Ninject 🥷

Ninject uses modern Python features to provide a simple and performant dependency injection framework.

• Installation
• Basic Usage
• Types of Providers
• Providers with Dependencies
• Providing Multiple Dependencies
• Providing Dependencies Concurrently
• Mixing Async and Sync Providers

Installation

pip install ninject

Basic Usage

First declare a Dependency and inject it into a dependent function.

from ninject import Dependency, inject

Message = Dependency("Message", str)


@inject
def print_message(*, message: Message = inject.ed):
    print(message)

Next, define a Context with a function that provides the Dependency.

from ninject import Context

context = Context()


@context.provides
def provide_message() -> Message:
    return Message("Hello, World!")

Finally, establish the context and call the function with the inject.ed dependency:

with context:
    print_message()

The output will be:

Hello, World!

Types of Providers

A provider is one of the following

• A function that returns a value
• A generator that yields a single value
• A context manager class that yields a value
• An async function that returns a value
• An async generator that yields a single value
• An async context manager class that yields a value

from ninject import Dependency, Context

Message = Dependency("Message", str)

context = Context()

# --- Sync Providers -------------------------------------


@context.provides
def sync_function() -> Message:
    return Message("Hello, World!")


@context.provides
def sync_generator() -> Message:
    try:
        yield Message("Hello, World!")
    finally:
        pass


@context.provides
class SyncContextManager:
    def __enter__(self) -> Message:
        return Message("Hello, World!")

    def __exit__(self, *args) -> None:
        pass


# --- Async Providers ------------------------------------


@context.provides
async def async_function() -> Message:
    return Message("Hello, World!")


@context.provides
async def async_generator() -> Message:
    try:
        yield Message("Hello, World!")
    finally:
        pass


@context.provides
class AsyncContextManager:
    async def __aenter__(self) -> Message:
        return Message("Hello, World!")

    async def __aexit__(self, *args) -> None:
        pass

Providers with Dependencies

Providers can have their own dependencies:

from ninject import Context, Dependency, inject

Greeting = Dependency("Greeting", str)
Recipient = Dependency("Recipient", str)
Message = Dependency("Message", str)


@inject
def print_message(*, message: Message = inject.ed):
    print(message)


context = Context()


@context.provides
def provide_greeting() -> Greeting:
    return Greeting("Hello")


@context.provides
def provide_recipient() -> Greeting:
    return Greeting("World")


@context.provides
def provide_message(
    *, greeting: Greeting = inject.ed, recipient: Recipient = inject.ed
) -> Message:
    return Message(f"{greeting}, {recipient}!")


if __name__ == "__main__":
    with context:
        print_message()

The output will be:

Hello, World!

Providing Multiple Dependencies

A single provider can supply multiple dependencies:

from ninject import Context, Dependency, inject

Greeting = Dependency("Greeting", str)
Recipient = Dependency("Recipient", str)
MessageContent = tuple[Greeting, Recipient]


@inject
def print_message(*, greeting: Greeting = inject.ed, recipient: Recipient = inject.ed):
    print(f"{greeting}, {recipient}!")


context = Context()


@context.provides
def provide_message_content() -> MessageContent:
    return "Hello", "World"


if __name__ == "__main__":
    with context:
        print_message()

You may also depend on MessageContent directly:

from ninject import Context, Dependency, inject

Greeting = Dependency("Greeting", str)
Recipient = Dependency("Recipient", str)
MessageContent = tuple[Greeting, Recipient]


@inject
def print_message(*, message_content: MessageContent = inject.ed):  # TypeError!
    greeting = message_content["greeting"]
    recipient = message_content["recipient"]
    print(f"{greeting}, {recipient}!")


context = Context()


@context.provides(MessageContent)
def provide_message_content() -> dict:
    return {"greeting": "Hello", "recipient": "World"}


if __name__ == "__main__":
    with context:
        print_message()

Providing Dependencies Concurrently

Ninject does not execute async providers concurrently since doing so can add a substantial amount of overhead to async function calls if it's unnecessary. If you want to satisfy dependencies concurrently you can leverage the ability to provide multiple dependencies at once. With that in mind, you can use asyncio.gather to run several async functions concurrently before returning the dependencies:

import asyncio
from ninject import Context, Dependency, inject

Greeting = Dependency("Greeting", str)
Recipient = Dependency("Recipient", str)


@dependencies
class MessageContent(TypedDict):
    greeting: Greeting
    recipient: Recipient


@inject
async def print_message(
    *, greeting: Greeting = inject.ed, recipient: Recipient = inject.ed
):
    print(f"{greeting}, {recipient}!")


context = Context()


async def get_message() -> str:
    return "Hello"


async def get_recipient() -> str:
    return "World"


@context.provides(MessageContent)
async def provide_message_content() -> dict:
    greeting, recipient = await asyncio.gather(get_message(), get_recipient())
    return {"greeting": greeting, "recipient": recipient}


if __name__ == "__main__":
    with context:
        asyncio.run(print_message())

Mixing Async and Sync Providers

To mix async and sync providers, the highest order dependent function must be async. So, in the example below, that highest order dependent async function is print_message. The fact that print_message is async is what allows the sync provide_message function to depend on the async provide_recipient function:

import asyncio
from ninject import Context, Dependency, inject

Greeting = Dependency("Greeting", str)
Recipient = Dependency("Recipient", str)

context = Context()


@context.provides(Recipient)
async def provide_recipient() -> str:
    return "World"


@context.provides(Message)
def provide_message(*, recipient: Recipient = inject.ed) -> str:
    return f"Hello, {recipient}!"


@inject
async def print_message(*, message: Message = inject.ed):
    print(message)


if __name__ == "__main__":
    with context:
        asyncio.run(print_message())

If print_message were sync, then the following error would be raised:

RuntimeError: Cannot use an async context manager in a sync context