That's a path parameter. You put a variable name in curly braces inside the path, and FastAPI extracts it for you:

@app.get("/users/{user_id}")
def get_user(user_id: int):
    return {"user_id": user_id, "name": f"User {user_id}"}

When someone requests GET /users/42, FastAPI extracts 42, converts it to an int (because of the type hint), and passes it as user_id. If someone requests /users/abc, FastAPI returns a 422 error automatically — it knows abc isn't a valid integer.

Exactly. That's one of FastAPI's superpowers. The type hint user_id: int does three things:

1. Validates — rejects non-integer values with a clear error
2. Converts — turns the string from the URL into a Python int
3. Documents — shows up in the auto-generated API docs

You can use str, int, float, or even bool:

@app.get("/items/{item_id}")
def get_item(item_id: int):
    return {"item_id": item_id}

@app.get("/files/{file_path:path}")
def get_file(file_path: str):
    return {"path": file_path}

The :path converter allows slashes in the parameter, so /files/data/reports/q1.csv captures data/reports/q1.csv.

Absolutely. Just add more variables:

@app.get("/courses/{course_id}/lessons/{lesson_id}")
def get_lesson(course_id: int, lesson_id: int):
    return {
        "course_id": course_id,
        "lesson_id": lesson_id,
        "title": f"Lesson {lesson_id} of Course {course_id}"
    }

GET /courses/5/lessons/3 extracts course_id=5 and lesson_id=3. The parameter names in the path must match the function argument names exactly.

Order matters. FastAPI checks routes in the order you define them. If you have:

@app.get("/users/me")
def current_user():
    return {"user": "current"}

@app.get("/users/{user_id}")
def get_user(user_id: int):
    return {"user_id": user_id}

Put /users/me first. Otherwise /users/{user_id} would try to convert me to an integer and fail. Fixed paths always go before parameterized paths.