diny

Dead simple dependency injection for Python.

pip install diny

Works with your existing code

Drop in @singleton / @inject and delete your glue code:

+from diny import inject, singleton
+
+@singleton
 class Config:
     def __init__(self):
         self.url = "postgres://localhost"

+@singleton
 class Database:
     def __init__(self, config: Config):
         self.conn = connect(config.url)

+@inject
 def list_users(db: Database):
     return db.query("SELECT * FROM users")

-# Lots of lines of glue code
-config = ...
-db = ...
-
-list_users(db)
+list_users()   # Config and Database built on first call, cached after

The cache is process-wide until you scope it.

Decorators

@singleton caches one instance per scope. @factory builds a fresh one at each site. @inject resolves a function's typed deps on call.

from diny import inject, singleton, factory
from uuid import uuid4

@singleton
class Database:
    def __init__(self, config: Config): ...

@factory
class RequestId:
    def __init__(self): self.value = uuid4()

@inject
def handler(db: Database, req: RequestId):
    # db is cached across calls; req is fresh every time
    ...

Plain params pass through untouched:

@inject
def get_user(user_id: int, db: Database):
    return db.query("SELECT * FROM users WHERE id = %s", user_id)

get_user(42)

Custom providers

For types you don't own or that need custom construction, use @provider(Type):

from diny import provider

@provider(Database)
def make_db(config: Config):
    return PostgresDB(config.url, pool_size=10)

# Now Database is auto-injected using make_db — no provide() needed
list_users()

The provider function's own typed parameters are injected too. Scope is determined by the call site (Singleton[T] / Factory[T]), defaulting to singleton. provide() overrides @provider within its scope.

Annotations

For classes you don't own — or to override a class's default scope at one call site — use Singleton[T] / Factory[T]:

from diny import Singleton, Factory

@inject
def handler(
    client: Singleton[ThirdPartyClient],   # cached
    id_a:   Factory[RequestId],            # fresh
    id_b:   Factory[RequestId],            # fresh, different instance
):
    ...

Site annotations beat class decorators — Singleton[RequestId] forces singleton even if RequestId is @factory.

Undecorated classes without a site annotation are passed through to the caller. Nothing is auto-injected behind your back.

Scoped overrides

Open a scope with provide() to override any dependency — classes, instances, or functions. This overrides both @singleton/@factory and @provider registrations within the scope:

from diny import provide

class FakeDatabase(Database):
    def __init__(self, config: Config):
        self.fake = True

with provide(Database=FakeDatabase):
    list_users()                     # uses FakeDatabase

with provide(Config(url="test://")):
    list_users()                     # uses this Config instance

Scopes nest. Use inherit=True to keep the parent's cached instances while overriding specific deps:

with provide(Config(url="prod")):
    handler()                          # prod config, fresh database

    with provide(Database=AdminDB, inherit=True):
        handler()                      # admin database, same config instance

    with provide(Config(url="test")):
        handler()                      # test config, fresh everything

Async

from diny import inject, aprovide

@inject
async def handler(db: Database):
    return await db.fetch_all()

async def main():
    async with aprovide():
        await handler()

Async provider functions work inside aprovide:

async def make_pool(config: Config):
    return await asyncpg.create_pool(config.url)

async with aprovide(Pool=make_pool):
    await handler()