FastAPI Day 1: Build Your First API in 5 Minutes and Speed Up Development

1. One‑line installation

Run pip install "fastapi[standard]". The standard extra bundles Uvicorn, interactive docs, and auto‑reload, so you don’t need to install each component manually. fastapi dev – development mode fastapi run – production mode

Separating these two commands is a design strength of FastAPI.

2. First API in four lines

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def root():
    return {"message": "Hello World"}

Save as main.py and start the server with fastapi dev main.py. The service listens on 127.0.0.1:8000 with auto‑reload enabled, so code changes take effect without a manual restart.

3. Automatic interactive documentation

Open http://127.0.0.1:8000/docs to see a full Swagger UI. You can expand any endpoint, click “Try it out”, fill parameters, and execute the request; the JSON response appears directly on the page. This UI is generated by FastAPI without any extra configuration.

Practical tip: When collaborating with front‑end developers, share the /docs URL so they can test endpoints without Postman or a separate test environment.

4. Four sample endpoints to illustrate routing

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def root():
    return {"message": "Hello World"}

@app.get("/health")
def health():
    return {"status": "ok"}

@app.get("/about")
def about():
    return {"name": "fastapi-14days", "version": "0.1.0"}

@app.get("/greet/{name}")
def greet(name: str):
    return {"message": f"Hello, {name}!"}

Key observations:

Path parameters are declared directly in the URL (e.g., {name}) and FastAPI infers their type from Python annotations.

No decorator arguments or class inheritance are needed; the HTTP method name itself acts as the decorator, which feels more intuitive for developers coming from Flask.

Each endpoint automatically updates the OpenAPI schema, the /docs UI, and the alternative /redoc UI.

Type errors are caught during request validation, preventing invalid data from reaching business logic.

5. Development vs. production commands

The two commands differ in semantics, which helps avoid confusion in a team setting.

fastapi dev : listens on 127.0.0.1, enables auto‑reload, intended for local debugging.

fastapi run : listens on 0.0.0.0, no auto‑reload, suited for container or server deployment.

6. What was accomplished on Day 1

Installed fastapi[standard] Wrote four endpoints (root, health check, project info, dynamic greeting)

Verified the /docs UI works

Ran pytest with five passing test cases

The entire workflow—from package installation to passing tests—took less than 20 minutes.

7. Next steps

Day 2 will cover path and query parameters, the most common input methods in FastAPI, and how type annotations are turned into validation rules.

Related links

Official documentation (Chinese): https://fastapi.tiangolo.com/zh/

GitHub repository: https://github.com/fastapi/fastapi