*     in this instance of the Java virtual machine
   */
  public Optional<Charset> charset() {
    // racy single-check idiom, this is safe because Optional is immutable.
    Optional<Charset> local = parsedCharset;
    if (local == null) {
      String value = null;
      local = Optional.absent();
      for (String currentValue : parameters.get(CHARSET_ATTRIBUTE)) {
        if (value == null) {

            </Export-Package>
            <Import-Package>
              com.google.common.util.concurrent.internal,
              javax.annotation;resolution:=optional,
              javax.crypto.*;resolution:=optional,
              sun.misc.*;resolution:=optional
            </Import-Package>
            <Bundle-DocURL>https://github.com/google/guava/</Bundle-DocURL>
          </instructions>
        </configuration>

本例中，查询参数 `q` 是可选的，默认值为 `None`。

/// check | "检查"

注意，**FastAPI** 可以识别出 `item_id` 是路径参数，`q` 不是路径参数，而是查询参数。

///

/// note | "笔记"

因为默认值为 `= None`，FastAPI 把 `q` 识别为可选参数。

FastAPI 不使用 `Optional[str]` 中的 `Optional`（只使用 `str`），但 `Optional[str]` 可以帮助编辑器发现代码中的错误。

///

## 查询参数类型转换

参数还可以声明为 `bool` 类型，FastAPI 会自动转换参数类型：


//// tab | Python 3.10+

```Python hl_lines="7"

			Description: "[DEPRECATED] authorization token for OPA endpoint" + defaultHelpPostfix(AuthToken),
			Optional:    true,
			Type:        "string",
			Sensitive:   true,
			Secret:      true,
		},
		config.HelpKV{
			Key:         config.Comment,
			Description: config.DefaultComment,
			Optional:    true,
			Type:        "sentence",
		},
	}

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <optional>true</optional>
    </dependency>
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <optional>true</optional>
    </dependency>
    <dependency>
      <groupId>org.jline</groupId>
      <artifactId>jansi-core</artifactId>

                        }
                    }
                    boolean optional = false;
                    if (!selector.isEmpty() && selector.charAt(0) == '?') {
                        optional = true;
                        selector = selector.substring(1);
                    }
                    projectActivation.addProjectActivation(selector, active, optional);
                }
            }
        }
    }

 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.maven.api.services;

import java.util.List;
import java.util.Optional;

import org.apache.maven.api.Repository;
import org.apache.maven.api.Version;
import org.apache.maven.api.annotations.Experimental;
import org.apache.maven.api.annotations.Nonnull;

/**
 *
 * @since 4.0.0

from typing import Optional

from fastapi import FastAPI, Security
from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
from fastapi.testclient import TestClient

app = FastAPI()

security = HTTPBearer(auto_error=False)


@app.get("/users/me")
def read_current_user(
    credentials: Optional[HTTPAuthorizationCredentials] = Security(security),
):
    if credentials is None:

from typing import Optional

from fastapi import FastAPI, Security
from fastapi.security import OAuth2AuthorizationCodeBearer
from fastapi.testclient import TestClient

app = FastAPI()

oauth2_scheme = OAuth2AuthorizationCodeBearer(
    authorizationUrl="authorize",
    tokenUrl="token",
    description="OAuth2 Code Bearer",
    auto_error=True,
)


@app.get("/items/")

from typing import Optional

from fastapi import FastAPI, Security
from fastapi.security import OAuth2PasswordBearer
from fastapi.testclient import TestClient

app = FastAPI()

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token", auto_error=False)


@app.get("/items/")
async def read_items(token: Optional[str] = Security(oauth2_scheme)):
    if token is None:
        return {"msg": "Create an account first"}
    return {"token": token}