Proper way to implement ABC SubClass

Question

I have an Interface class which defines the requirements to an active "in-use" class:

class Portfolio(ABC):
    @abstractmethod
    def update_portfolio(self):
        raise NotImplementedError

    @abstractmethod
    def update_from_fill(self):
        raise NotImplementedError

    @abstractmethod
    def check_signal(self, signal_event):
        raise NotImplementedError

The methods update_portfolio and update_from_fill are both methods which will be the same in 99% of the required cases. Only the check_signal method will vary. Therefore, to avoid having to write the same code again and again, I have defined a base class with default methods for update_portfolio and update_from_fill:

class BaseBacktestPortfolio(Portfolio):
    def __init__(self, ...):
        ...

    def update_portfolio(self, ...):
        ...

    def update_from_fill(self, ...):
        ...

Then, finally, I have a class inheriting from the BacktestPortfolio class which specifies the correct implementation of the check_signal method:

class USBacktestPortfolio(BaseBacktestPortfolio):
    def check_signal(self, ...):
        ...

Now, the problem is that my editor complains about the BacktestPortfolio classing not having all the required abstract methods. I could ignore this, of course, but the perfect scenario would be if I could make sure that it is not possible to instantiate an object form the BacktestPortfolio class.

Is this possible? And/or is there a more correct way to implement a structure like this?

Answer 1

You can make the BaseBacktestPortfolio also as Abstract class.

from abc import ABC, abstractmethod

class Portfolio(ABC):
    @abstractmethod
    def update_portfolio(self):
        pass

    @abstractmethod
    def update_from_fill(self):
        pass

    @abstractmethod
    def check_signal(self, signal_event):
        pass

class BaseBacktestPortfolio(Portfolio, ABC):

    def update_portfolio(self):
        print("updated portfolio")

    def update_from_fill(self):
        print("update from fill")

    @abstractmethod
    def check_signal(self):
        pass

class USBacktestPortfolio(BaseBacktestPortfolio):
    def check_signal(self):
        print("checked signal")

Also notice that you don't need raise NotImplementedError inside abstract method. You can just pass. Its more Pythonic :)