uncalled-for

Async dependency injection for Python functions.

Declare what your function needs as parameter defaults. They show up resolved when the function runs. No ceremony, no container, no configuration.

from uncalled_for import Depends

async def get_db():
    db = await connect()
    try:
        yield db
    finally:
        await db.close()

async def handle_request(db: Connection = Depends(get_db)):
    await db.execute("SELECT 1")

Features

• Zero dependencies — standard library only
• Async-native — built on AsyncExitStack and ContextVar
• Context manager lifecycle — sync and async generators get proper cleanup
• Nested dependencies — dependencies can depend on other dependencies
• Caching — each dependency resolves once per call, even if referenced multiple times
• Call arguments — a factory can reference the arguments of the call it serves

Annotated dependencies

Depends, Shared and CallArgument also work as Annotated metadata, so you can name a dependency once and reuse it:

DB = Annotated[Connection, Depends(get_db)]

async def handle_request(db: DB):
    await db.execute("SELECT 1")

This resolves like db: Connection = Depends(get_db), but to Python db is still a required parameter. A custom dependency supplies a value from metadata only if it subclasses Provider; other dependencies there wrap the call.

When something else calls the function with type-checked arguments, such as a task queue that schedules it, default the parameter to Provided so those callers can leave it out:

async def handle_request(db: DB = Provided): ...

A type alias works the same way: type DB = Annotated[Connection, Depends(get_db)].

Call arguments

A factory sometimes needs a value from the call itself, not from another dependency. CallArgument declares that reference:

from uncalled_for import CallArgument, Depends

def load_user(user_id: str = CallArgument()) -> User:
    return users[user_id]

async def handle_request(
    user_id: str,
    user: User = Depends(load_user),
):
    ...

The bare form takes the name of the parameter it is declared on. CallArgument("name") names a different parameter of the outer function. The reference sees the same value the outer function receives, whether the caller passed it or another dependency on the outer signature produced it. CallArgument(optional=True) yields None when the outer function has no such argument. References that form a cycle raise CycleError.

Bindings

Depends accepts keyword bindings, so you can wire up an ordinary function without changing it:

async def handle_request(
    user_id: str,
    user: User = Depends(load_user, user_id=CallArgument("user_id")),
):
    ...

A binding that is a Dependency, such as CallArgument(...) or another Depends(...), resolves first and the factory receives its value. Any other value passes through as it is. A binding replaces the factory parameter's own default, which is then never resolved. Two dependencies on the same factory share one cached result only when their bindings match.

Install

pip install uncalled-for