self.task = task

    # First approach.  Works, but no warnings if don't implement setter in subclass
    @property
    @abstractmethod
    def start(self):
        pass

    @start.setter
    @abstractmethod
    def start(self, value):
        pass

    # Second approach.  "This method for 'end' may look slight messier, but raises errors if not implemented.
    @abstractmethod
    def get_end(self):
        pass

    @abstractmethod
    def set_end(self, value):
        pass

    end = property(get_end, set_end)

    def test_elapsed_time(self):
        return self.get_end() - self.start


class FirstStep(Step):
    @property
    def start(self):
        return self.task.start_dt

    # No warnings if this is commented out.
    @start.setter
    def start(self, value):
        self.task.start_dt = value

    def get_end(self):
        return self.task.end_dt

    def set_end(self, value):
        self.task.end_dt = value

Answer 1

I suspect this is a bug in the interaction of abstract methods and properties.

In your base class, the following things happen, in order:

1. You define an abstract method named start.
2. You create a new property that uses the abstract method from 1) as its getter. The name start now refers to this property, with the only reference to the original name now held by Self.start.fget.
3. Python saves a temporary reference to start.setter, because the name start is about to be bound to yet another object.
4. You create a second abstract method named start
5. The reference from 3) is given the abstract method from 4) to define a new property to replace the once currently bound to the name start. This property has as its getter the method from 1 and as its setter the method from 4). Now start refers to this property; start.fget refers to the method from 1); start.fset refers to the method from 4).

At this point, you have a property, whose component functions are abstract methods. The property itself was not decorated as abstract, but the definition of property.__isabstractmethod__ marks it as such because all its component methods are abstract. More importantly, you have the following entries in Step.__abstractmethods__:

1. start, the property
2. end, the property
3. set_end, the setter for end
4. gen_end, the getter for end

Note that the component functions for the start property are missing, because __abstractmethods__ stores names of, not references to, things that need to be overriden. Using property and the resulting property's setter method as decorators repeatedly replace what the name start refers to.

Now, in your child class, you define a new property named start, shadowing the inherited property, which has no setter and a concrete method as its getter. At this point, it doesn't matter if you provide a setter for this property or not, because as far as the abc machinery is concerned, you have provided everything it asked for:

1. A concrete method for the name start
2. Concrete methods for the names get_end and set_end
3. Implicitly a concrete definition for the name end, because all of the underlying functions for the property end have been provided concrete definitions.

Answer 2

@chepner answered and explained it well. Based on that, I came up with a way around it that is... well... you decide. Sneaky at best. But it achieves my 3 main goals:

1. Raises exceptions for unimplemented setters in subclasses
2. Supports the python property semantics (vs. functions etc)
3. Avoids boilerplate re-declaring every property in every subclass which still might not have solved #1 anyway.

Just declare the abstract get/set functions in the base class (not the property). Then add a @classmethod initializer to the base class that creates the actual properties using those abstract methods, but at that point, they'll be concrete methods on the subclass.

It's a one liner after the subclass declaration to init the properties. Nothing enforces that call being made, so it's not ironclad. Not a big savings in this example, but I'll have many properties. The end results doesn't look as dirty as I thought it would. Would like to hear comments or warnings of things I'm overlooking.

from abc import abstractmethod, ABC

class ParentTask(object):
    def __init__(self):
        self.first_step = FirstStep(self)
        self.second_step = SecondStep(self)
        print(self.first_step.end)
        print(self.second_step.end)


class Step(ABC):
    def __init__(self, task):
        self.task = task

    @classmethod
    def init_properties(cls):
        cls.end = property(cls.get_end, cls.set_end)

    @abstractmethod
    def get_end(self):
        pass

    @abstractmethod
    def set_end(self, value):
        pass


class FirstStep(Step):
    def get_end(self):
        return 1

    def set_end(self, value):
        self.task.end = value


class SecondStep(Step):
    def get_end(self):
        return 2

    def set_end(self, value):
        self.task.end = value


FirstStep.init_properties()
SecondStep.init_properties()
ParentTask()