@app.get("/items/{item_id}")
def main(
    item_id: int = Path(default=..., gt=0),
    query: str = Query(default=..., max_length=10),
    session: str = Cookie(default=..., min_length=3),
    x_trace: str = Header(default=..., title="Tracing header"),
):
    return {"message": "Hello World"}
```

...all these parameters are required because the default value is `...` (Ellipsis).

<!--{
	"Title": "The Go Programming Language Specification",
	"Subtitle": "Language version go1.22 (April 25, 2024)",
	"Path": "/ref/spec"
}-->

<h2 id="Introduction">Introduction</h2>

<p>
This is the reference manual for the Go programming language.
The pre-Go1.18 version, without generics, can be found
<a href="/doc/go1.17_spec.html">here</a>.
For more information and other documents, see <a href="/">go.dev</a>.
</p>

<p>