/// note | 注意

`PATCH` 相較於 `PUT` 較少被使用、也較不為人知。

許多團隊甚至在部分更新時也只用 `PUT`。

你可以依需求自由選用，**FastAPI** 不會強制規範。

但本指南會大致示範它們各自的設計用法。

///

### 使用 Pydantic 的 `exclude_unset` 參數 { #using-pydantics-exclude-unset-parameter }

如果要接收部分更新，在 Pydantic 模型的 `.model_dump()` 中使用 `exclude_unset` 參數非常實用。

例如 `item.model_dump(exclude_unset=True)`。

這會產生一個只包含建立 `item` 模型時實際設定過之欄位的 `dict`，不含預設值。

// if empty then c.prefixPath is used i.e "/skydns"
func CoreDNSPath(prefix string) EtcdOption {
	return func(args *CoreDNS) {
		args.prefixPath = prefix
	}
}

// NewCoreDNS - initialize a new coreDNS set/unset values.
func NewCoreDNS(cfg clientv3.Config, setters ...EtcdOption) (Store, error) {
	etcdClient, err := clientv3.New(cfg)
	if err != nil {
		return nil, err
	}

	args := &CoreDNS{
		etcdClient: etcdClient,

        String s = session.toString();
        assertTrue(s.contains("targetHost=server.example"));
        assertTrue(s.contains("targetDomain=EXAMPLE"));
    }

    @Test
    @DisplayName("getExpiration: null when unset, value when positive")
    void testGetExpiration() {
        SmbSessionImpl session = newSession();
        assertNull(session.getExpiration());
        setField(session, "expiration", 123L);

	}

	if cap(t.requestsPool) != apiRequestsMaxPerNode {
		// Only replace if needed.
		// Existing requests will use the previous limit,
		// but new requests will use the new limit.
		// There will be a short overlap window,
		// but this shouldn't last long.
		t.requestsPool = make(chan struct{}, apiRequestsMaxPerNode)
	}
	listQuorum := cfg.ListQuorum

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @Override
    public @Nullable ValueReference<Object, Object> getValueReference() {

     * for the current build session and do not modify the underlying project model.
     *
     * @param project the project to modify
     * @param key they property's key
     * @param value the value or {@code null} to unset the property
     */
    void setProperty(@Nonnull Project project, @Nonnull String key, @Nullable String value);

    /**
     * Returns the original project being built when the input project is a forked project.

	// MINIO_KMS_SECRET_KEY_FILE - either to the argument passed to
	// the container or to a default string (e.g. "minio_master_key").
	//
	// We have to distinguish a explicit config from an implicit. Hence,
	// we unset the env. vars if they are set but empty or contain a path
	// which does not exist. The downside of this check is that if
	// MINIO_KMS_SECRET_KEY_FILE is set to a path that does not exist,

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This wrapper uses syscall.Flock to prevent concurrent adb commands,
// so for now it only builds on platforms that support that system call.
// TODO(#33974): use a more portable library for file locking.

//go:build darwin || dragonfly || freebsd || illumos || linux || netbsd || openbsd

                    if (downloadMonitor != null) {
                        wagon.addTransferListener(downloadMonitor);
                    }
                }

                // unset the firstRun flag, so we don't get caught in an infinite loop...
                firstRun = false;
            }
        } catch (ConnectionException e) {

* `tags: List[str] = []` 默认值为一个空列表：`[]`。

但如果它们并没有被实际存储，你可能希望在结果中省略这些默认值。

例如，当你在 NoSQL 数据库中保存了具有许多可选属性的模型，但又不想发送充满默认值的冗长 JSON 响应。

### 使用 `response_model_exclude_unset` 参数 { #use-the-response-model-exclude-unset-parameter }

你可以设置*路径操作装饰器*参数 `response_model_exclude_unset=True`：

{* ../../docs_src/response_model/tutorial004_py310.py hl[22] *}

这样响应中将不会包含那些默认值，而只包含实际设置的值。