* Have installs and upgrades affected by this feature?
        * Is the feature still being worked on?
        * Is the default behavior altered?
        * Is this turned on by default?
        * How many users are affected by this change?

## Feature implementation

Release managers will continue to have a final say in what gets merged or not, unless directed by the TOC. See the next

 * returned.
 *
 * <p>All default instances returned by {@link #get} are generics-safe. Clients won't get type
 * errors for using {@code get(Comparator.class)} as a {@code Comparator<Foo>}, for example.
 * Immutable empty instances are returned for collection types; {@code ""} for string; {@code 0} for
 * number types; reasonable default instance for other stateless types. For mutable types, a fresh

   *
   * @see <a href=
   *     "http://code.google.com/webtoolkit/doc/latest/DevGuideServerCommunication.html#DevGuideSerializableTypes">
   *     Documentation about GWT serialization</a>
   */
  boolean serializable() default false;

  /**
   * When {@code true}, the annotated type is emulated in GWT. The emulated source (also known as
   * super-source) is different from the implementation used by the JVM.
   *
   * @see <a href=

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    q: str | None = Query(default=None, title="Query string", min_length=3),
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

RESOURCE NAME     TYPE           STATUS     VALID CERT     SERIAL NUMBER                        NOT AFTER                NOT BEFORE
default           Cert Chain     ACTIVE     false          6fbee254c22900615cb1f74e3d2f1713     2023-05-16T01:32:52Z     2023-05-15T01:30:52Z

     * @return an URL handler using this context
     */
    URLStreamHandler getUrlHandler ();


    /**
     * @return whether default credentials are available
     */
    boolean hasDefaultCredentials ();


    /**
     * @return a child context using the configured default credentials
     */
    CIFSContext withDefaultCredentials ();


    /**

    public static RepositoryPolicy toRepositoryPolicy(org.apache.maven.model.RepositoryPolicy policy) {
        boolean enabled = true;
        String checksums = toRepositoryChecksumPolicy(RepositoryPolicy.CHECKSUM_POLICY_WARN); // the default
        String updates = RepositoryPolicy.UPDATE_POLICY_DAILY;

        if (policy != null) {
            enabled = policy.isEnabled();
            if (policy.getUpdatePolicy() != null) {

  // VolumeClaimTemplates when the StatefulSet is deleted. The default policy
  // of `Retain` causes PVCs to not be affected by StatefulSet deletion. The
  // `Delete` policy causes those PVCs to be deleted.
  optional string whenDeleted = 1;

  // whenScaled specifies what happens to PVCs created from StatefulSet
  // VolumeClaimTemplates when the StatefulSet is scaled down. The default

		if _, err := uuid.Parse(vid); err != nil {
			return opts, InvalidVersionID{
				Bucket:    bucket,
				Object:    object,
				VersionID: vid,
			}
		}
	}

	// default case of passing encryption headers to backend
	opts, err = getDefaultOpts(r.Header, false, nil)
	if err != nil {
		return opts, err
	}
	opts.PartNumber = partNumber
	opts.VersionID = vid

```

由于我们必须用 `Query(default=None)` 替换默认值 `None`，`Query` 的第一个参数同样也是用于定义默认值。

所以：

```Python
q: Union[str, None] = Query(default=None)
```

...使得参数可选，等同于：

```Python
q: str = None
```

但是 `Query` 显式地将其声明为查询参数。

然后，我们可以将更多的参数传递给 `Query`。在本例中，适用于字符串的 `max_length` 参数：

```Python
q: Union[str, None] = Query(default=None, max_length=50)
```