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: I suspect this is a bug in the interaction of abstract methods and properties. In your base class, the following things happen, in order:  You define an abstract method named start. 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. Python saves a temporary reference to start.setter, because the name start is about to be bound to yet another object. You create a second abstract method named start 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__:  start, the property end, the property set_end, the setter for end 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:  A concrete method for the name start Concrete methods for the names get_end and set_end Implicitly a concrete definition for the name end, because all of the underlying functions for the property end have been provided concrete definitions.