Cyclic Dependency Example

In this example you’ll learn how the injectable framework can make it easier to deal with circular references.

We use qualifiers to register ours cyclic-dependent services which will enable us to refer to these dependencies by their qualifier string when injecting the services into our CyclicDependency example class instead of having to import them and possibly falling into a cyclic import loop.

For each of the services we inject the other one with a lazy modifier which will prevent us from falling into an instantiation loop as lazy dependencies are only instantiated when its attributes are accessed or its methods are invoked.

See also

The Lazy Injection Example details how lazy injection works.

cyclic_dependency_example.py

from examples import Example
from injectable import Autowired, autowired, load_injection_container


class CyclicDependency(Example):
    @autowired
    def __init__(self, service_a: Autowired("A"), service_b: Autowired("B")):
        self.service_a = service_a
        self.service_b = service_b

    def run(self):
        print(self.service_a.get_some_property_from_b)
        # some property from B

        print(self.service_b.get_some_property_from_a)
        # some property from A


def run_example():
    load_injection_container()
    example = CyclicDependency()
    example.run()


if __name__ == "__main__":
    run_example()

service_a.py

from injectable import injectable, Autowired, autowired


@injectable(qualifier="A")
class ServiceA:
    @autowired
    def __init__(self, service_b: Autowired("B", lazy=True)):
        self.service_b = service_b
        self.some_property = "some property from A"

    @property
    def get_some_property_from_b(self):
        return self.service_b.some_property

service_b.py

from injectable import injectable, autowired, Autowired


@injectable(qualifier="B")
class ServiceB:
    @autowired
    def __init__(self, service_a: Autowired("A", lazy=True)):
        self.service_a = service_a
        self.some_property = "some property from B"

    @property
    def get_some_property_from_a(self):
        return self.service_a.some_property