# Templates { #templates }

You can use any template engine you want with **FastAPI**.

A common choice is Jinja2, the same one used by Flask and other tools.

There are utilities to configure it easily that you can use directly in your **FastAPI** application (provided by Starlette).

## Install dependencies { #install-dependencies }

/**
 * Implementation of the RC4 (ARCFOUR) stream cipher algorithm.
 * This class provides RC4 encryption/decryption functionality used in SMB1 protocol operations.
 */
public class RC4 {

    byte[] s;
    int i, j;

    /**
     * Default constructor for RC4 cipher.
     * Call init() to initialize with a key before use.
     */
    public RC4() {
    }

    /**

		return tr
	}
}

// uaTransport - User-Agent  transport
type uaTransport struct {
	ua string
	rt http.RoundTripper
}

func (u *uaTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	req2 := req.Clone(req.Context())
	req2.Header.Set("User-Agent", u.ua)
	return u.rt.RoundTrip(req2)
}

// WithUserAgent wraps an existing transport with custom User-Agent

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * A simplistic set which implements the bare minimum so that it can be used in tests without
 * relying on any specific Set implementations. Slow. Explicitly allows null elements so that they
 * can be used in the testers.
 *
 * @author Regina O'Dell
 */
@GwtCompatible
@NullMarked

  /**
   * Creates a new container containing the given elements. TODO: would be nice to figure out how to
   * use E... or E[] as a parameter type, but this doesn't seem to work because Java creates an
   * array of the erased type.
   */
  T create(Object... elements);

  /**
   * Helper method to create an array of the appropriate type used by this generator. The returned
   * array will contain only nulls.
   */
  E[] createArray(int length);

    /**
     * Ctor that creates "managed" ClassWorld. This constructor is not used in "normal" circumstances.
     */
    public ClingSupport() {
        this(new ClassWorld(CORE_CLASS_REALM_ID, Thread.currentThread().getContextClassLoader()), true);
    }

    /**
     * Ctor to be used when running in ClassWorlds Launcher.
     */
    public ClingSupport(ClassWorld classWorld) {

**FastAPI** will use that *temporal* response to extract the cookies (also headers and status code), and will put them in the final response that contains the value you returned, filtered by any `response_model`.

You can also declare the `Response` parameter in dependencies, and set cookies (and headers) in them.

## Return a `Response` directly { #return-a-response-directly }

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

/**
 * A simplistic set which implements the bare minimum so that it can be used in tests without
 * relying on any specific Set implementations. Slow. Explicitly allows null elements so that they
 * can be used in the testers.
 *
 * @author Regina O'Dell
 */
@GwtCompatible
@NullMarked

   * HTTP/3 is the third and upcoming major version of the Hypertext Transfer Protocol used to
   * exchange information. HTTP/3 runs over QUIC, which is published as RFC 9000.
   *
   * HTTP/3 is not natively supported by OkHttp, but provided to allow a theoretical interceptor
   * that provides support.
   */
  HTTP_3("h3"),
  ;

  /**
   * Returns the string used to identify this protocol for ALPN, like "http/1.1", "spdy/3.1" or
   * "h2".

{* ../../docs_src/metadata/tutorial004_py39.py hl[3:16,18] *}

Notice that you can use Markdown inside of the descriptions, for example "login" will be shown in bold (**login**) and "fancy" will be shown in italics (_fancy_).

/// tip

You don't have to add metadata for all the tags that you use.

///

### Use your tags { #use-your-tags }