/// warning | Advertencia

Antes de establecer `convert_underscores` a `False`, ten en cuenta que algunos proxies y servidores HTTP no permiten el uso de headers con guiones bajos.

///

## Headers duplicados

Es posible recibir headers duplicados. Eso significa, el mismo header con múltiples valores.

Puedes definir esos casos usando una lista en la declaración del tipo.

    assertThat(connect.headers["Private"]).isNull()
    assertThat(connect.headers["User-Agent"]).isEqualTo(USER_AGENT)
    assertThat(connect.headers["Proxy-Connection"]).isEqualTo("Keep-Alive")
    assertThat(connect.headers["Host"]).isEqualTo("android.com:443")
    val get = server.takeRequest()
    assertThat(get.headers["Private"]).isEqualTo("Secret")
    assertThat(get.headers["User-Agent"]).isEqualTo("App 1.0")

**FastAPI** will **extract** the data for **each field** from the **headers** in the request and give you the Pydantic model you defined.

## Check the Docs { #check-the-docs }

You can see the required headers in the docs UI at `/docs`:

<div class="screenshot">
<img src="/img/tutorial/header-param-models/image01.png">
</div>

## Forbid Extra Headers { #forbid-extra-headers }

* `allow_headers` - Una lista de headers de request HTTP que deberían estar soportados para requests cross-origin. Por defecto es `[]`. Puedes usar `['*']` para permitir todos los headers. Los headers `Accept`, `Accept-Language`, `Content-Language` y `Content-Type` siempre están permitidos para <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests"...

  const val FLAG_ACK = 0x1 // Used for settings and ping.
  const val FLAG_END_STREAM = 0x1 // Used for headers and data.
  const val FLAG_END_HEADERS = 0x4 // Used for headers and continuation.
  const val FLAG_END_PUSH_PROMISE = 0x4
  const val FLAG_PADDED = 0x8 // Used for headers and data.
  const val FLAG_PRIORITY = 0x20 // Used for headers.
  const val FLAG_COMPRESSED = 0x20 // Used for data.

  /** Lookup table for valid frame types. */

    assertThat(recorded.headers["Accept"]).isEqualTo("text/plain")
    assertThat(recorded.headers["Accept-Encoding"]).isNull() // No built-in gzip.
    assertThat(recorded.headers["Connection"]).isEqualTo("Upgrade, HTTP2-Settings")
    if (PlatformVersion.majorVersion < 19) {
      assertThat(recorded.headers["Content-Length"]).isEqualTo("0")
    }
    assertThat(recorded.headers["HTTP2-Settings"]).isNotNull()

**FastAPI** **extraerá** los datos para **cada campo** de los **headers** en el request y te dará el modelo Pydantic que definiste.

## Revisa la Documentación

Puedes ver los headers requeridos en la interfaz de documentación en `/docs`:

<div class="screenshot">
<img src="/img/tutorial/header-param-models/image01.png">
</div>

## Prohibir Headers Extra

    @Override public Response intercept(Chain chain) throws IOException {
      Request request = chain.request();
      Headers newHeaders = request.headers()
          .newBuilder()
          .add("Date", new Date())
          .build();
      Request newRequest = request.newBuilder()
          .headers(newHeaders)
          .build();
      return chain.proceed(newRequest);
    }
  }

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3.internal.http1

import okhttp3.Headers
import okio.BufferedSource

/**
 * Parse all headers delimited by "\r\n" until an empty line. This throws if headers exceed 256 KiB.
 */
class HeadersReader(
  val source: BufferedSource,
) {
  private var headerLimit = HEADER_LIMIT.toLong()

// getCanonicalHeaders generate a list of request headers with their values
func getCanonicalHeaders(signedHeaders http.Header) string {
	var headers []string
	vals := make(http.Header, len(signedHeaders))
	for k, vv := range signedHeaders {
		k = strings.ToLower(k)
		headers = append(headers, k)
		vals[k] = vv
	}
	sort.Strings(headers)

	var buf bytes.Buffer
	for _, k := range headers {
		buf.WriteString(k)
		buf.WriteByte(':')