--- name: fastapi-fd473d67852e description: >- FastAPI is a Python web framework for building HTTP and WebSocket APIs from type-annotated functions, with request validation by Pydantic and automatic OpenAPI docs at /docs. Use this skill to create a FastAPI project, define routes and Pydantic schemas, use Depends dependency injection and Annotated parameters, add auth headers, background tasks, middleware, lifespan startup, WebSockets, tests with TestClient or httpx, and run the app with the fastapi CLI. license: Apache-2.0 compatibility: 'Python 3.10+ (fastapi 0.162 requires >=3.10), pip or uv' metadata: author: terminal-skills version: 1.1.0 category: development repository: https://github.com/fastapi/fastapi tags: "python, api, async, openapi, pydantic" --- # FastAPI ## Overview FastAPI is built on Starlette (ASGI) and Pydantic v2. Path operation functions declare their inputs with type hints (path, query, header, body); FastAPI validates them, returns 422 with details on bad input, serialises the return value through `response_model`, and serves interactive docs at `/docs` (Swagger UI) and `/redoc` plus the schema at `/openapi.json`. The code below was run with fastapi 0.162.0, Starlette 1.10, Pydantic 2.14 and pytest 9. ## Instructions ### 1. Install and layout ```bash python -m venv .venv && source .venv/bin/activate pip install "fastapi[standard]" # adds the fastapi CLI, uvicorn, httpx, email-validator ``` ``` shop-api/ ├── app/ │ ├── __init__.py │ ├── main.py # FastAPI() app, lifespan, include_router │ ├── deps.py # shared dependencies │ └── routers/ │ ├── __init__.py │ ├── items.py │ └── ws.py ├── tests/ │ └── test_items.py └── conftest.py # anyio_backend fixture ``` ### 2. App and lifespan Use `lifespan` for startup and shutdown work (`@app.on_event` is deprecated). ```python # app/main.py from contextlib import asynccontextmanager from fastapi import FastAPI from .routers import items, ws @asynccontextmanager async def lifespan(app: FastAPI): app.state.items = {} # startup: open pools, load caches yield app.state.items.clear() # shutdown: close connections app = FastAPI(title="Shop API", version="1.0.0", lifespan=lifespan) app.include_router(items.router) app.include_router(ws.router) ``` ### 3. Dependencies Declare parameters and dependencies with `Annotated`; give reusable ones an alias. A `Header()` parameter named `x_api_key` reads the `X-API-Key` header (underscores become hyphens). ```python # app/deps.py import os from typing import Annotated from fastapi import Depends, Header, HTTPException, Request def get_store(request: Request) -> dict[int, dict]: return request.app.state.items Store = Annotated[dict[int, dict], Depends(get_store)] async def current_user(x_api_key: Annotated[str, Header()]) -> str: # keys come from the environment, e.g. SHOP_API_KEYS="key-maria-7f3a:maria,key-dmitri-91bc:dmitri" keys = dict(pair.split(":") for pair in os.environ.get("SHOP_API_KEYS", "").split(",") if pair) user = keys.get(x_api_key) if user is None: raise HTTPException(status_code=401, detail="Invalid API key") return user CurrentUser = Annotated[str, Depends(current_user)] ``` ### 4. Routes, schemas, background tasks ```python # app/routers/items.py from typing import Annotated from fastapi import APIRouter, BackgroundTasks, HTTPException, Path, Query, status from pydantic import BaseModel, Field from ..deps import CurrentUser, Store router = APIRouter(prefix="/items", tags=["items"]) class ItemCreate(BaseModel): name: str = Field(min_length=1, max_length=100) price: float = Field(gt=0) description: str | None = None class ItemResponse(ItemCreate): id: int def audit(user: str, action: str) -> None: print(f"audit: {user} {action}") @router.get("/", response_model=list[ItemResponse]) async def list_items( store: Store, skip: Annotated[int, Query(ge=0)] = 0, limit: Annotated[int, Query(ge=1, le=100)] = 20, ): return list(store.values())[skip : skip + limit] @router.post("/", response_model=ItemResponse, status_code=status.HTTP_201_CREATED) async def create_item(item: ItemCreate, store: Store, user: CurrentUser, tasks: BackgroundTasks): new_id = max(store, default=0) + 1 store[new_id] = {"id": new_id, **item.model_dump()} tasks.add_task(audit, user, f"created item {new_id}") # runs after the response is sent return store[new_id] @router.get("/{item_id}", response_model=ItemResponse) async def get_item(item_id: Annotated[int, Path(gt=0)], store: Store): if item_id not in store: raise HTTPException(status_code=404, detail="Item not found") return store[item_id] ``` For ORM objects add `model_config = {"from_attributes": True}` to the response schema. Validators use Pydantic v2 names: `field_validator` with `@classmethod`, and `EmailStr` needs `email-validator` (included in `fastapi[standard]`). ### 5. WebSocket and middleware ```python # app/routers/ws.py from fastapi import APIRouter, WebSocket, WebSocketDisconnect router = APIRouter() @router.websocket("/ws/{client_id}") async def echo(websocket: WebSocket, client_id: str): await websocket.accept() try: while True: text = await websocket.receive_text() await websocket.send_text(f"{client_id}: {text}") except WebSocketDisconnect: pass ``` ```python # in app/main.py, after app = FastAPI(...) import time from fastapi import Request @app.middleware("http") async def add_timing_header(request: Request, call_next): start = time.perf_counter() response = await call_next(request) response.headers["X-Process-Time"] = f"{time.perf_counter() - start:.4f}" return response ``` ### 6. Tests `TestClient` as a context manager runs the lifespan; without the `with` block `app.state.items` would not exist. For async tests, `pytest.mark.anyio` needs an `anyio_backend` fixture, otherwise it also tries the `trio` backend. ```python # conftest.py import pytest @pytest.fixture def anyio_backend(): return "asyncio" ``` ```python # tests/test_items.py import pytest from fastapi.testclient import TestClient from httpx import ASGITransport, AsyncClient from app.main import app HEADERS = {"X-API-Key": "key-maria-7f3a"} def test_create_and_read_item(monkeypatch): monkeypatch.setenv("SHOP_API_KEYS", "key-maria-7f3a:maria") with TestClient(app) as client: r = client.post("/items/", json={"name": "Desk lamp", "price": 79.0}, headers=HEADERS) assert r.status_code == 201 assert client.get(f"/items/{r.json()['id']}").json()["name"] == "Desk lamp" assert client.post("/items/", json={"name": "x", "price": 1}).status_code == 422 with client.websocket_connect("/ws/maria") as ws: ws.send_text("hi") assert ws.receive_text() == "maria: hi" @pytest.mark.anyio async def test_async_client(monkeypatch): monkeypatch.setenv("SHOP_API_KEYS", "key-maria-7f3a:maria") async with app.router.lifespan_context(app): async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as c: r = await c.post("/items/", json={"name": "Notebook", "price": 4.5}, headers=HEADERS) assert r.status_code == 201 ``` Run with `python -m pytest -q`. ### 7. Run ```bash fastapi dev app/main.py --port 8000 # development: auto-reload, binds 127.0.0.1 fastapi run app/main.py --host 0.0.0.0 --port 8000 --workers 4 # production-style, no reload ``` `fastapi dev` listens on 127.0.0.1 only. Pass `--host 0.0.0.0` only inside a container or behind a firewall or proxy. `uvicorn app.main:app` also works. ## Examples ### Example 1: CRUD endpoint for products Request: "Add a products API with create, list and get, validated, with 404 for unknown ids." Copy the items router above with `ProductCreate`/`ProductResponse`, include it in `main.py`, and run `fastapi dev app/main.py`. Result: `POST /items/` with header `X-API-Key: key-maria-7f3a` returns 201 and `{"name":"Desk lamp","price":79.0,"description":null,"id":1}`; `GET /items/99` returns 404 `{"detail":"Item not found"}`; a negative price returns 422; `/docs` lists all three operations. ### Example 2: Protect routes with an API key dependency Request: "Only users with a valid key may create items." Use `CurrentUser` from `deps.py` on the write routes. Result: a missing header returns 422 (the required header is part of the schema), an unknown key returns 401 `Invalid API key`, a valid key goes through. For OAuth2 or JWT use `fastapi.security` (`OAuth2PasswordBearer`) and the official security tutorial. ## Guidelines - Use `async def` only when the body awaits non-blocking I/O; plain `def` routes run in a thread pool. Blocking calls (requests, sync DB drivers) inside `async def` stall the whole event loop. - Always set `response_model` (or a return type) so internal fields such as password hashes are not serialised. - Keep secrets in environment variables or settings (`pydantic-settings`), not in code. - Background tasks run in the same process and are lost on a crash; use a queue (Celery, ARQ, Dramatiq) for important work. - Restrict CORS to known origins with `CORSMiddleware`; do not combine `allow_origins=["*"]` with credentials. - Upgrade note: since 0.162 an omitted `bool` form field no longer defaults to `False` (declare `= False` explicitly for HTML checkboxes), and since 0.160 a parameter model's `extra` setting applies to every submitted query/header/cookie/form field. Read the release notes before bumping. - Do not rely on `/docs` in production if the API is private; set `docs_url=None` on `FastAPI(...)`. --- Source attribution: TerminalSkills Original project: https://github.com/TerminalSkills/skills/blob/3bfbf7f76459d89422eddc0e5f572ad0e6c393c9/skills/fastapi/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. Definitions. "License" shall mean the terms and conditions for use, reproduction, and distribution as defined by Sections 1 through 9 of this document. "Licensor" shall mean the copyright owner or entity authorized by the copyright owner that is granting the License. "Legal Entity" shall mean the union of the acting entity and all other entities that control, are controlled by, or are under common control with that entity. For the purposes of this definition, "control" means (i) the power, direct or indirect, to cause the direction or management of such entity, whether by contract or otherwise, or (ii) ownership of fifty percent (50%) or more of the outstanding shares, or (iii) beneficial ownership of such entity. "You" (or "Your") shall mean an individual or Legal Entity exercising permissions granted by this License. "Source" form shall mean the preferred form for making modifications, including but not limited to software source code, documentation source, and configuration files. "Object" form shall mean any form resulting from mechanical transformation or translation of a Source form, including but not limited to compiled object code, generated documentation, and conversions to other media types. "Work" shall mean the work of authorship, whether in Source or Object form, made available under the License, as indicated by a copyright notice that is included in or attached to the work (an example is provided in the Appendix below). "Derivative Works" shall mean any work, whether in Source or Object form, that is based on (or derived from) the Work and for which the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work of authorship. For the purposes of this License, Derivative Works shall not include works that remain separable from, or merely link (or bind by name) to the interfaces of, the Work and Derivative Works thereof. "Contribution" shall mean any work of authorship, including the original version of the Work and any modifications or additions to that Work or Derivative