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 the some_service argument with the "foo" qualifier and we optionally autowire the bunch_of_services argument with a list of all injectables that satisfy the "bar" qualifier.

In this example, both qualifiers, "foo" and "bar", aren’t declared by any injectable though as we declared both injections as optional, the __init__ method won’t fail and instead will inject the value None to some_service and an empty list [] to bunch_of_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 Optional, List

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


class OptionalInjection(Example):
    @autowired
    def __init__(
        self,
        some_service: Autowired(Optional["foo"]),
        bunch_of_services: Autowired(Optional[List["bar"]]),
    ):
        self.some_service = some_service
        self.bunch_of_services = bunch_of_services

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

        print(self.bunch_of_services)
        # []


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


if __name__ == "__main__":
    run_example()