aiodine 0.2.0

Async-first dependency injection library for Python

aiodine

aiodine provides async-first dependency injection in the style of Pytest fixtures for Python 3.6+.

Installation

pip install aiodine

Usage

Note: this section is under construction.

aiodine revolves around two concepts: providers and consumers.

Providers are:

• Explicit: once a provider is defined, a consumer can use it by declaring it as a function parameter.
• Modular: a provider can itself use other provider.
• Flexible: providers are reusable within the scope of a function or a whole session, and support a variety of syntaxes (asynchronous or synchronous, function or generator) to make provisioning resources fun again.

Providers

Providers make a resource available to consumers within a certain scope. They are created by decorating a provider function with @aiodine.provider.

Here's a "hello world" provider:

import aiodine

@aiodine.provider
async def hello():
    return "Hello, aiodine!"

Tip: synchronous provider functions are also supported!

Providers are available in two scopes:

• function: the provider's value is re-computed everytime it is consumed.
• session: the provider's value is computed only once (the first time it is consumed) and is reused in subsequent calls.

By default, providers are function-scoped.

Consumers

Once a provider has been declared, it can be used by consumers. A consumer is built by decoratinga consumer function with @aiodine.consumer. A consumer can declare a provider as one of its parameters and aiodine will inject it at runtime.

Here's an example consumer:

@aiodine.consumer
async def show_friendly_message(hello):
    print(hello)

Tip: synchronous consumer functions are also supported!

All aiodine consumers are asynchronous, so you'll need to run them in an asynchronous context:

from asyncio import run

async def main():
    await show_friendly_message()

run(main())  # "Hello, aiodine!"

Of course, a consumer can declare non-provider parameters too. These are then regular parameters and will have to be passed when calling the consumer.

@aiodine.consumer
async def show_friendly_message(hello, repeat=1):
    for _ in range(repeat):
        print(hello)

async def main():
    await show_friendly_message(repeat=10)

Providers consuming other providers

Providers can also consume other providers. To do so, providers need to be frozen so that the dependency graph can be correctly resolved:

import aiodine

@aiodine.provider
async def email():
    return "[e-mail]"

@aiodine.provider
async def send_email(email):
    print(f"Sending email to {email}…")

aiodine.freeze()  # <- Ensures that `send_email` has resolved `email`.

A context manager is also available:

import aiodine

with aiodine.exit_freeze():
    @aiodine.provider
    async def email():
        return "[e-mail]"

    @aiodine.provider
    async def send_email(email):
        print(f"Sending email to {email}…")

Note: thanks to this, providers can be declared in any order.

Generator providers

Generator providers can be used to perform cleanup operations after a provider has gone out of scope.

import os
import aiodine

@aiodine.provider
async def testing():
    initial = os.getenv("APP_ENV")
    os.environ["APP_ENV"] = "TESTING"
    try:
        yield
    finally:
        os.environ.pop("APP_ENV")
        if initial is not None:
            os.environ["APP_ENV"] = initial

Note: session generator providers will only be cleaned up if using them in the context of a session. See Sessions for details.

Tip: synchronous generator providers are also supported!

Lazy async providers

When the provider function is asynchronous, its return value is awaited before being injected into the consumer. In other words, providers are eager by default.

You can mark a provider as lazy in order to defer awaiting the provided value to the consumer. This is useful when the provider needs to be conditionally evaluated.

from asyncio import sleep
import aiodine

@aiodine.provider(lazy=True)
async def expensive_computation():
    await sleep(10)
    return 42

@aiodine.consumer
async def compute(expensive_computation, cache=None):
    if cache:
        return cache
    return await expensive_computation

Factory providers

Instead of returning a value, a factory provider returns a function. This allows to build providers that can be reused for a variety of inputs.

This is a design pattern more than anything else. In fact, there's no extra code in aiodine to support this feature.

The following example defines a factory provider for a (simulated) database query:

import aiodine

@aiodine.provider(scope="session")
async def notes():
    # Some hard-coded sticky notes.
    return [
        {"id": 1, "text": "Groceries"},
        {"id": 2, "text": "Make potatoe smash"},
    ]

@aiodine.provider
async def get_note(notes):
    async def _get_note(pk: int) -> list:
        try:
            # TODO: fetch from a database instead?
            return next(note for note in notes if note["id"] == pk)
        except StopIteration:
            raise HTTPError(404, detail=f"Note with ID {pk} does not exist.")

    return _get_note

Example usage in a consumer:

@aiodine.consumer
async def show_note(pk: int, get_note):
    print(await get_note(pk))

Tip: you can combine factory providers with generator providers to cleanup any resources the factory needs to use.

Here's an example that provides temporary files and removes them on cleanup:

import os
import aiodine

@aiodine.provider(scope="session")
def tmpfile():
    files = set()

    async def _create_tmpfile(path: str):
        with open(path, "w") as tmp:
            files.add(path)
            return tmp

    yield _create_tmpfile

    for path in files:
        os.remove(path)

Sessions

A session is the context in which session providers live.

More specifically, session providers (resp. generator session providers) are instanciated (resp. setup) when entering a session, and destroyed (resp. cleaned up) when exiting the session.

To enter a session, use:

await aiodine.enter_session()

To exit it:

await aiodine.exit_session()

An async context manager syntax is also available:

async with aiodine.session():
    ...

FAQ

Why "aiodine"?

aiodine contains aio as in asyncio, and di as in Dependency Injection The last two letters are only there to make this library's name pronounce like iodine, the chemical element.

Changelog

See CHANGELOG.md.

License

MIT