Decoupled chain of responsibility in Python

Question

I've started to learn Python today. I'm trying to write decoupled middleware or chain of responsibility pattern.

In my app I have some two classes:

• abstract class or interface named Processable
• class ProcessorChain, which must locate implementations of the Processable, create objects of these implementations and call methods of these objects

In client code, user can:

• create his own class based on Processable
• write full path to his class to some global setting
• and be sure what it will be processed in queue by my app processor chain :-)

So, the same thing we got using service container, when we call services by tag. In Symfony, for example, we use service tags and CompilerPass to process a group of decoupled objects which implement one interface.

The question is: how to get same in Python?

I've already learnt about __subclasses__ method, but it works only if sub-classes was imported. So, I've learnt about importlib.importmodule, then I got what I wanted.

The folder structure is:

• patterns
  • main.py
  • settings.py
  • Chain
    • chain.py
    • processor.py

patterns.Chain.chain.py

import importlib
from abc import ABCMeta, abstractmethod
from patterns import settings


class Processable(metaclass=ABCMeta):
    """Meta class define required properties and methods"""
    __metaclass__ = ABCMeta

    @abstractmethod
    def execute(self) -> str:
        return 'Base class processing...'


class ProcessorChain:
    @staticmethod
    def process_directly():

        # load processors from settings list
        for definition in settings.CHAIN_PROCESSORS:

            # parse module and package
            m, p = definition.rsplit('.', 1)
            module = importlib.import_module(m, p)

            # get class
            cls = getattr(module, p)

            # check if the class is a child of Processable
            if issubclass(cls, Processable):

                # create an instance of the class
                processor = cls()

                # and do something
                print(processor.execute())

    @staticmethod
    def process_subclass():

        # still requires make dynamic imports
        for definition in settings.CHAIN_PROCESSORS:
            # parse module and package
            m, p = definition.rsplit('.', 1)
            # import a module
            importlib.import_module(m, p)

        # load all subclasses of Processable
        for processor in Processable.__subclasses__():
            # and do something
            print(processor().execute())

patterns.Chain.processor.py

from .chain import Processable


class FirstProcessor(Processable):
    """Provides required properties and methods"""
    context = 1

    def execute(self):
        return super(FirstProcessor, self).execute() + ' (implementation from meta class)'


class SecondProcessor(Processable):
    """Provides required properties and methods"""
    context = 2

    def execute(self):
        return str(self.context) + ' processing...' + ' (its own implementation)'

patterns.settings.py

CHAIN_PROCESSORS = [
    'patterns.Chain.processor.FirstProcessor',
    'patterns.Chain.processor.SecondProcessor'
]

patterns.main.py

from patterns.Chain.chain import ProcessorChain


ProcessorChain().process_directly()

The run of main.py as expected prints:

Base class processing... (implementation from meta class)
2 processing... (its own implementation)

https://github.com/xcono/py3-trials/tree/master/patterns

But, actually, I don't know is what pythonic way? May be there is a better way to acheive decoupled chain? May be better to use event listeners/dispatchers?

Dear Pythonists, please, tell the truth, how do you cook it?

Answer 1

This looks unfinished to me and far from ready for review.

1. The docstring for the class Processable doesn't describe the class. What kind of thing is an instance of Processable?

2. The class Processable is misleadingly named — its instances are things that can process not things that can be processed so a name like Processor would be better.

3. The execute method has no docstring. What does it do?

4. The purpose of the execute method is surely to process a request, but in that case shouldn't it take the request as a parameter?

5. Why does the execute method return a string? This seems to be an unnecessary limitation: a processing chain might need to return a more complex object.

6. In the chain of responsibility pattern, a processor's default behaviour is to forward the request to the next processor in the chain. But this seems to be omitted from the implementation here. Where is the next processor stored? How is the request forwarded?

7. It is not clear what happens if none of the processors in the chain want to handle a request.

8. staticmethod is nearly always an anti-pattern in Python. In some languages (like Java) all functions have to belong to a class. But in Python you can just write a function, and there is no need to make a class whose only job is to act as a container for some functions.

9. There are no docstrings for process_directly and process_subclass. What do these functions do?

10. process_directly is coupled tightly to settings.CHAIN_PROCESSORS. It would be better for this to be a parameter, in case the application needs to construct its processor chain in some other way.

11. process_directly constructs a Processable instance and calls its execute method. But doesn't there need to be a request somewhere?

12. process_directly throws away the Processable instance after it is finished. This means that if you call it again it has to go through the construction logic again, which is a waste of time, and it makes it hard to implement stateful processors (because the objects don't survive from one execution to the next).

13. process_directly calls print on the result. It seems unlikely that this is what real processor chains will want to do with the result.

14. If the test issubclass(cls, Processable) fails, nothing happens. This means that typos and other mistakes in the setting will be silently ignored. It would be better to raise an exception.

15. I don't understand the purpose of process_subclass.

16. There's duplicated code (for loading a class from a module) between process_directly and process_subclass.