I faced the dreaded circular import issue when doing some simple research. I tried the “import ” solution but doesn’t seem to work. What is the solution to make it work?

Example code (all 3 files are located in the same directory, no other files present in there):

my_subject.py:

from abc import ABC, abstractmethod
from random import randrange
import my_observer


class Subject(ABC):
    @abstractmethod
    def attach(self, observer: my_observer.Observer) -> None:
        pass

    @abstractmethod
    def detach(self, observer: my_observer.Observer) -> None:
        pass

    @abstractmethod
    def notify(self) -> None:
        pass


class ConcreteSubject(Subject):
    _state: int = None
    _observers: list[my_observer.Observer] = []

    def attach(self, observer: my_observer.Observer) -> None:
        print("Subject: Attached an observer.")
        self._observers.append(observer)

    def detach(self, observer: my_observer.Observer) -> None:
        self._observers.remove(observer)

    def notify(self) -> None:
        print("Subject: Notifying observers...")
        for observer in self._observers:
            observer.update(self)

    def some_business_logic(self) -> None:
        print("nSubject: I'm doing something important.")
        self._state = randrange(0, 10)

        print(f"Subject: My state has just changed to: {self._state}")
        self.notify()

my_observer.py:

from abc import ABC, abstractmethod
import my_subject


class Observer(ABC):
    @abstractmethod
    def update(self, subject: my_subject.Subject) -> None:
        pass


class ConcreteObserverA(Observer):
    def update(self, subject: my_subject.Subject) -> None:
        if subject._state < 3:
            print("ConcreteObserverA: Reacted to the event")


class ConcreteObserverB(Observer):
    def update(self, subject: my_subject.Subject) -> None:
        if subject._state == 0 or subject._state >= 2:
            print("ConcreteObserverB: Reacted to the event")

observer_main.py:

import my_subject 
import my_observer 

if __name__ == "__main__":
    # The client code.

    subject = my_subject.ConcreteSubject()

    observer_a = my_observer.ConcreteObserverA()
    subject.attach(observer_a)

    observer_b = my_observer.ConcreteObserverB()
    subject.attach(observer_b)

    subject.some_business_logic()
    subject.some_business_logic()

    subject.detach(observer_a)

    subject.some_business_logic()

terminal output when running observer_main.py:

Traceback (most recent call last):
  File "/home/samprass/dev/python/practising/design_patterns/observer/observer_main.py", line 1, in <module>
    import my_subject 
  File "/home/samprass/dev/python/practising/design_patterns/observer/my_subject.py", line 3, in <module>
    import my_observer
  File "/home/samprass/dev/python/practising/design_patterns/observer/my_observer.py", line 18, in <module>
    class Observer(ABC):
  File "/home/samprass/dev/python/practising/design_patterns/observer/my_observer.py", line 20, in Observer
    def update(self, subject: my_subject.Subject) -> None:
AttributeError: partially initialized module 'my_subject' has no attribute 'Subject' (most likely due to a circular import)

I tried to use simple “import ” instead of “from import “