**FastAPI** se basa completamente en estas anotaciones de tipos, dándole muchas ventajas y beneficios.

Pero incluso si nunca usas **FastAPI**, te beneficiaría aprender un poco sobre ellas.

/// note | Nota

Si eres un experto en Python, y ya sabes todo sobre las anotaciones de tipos, salta al siguiente capítulo.

///

## Motivación { #motivation }

    we ignored the problem at request-building time, only to crash later with a
    `NullPointerException`.
 *  Fix: Include a backwards-compatible `OkHttp-Response-Source` header with
    `OkUrlFactory `responses.
 *  Fix: Don't include a default User-Agent header in requests made with the Call
    API. Requests made with OkUrlFactory will continue to have a default user
    agent.
 *  New: Guava-like API to create headers:

Ahora utilizaremos una dependencia simple para leer un header `X-Token` personalizado:

{* ../../docs_src/bigger_applications/app_an_py39/dependencies.py hl[3,6:8] title["app/dependencies.py"] *}

/// tip | Consejo

Estamos usando un header inventado para simplificar este ejemplo.

				AllowQuotedRecordDelimiter: false,
				unmarshaled:                true,
			}
			if !c.header {
				args.FileHeaderInfo = none
			}
			inr := io.Reader(bytes.NewReader(input))
			if c.sendErr != nil {
				inr = io.MultiReader(inr, errReader{c.sendErr})
			}
			r, _ := NewReader(io.NopCloser(inr), &args)
			fields := 0
			for {
				record, err = r.Read(record)

Pero ahora, al tener `Query(max_length=50)` dentro de `Annotated`, le estamos diciendo a FastAPI que queremos que tenga **validación adicional** para este valor, queremos que tenga un máximo de 50 caracteres. 😎

/// tip | Consejo

index.reindex.refresh=false
# Timeout for reindex operations.
index.reindex.timeout=1m
# Scroll timeout for reindex operations.
index.reindex.scroll=5m
# Maximum number of documents for reindex operations.
index.reindex.max_docs=

# query

# Maximum length of search queries.
query.max.length=1000
# Timeout (ms) for search queries.
query.timeout=10000
# Whether to enable logging for query timeouts.

[<a href="#Go_1.18">Go 1.18</a>].
</p>

<pre>
type Reader interface {
	Read(p []byte) (n int, err error)
	Close() error
}

type Writer interface {
	Write(p []byte) (n int, err error)
	Close() error
}

// ReadWriter's methods are Read, Write, and Close.
type ReadWriter interface {
	Reader  // includes methods of Reader in ReadWriter's method set

When writing request headers, use `header(name, value)` to set the only occurrence of `name` to `value`. If there are existing values, they will be removed before the new value is added. Use `addHeader(name, value)` to add a header without removing the headers already present.

Uma dessas variáveis ​​é a variável `PATH`.

/// tip | Dica

Você pode aprender mais sobre a variável de ambiente `PATH` na seção [Variáveis ​​de ambiente](environment-variables.md#path-environment-variable){.internal-link target=_blank}.

///

Una de esas variables es la variable `PATH`.

/// tip | Consejo

Puedes aprender más sobre la variable de entorno `PATH` en la sección [Variables de Entorno](environment-variables.md#path-environment-variable){.internal-link target=_blank}.

///