fastapi-cache-di

Speed up FastAPI startup by caching its dependency-tree introspection.

At startup, FastAPI re-introspects every route's dependency tree with no caching, so dependencies shared across many routes (auth checks, DB handles, role guards …) are parsed from scratch again and again. fastapi-cache-di memoizes that work for the duration of route loading, turning O(routes × tree-depth) into O(routes + unique-deps), then restores FastAPI's originals.

Install

pip install fastapi-cache-di
# or
uv add fastapi-cache-di

Usage

Wrap the code that loads your routes with the context manager:

from fastapi import APIRouter, FastAPI

from fastapi_cache_di import fastapi_deps_cache

app = FastAPI()
users = APIRouter()
orders = APIRouter()
billing = APIRouter()

with fastapi_deps_cache():
    # Registering routes introspects each dependency tree; caching is active here.
    app.include_router(users)
    app.include_router(orders)
    app.include_router(billing)
# originals restored here; cache memory freed

Manual patch/unpatch, sharing and inspecting the cache, and the cache-key details are covered in the documentation.

Calling loop-safe dependencies inline

At request time, FastAPI calls every dependency that is neither a coroutine nor a generator through the threadpool: every Depends(SomeClass) and every plain def dependency costs a round trip to a worker thread, often far more than a class whose __init__ only stores its arguments.

patch_fastapi_run_in_threadpool() lets such dependencies run directly on the event loop. It is opt-in, because the library cannot tell whether a call blocks:

• loop_safe marks one sync function or class (it returns the same object, so Depends(...) and dependency_overrides keep working);
• inline_classes=True treats every class as loop-safe;
• everything else still goes through the threadpool, as do sync generator dependencies and sync endpoints.

from typing import Annotated

from fastapi import Depends, FastAPI

from fastapi_cache_di import loop_safe, patch_fastapi_run_in_threadpool

patch_fastapi_run_in_threadpool(inline_classes=True)


@loop_safe
def get_tenant_id() -> str:
    return "acme"


class UserRepository:
    def __init__(self, tenant_id: Annotated[str, Depends(get_tenant_id)]) -> None:
        self.tenant_id = tenant_id


app = FastAPI()


@app.get("/users")
async def list_users(repo: Annotated[UserRepository, Depends()]) -> str:
    return repo.tenant_id

A loop-safe callable must never block: no I/O, no lock, no heavy computation. A blocking call on the event loop stalls every in-flight request. See the documentation for overrides and the marker shared with fastapi-standalone-di.

Skipping the sub-tree of cached dependencies

At request time, FastAPI solves the whole sub-tree of a dependency before checking whether its value is already in the request's cache, so a dependency shared across a deep tree is solved again at every occurrence. patch_fastapi_solve_dependencies() stops that walk as soon as the value is cached. It is opt-in and stays active for the app's lifetime:

from fastapi import FastAPI

from fastapi_cache_di import patch_fastapi_solve_dependencies

patch_fastapi_solve_dependencies()

app = FastAPI()

The values injected into endpoints are unchanged; see the documentation for the details.

Documentation

Full documentation is available at toilal.github.io/fastapi-cache-di (usage guide, internals, and API reference).

Requirements

• Python ≥ 3.12
• FastAPI ≥ 0.112.4

License

MIT © Rémi Alvergnat

CHANGELOG

v0.3.0 (2026-10-16)

Bug Fixes

• Patch get_flat_dependant only where FastAPI still defines it (#14, 30fbb28)

• Support FastAPI 0.141+ where get_flat_dependant no longer exists (#14, 30fbb28)

Features

• Call loop-safe dependencies on the event loop instead of a threadpool hop (#16, a37d214)

• Skip cached sub-trees when dependency_overrides is set too (#15, bb8a2f7)

• Skip re-walking the sub-tree of already-cached dependencies at request time (#15, bb8a2f7)

v0.2.1 (2026-07-06)

Bug Fixes

• Ship py.typed marker for PEP 561 type information (#11, 1a7eb6f)

v0.2.0 (2026-07-05)

Features

• Support Python 3.11 (#10, d593d6f)

v0.1.0 (2026-07-05)

• Initial Release

Changelog entries are generated automatically by python-semantic-release from Conventional Commits on release.