Query Params & Validation: Making APIs Flexible and Secure

What are Query Parameters?

Query parameters let users filter and customize API responses
without changing the endpoint itself.

Instead of /products/electronics, you write:
/products?category=electronics&limit=5

Clean, flexible, and powerful.

What is Input Validation?

Validation ensures bad data never enters your system.
If a user sends an empty search term or a negative price,
your API should reject it immediately, not crash later.

My 3 Endpoints for Day 5

1. Search with Validation

@app.get("/search")
def search(q: str = Query(min_length=1, description="Search term")):
    return {
        "query": q,
        "message": f"Searching for: {q}"
    }

If q is empty : FastAPI automatically returns a 422 error.
No extra code needed. That's Pydantic doing the work.

2. Products with Optional Filters

@app.get("/products")
def get_products(
    category: str = Query(default="all"),
    limit: int = Query(default=10, ge=1, le=100)
):
    return {
        "category": category,
        "limit": limit,
        "message": f"Fetching {limit} products from: {category}"
    }

limit has a minimum of 1 and maximum of 100

users can't request 0 or 10,000 products.

3. POST with Pydantic Model

class Item(BaseModel):
    name: str
    price: float
    quantity: int

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

Pydantic automatically validates types
if price is a string instead of a float, it's rejected.

Screenshots

Lessons Learned

Validation isn't optional in production APIs.
Never trust user input, always validate at the boundary
before it touches your business logic or database.

Day 5 done. 25 more to go.

GDGoCBowen30dayChallenge