Dependencies Precedence Example

In this example you’ll see the use of declaring an injectable as primary and the use of explicitly declared qualifiers.

We declare an abstract base class AbstractService which exposes an abstract combine method. The classes SumService and MultiplyService both implement AbstractService and are declared as injectable though we specify different qualifiers for each and also declare SumService as primary.

Now, when we inject an AbstractService into our DependenciesPrecedence example class we will get an instance of the SumService class since we declared it as primary. We inject a MultipleService instance as well by using the explicit qualifier attributed to it.

Note

If SumService wasn’t declared as primary then injecting AbstractService would have failed and raised an error indicating there was ambiguity in resolving the specified dependency.

See also

The Qualifier Overloading Example shows how to get all instances which resolves a dependency instead of just the primary one.

dependencies_precedence_example.py

from examples import Example
from examples.dependencies_precedence.abstract_service import AbstractService
from injectable import (
    autowired,
    Autowired,
    load_injection_container,
)


class DependenciesPrecedence(Example):
    @autowired
    def __init__(
        self,
        abstract_service_1: Autowired(AbstractService),
        abstract_service_2: Autowired("multiply"),
    ):
        self.abstract_service_1 = abstract_service_1
        self.abstract_service_2 = abstract_service_2

    def run(self):
        self.abstract_service_1.combine(4, 2)
        # 4 + 2 = 6

        self.abstract_service_2.combine(4, 2)
        # 4 * 2 = 8


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


if __name__ == "__main__":
    run_example()

abstract_service.py

from abc import ABC, abstractmethod


class AbstractService(ABC):
    @abstractmethod
    def combine(self, a, b):
        ...

sum_service.py

from examples.dependencies_precedence.abstract_service import AbstractService
from injectable import injectable


@injectable(qualifier="sum", primary=True)
class SumService(AbstractService):
    def combine(self, a, b):
        print(f"{a} + {b} = {a + b}")

multiply_service.py

from examples.dependencies_precedence.abstract_service import AbstractService
from injectable import injectable


@injectable(qualifier="multiply")
class MultiplyService(AbstractService):
    def combine(self, a, b):
        print(f"{a} * {b} = {a * b}")