Optional Injection Example

In this example you’ll see how to declare an injection as optional using typing.Optional.

When a dependency is not found for injection you’ll receive an injectable.InjectionError. This may not be what you want if it is expected and OK that in some situations the dependency simply won’t be present.

In our OptionalInjection example class we optionally autowire ServiceXYZ and we optionally autowire a list of all ServiceXYZ injectables. We declare the ServiceXYZ as injectable but in the "XYZ" namespace.

In this example, both autowirings, optional_service and bunch_of_optional_services, aren’t satisfied by any injectable since there are no ServiceXYZ declared in the default namespace. But since we declared both injections as optional, the __init__ method won’t fail and instead will inject the value None to optional_service and an empty list [] to bunch_of_optional_services.

Note

The typing.Optional type shall be the outermost declared type, so Autowired(Optional[List[...]]) will work while Autowired(List[Optional[...]]) won’t.

See also

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

optional_injection_example.py

from typing import Annotated, Optional, List

from examples import Example
from examples.optional_injection.service_xyz import ServiceXYZ
from injectable import autowired, Autowired, load_injection_container


class OptionalInjection(Example):
    @autowired
    def __init__(
        self,
        optional_service: Annotated[Optional[ServiceXYZ], Autowired],
        bunch_of_optional_services: Annotated[Optional[List[ServiceXYZ]], Autowired],
    ):
        self.optional_service = optional_service
        self.bunch_of_optional_services = bunch_of_optional_services

    def run(self):
        print(self.optional_service)
        # None

        print(self.bunch_of_optional_services)
        # []


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


if __name__ == "__main__":
    run_example()

service_xyz.py

from injectable import injectable


@injectable(namespace="XYZ")
class ServiceXYZ: ...