Step 1 — Why Python · FastAPI

Python is the agreed-upon language for data, ML, and automation. FastAPI adds lightweight, type-safe web APIs to the stack.

1. Where Python leads

Data processing (pandas, numpy, polars)
ML (PyTorch, TensorFlow, scikit-learn)
Crawling (Playwright, Scrapy, BeautifulSoup)
Scripting / automation
Backend APIs (FastAPI, Django, Flask)

For system-level extreme performance, use Rust / C++ / Go.

2. Five FastAPI wins

Type hints double as validation + OpenAPI
Native async (async def)
Auto /docs (Swagger) and /redoc
Fast — Starlette + Pydantic, Node-class speed
Small surface — start with 1/10 of Spring

3. First API

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float
    tags: list[str] = []

@app.get("/")
def hello():
    return {"message": "hello from FastAPI"}

@app.post("/items")
def create_item(item: Item):
    return {"created": item, "id": 1}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "query": q}

uv run uvicorn main:app --reload

/docs → Swagger UI (interactive)
/redoc → ReDoc
/openapi.json → spec

4. uv — the Python package manager

curl -LsSf https://astral.sh/uv/install.sh | sh   # macOS/Linux
powershell -c "irm https://astral.sh/uv/install.ps1 | iex"   # Windows

uv init my-api
cd my-api
uv add fastapi "uvicorn[standard]" pydantic
uv add --dev pytest ruff
uv run uvicorn main:app --reload

uv replaces pip + venv + Poetry. Rust-based, 10–100x faster, reproducible lockfile.

5. Pydantic validation

from pydantic import BaseModel, Field, EmailStr

class User(BaseModel):
    id: int
    email: EmailStr
    nickname: str = Field(min_length=2, max_length=30)
    age: int | None = Field(default=None, ge=0, le=150)

Bad input → FastAPI returns 422 automatically.

6. Dependency injection

from fastapi import Depends

def get_db():
    db = SessionLocal()
    try: yield db
    finally: db.close()

@app.get("/users")
def list_users(db = Depends(get_db)):
    return db.query(User).all()

Easy to swap for mocks in tests.

7. Async

import httpx

@app.get("/external")
async def call_external():
    async with httpx.AsyncClient() as client:
        r = await client.get("https://api.example.com/data")
    return r.json()

Handles thousands of concurrent requests.

8. vs Spring

Axis	Spring Boot 4	FastAPI
Types	Java static	Python hints (optional)
Startup	5–10s	~0.5s
Learning curve	steep	gentle
Ecosystem	enterprise everything	data / ML
RAM	200 MB+	~50 MB

Small-to-mid APIs and data pipelines → FastAPI. Large enterprise → Spring.

9. Gotchas

uvicorn vs gunicorn — dev vs prod (gunicorn + uvicorn workers)
Mixing sync and async → blocks the event loop
Pydantic v1 vs v2 — 2026 is v2. .dict() → .model_dump()
.env not loading — use pydantic-settings or python-dotenv

10. Try it

uv run uvicorn main:app --reload
curl http://localhost:8000
curl -X POST http://localhost:8000/items -H "Content-Type: application/json" \
     -d '{"name":"apple","price":1000}'

Then visit http://localhost:8000/docs.

Deeper

Step 2 — expand this single file into a maintainable folder structure.