# A First FastAPI App — Django / FastAPI

Source: https://www.skillbyai.com/en/django-fastapi/i-fastapi-hello

> Type hints become validation.

## Path and query parameters

In FastAPI, a decorated function is an endpoint; its **type-annotated parameters** define path and query parameters, converted and validated automatically. A path parameter declared as `int` rejects non-numeric input with **422 Unprocessable Entity** and a structured error explaining what was wrong. Return dicts or Pydantic models and FastAPI serialises them to JSON. Run it with an ASGI server such as uvicorn (`uvicorn main:app --reload`) and open `/docs` for interactive documentation.

## Typed parameters and automatic validation, run

I ran this with Python 3.12, FastAPI 0.142.2, Pydantic 2.13 and Starlette 1.7, calling the app through FastAPI's TestClient (no server needed). A default query parameter, an overridden one, and a 422 error when "seven" cannot be parsed as an integer.

```python
from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/products/{product_id}")
def get_product(product_id: int, currency: str = "INR"):   # path + query params, typed
    return {"id": product_id, "currency": currency}

client = TestClient(app)          # calls the app in-process, no server needed
for url in ["/products/7", "/products/7?currency=USD", "/products/seven"]:
    r = client.get(url)
    print(r.status_code, r.json())
```

Output:

```
200 {'id': 7, 'currency': 'INR'}
200 {'id': 7, 'currency': 'USD'}
422 {'detail': [{'type': 'int_parsing', 'loc': ['path', 'product_id'], 'msg': 'Input should be a valid integer, unable to parse string as an integer', 'input': 'seven'}]}
```

## Use TestClient while learning

TestClient calls your app in-process, perfect for experiments and tests without starting a server.

**Quiz:** What status does FastAPI return when a typed parameter cannot be parsed?

- [ ] 301 Moved
- [ ] 500 Internal Server Error
- [ ] 200 OK
- [x] 422 Unprocessable Entity

*Answer:* 422 Unprocessable Entity. Validation errors are 422 by default.