⚠️ Path Parameter Conflict (Route Hijacking) — With Query Params
FastAPI matches routes using the path only, not parameter names or query parameters.
👉 Query parameters (?category=...) do NOT help FastAPI choose between routes.
❌ Conflicting Example
@app.get("/books/{book_title}")
@app.get("/books/{book_author}")Both match:
/books/something👉 FastAPI cannot distinguish them
👉 The first one defined captures all requests
👉 The second route never runs (hijacked)
⚠️ Why Query Parameters Didn’t Help
Even if this endpoint expects a query param:
@app.get("/books/{book_author}")
async def read_author_and_category(book_author: str, category: str):Request:
/books/Author One?category=science🚫 FastAPI still matches only by:
/books/Author One👉 It ignores ?category=science during routing
👉 So the request may go to /books/{book_title} instead
✅ Why Adding / Worked
Changing to:
@app.get("/books/{book_author}/")creates a different path:
/books/something/Now FastAPI can distinguish:
/books/something
/books/something/👉 The correct endpoint receives the request.
⚠️ Important Caution
Query parameters do NOT affect route matching
Parameter names do NOT matter
Trailing slash creates a new route but can confuse users
Order and path structure matter most
✅ Best Practice
Use clear, unique paths instead of relying on /:
/books/title/{book_title}
/books/author/{book_author}📌 Quick Rule
👉 Routing = based on path only
👉 Query params are processed after a route is selected
👉 Use distinct endpoints to avoid hijacking
🚀 Clean paths prevent confusing bugs in real APIs