Python3 - How does one define an abstract subclass from an existing abstract class?

Question

I initially defined the following abstract class:

from abc import ABC, abstractmethod    
class Primitive(ABC):

Now I want to create another abstract class that inherits from Primitive:

class InstrumentName(Primitive)

I need this class to be abstract since I ultimately want to create the following two concrete classes:

class CurrencyInstrumentName(InstrumentName)
class MetalInstrumentName(InstrumentName)

I have read the documentation and searched SO, but they mostly pertain to sublcassing concrete classes from abstract classes, or discussing how Python handles abstraction

Answer 1

Just subclass, you don't need to do anything special.

A class only becomes concrete when there are no more abstractmethod and abstractproperty objects left in the implementation.

Let's illustrate this:

from abc import ABC, abstractmethod    
class Primitive(ABC):
    @abstractmethod
    def foo(self):
        pass

    @abstractmethod
    def bar(self):
        pass

class InstrumentName(Primitive):
    def foo(self):
        return 'Foo implementation'

Here, InstrumentName is still abstract, because bar is left as an abstractmethod. You can't create an instance of that subclass:

>>> InstrumentName()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class InstrumentName with abstract methods bar

Subclasses can also add @abstractmethod or @abstractproperty methods as needed.

Under the hood, all subclasses inherit the ABCMeta metaclass that enforces this, and it simply checks if there are any @abstractmethod or @abstractproperty attributes left on the class.