Works thereof, that is intentionally submitted to the Licensor for inclusion in the Work by the copyright owner or by an individual or Legal Entity authorized to submit on behalf of the copyright owner. For the purposes of this definition, "submitted" means any form of electronic, verbal, or written communication sent to the Licensor or its representatives, including but not limited to communication on electronic mailing lists, source code control systems, and issue tracking systems that are managed by, or on behalf of, the Licensor for the purpose of discussing and improving the Work, but excluding communication that is conspicuously marked or otherwise designated in writing by the copyright owner as "Not a Contribution." "Contributor" shall mean Licensor and any individual or Legal Entity on behalf of whom a Contribution has been received by the Licensor and subsequently incorporated within the Work. 2. Grant of Copyright License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare Derivative Works of, publicly display, publicly perform, sublicense, and distribute the Work and such Derivative Works in Source or Object form. 3. Grant of Patent License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted. If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed. 4. Redistribution. You may reproduce and distribute copies of the Work or Derivative Works thereof in any medium, with or without modifications, and in Source or Object form, provided that You meet the following conditions: (a) You must give any other recipients of the Work or Derivative Works a copy of this License; and (b) You must cause any modified files to carry prominent notices stating that You changed the files; and (c) You must retain, in the Source form of any Derivative Works that You distribute, all copyright, patent, trademark, and attribution notices from the Source form of the Work, excluding those notices that do not pertain to any part of the Derivative Works; and (d) If the Work includes a "NOTICE" text file as part of its distribution, then any Derivative Works that You distribute must include a readable copy of the attribution notices contained within such NOTICE file, excluding any notices that do not pertain to any part of the Derivative Works, in at least one of the following places: within a NOTICE text file distributed as part of the Derivative Works; within the Source form or documentation, if provided along with the Derivative Works; or, within a display generated by the Derivative Works, if and wherever such third-party notices normally appear. The contents of the NOTICE file are for informational purposes only and do not modify the License. You may add Your own attribution notices within Derivative Works that You distribute, alongside or as an addendum to the NOTICE text from the Work, provided that such additional attribution notices cannot be construed as modifying the License. You may add Your own copyright statement to Your modifications and may provide additional or different license terms and conditions for use, reproduction, or distribution of Your modifications, or for any such Derivative Works as a whole, provided Your use, reproduction, and distribution of the Work otherwise complies with the conditions stated in this License. 5. Submission of Contributions. Unless You explicitly state otherwise, any Contribution intentionally submitted for inclusion in the Work by You to the Licensor shall be under the terms and conditions of this License, without any additional terms or conditions. Notwithstanding the above, nothing herein shall supersede or modify the terms of any separate license agreement you may have executed with Licensor regarding such Contributions. 6. Trademarks. This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file. 7. Disclaimer of Warranty. Unless required by applicable law or agreed to in writing, Licensor provides the Work (and each Contributor provides its Contributions) on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. You are solely responsible for determining the appropriateness of using or redistributing the Work and assume any risks associated with Your exercise of permissions under this License. 8. Limitation of Liability. In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall any Contributor be liable to You for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising as a result of this License or out of the use or inability to use the Work (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if such Contributor has been advised of the possibility of such damages. 9. Accepting Warranty or Additional Liability. While redistributing the Work or Derivative Works thereof, You may choose to offer, and charge a fee for, acceptance of support, warranty, indemnity, or other liability obligations and/or rights consistent with this License. However, in accepting such obligations, You may act only on Your own behalf and on Your sole responsibility, not on behalf of any other Contributor, and only if You agree to indemnify, defend, and hold each Contributor harmless for any liability incurred by, or claims asserted against, such Contributor by reason of your accepting any such warranty or additional liability. END OF TERMS AND CONDITIONS Copyright 2025 Terminal Skills Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: TerminalSkills Original project: https://github.com/TerminalSkills/skills/blob/3bfbf7f76459d89422eddc0e5f572ad0e6c393c9/skills/fastapi/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Backend Developer, Data Engineer, ML / AI Engineer Specialties: API, Backend, Cross Platform Tools: Uvicorn Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: 957943df72c544423f5ffbd0f167d83fd645592fa4963789a61bb88d8390be29