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 }

Make sure you create a [virtual environment](../virtual-environments.md), activate it, and install `jinja2`:

<div class="termy">

     <span style="background-color:#007166"><font color="#D3D7CF"> INFO </font></span>  Waiting for application startup.
     <span style="background-color:#007166"><font color="#D3D7CF"> INFO </font></span>  Application startup complete.
```

</div>

/// tip

For production you would use `fastapi run` instead of `fastapi dev`. 🚀

///

### Choosing between application and network interceptors

Each interceptor chain has relative merits.

**Application interceptors**

 * Don't need to worry about intermediate responses like redirects and retries.
 * Are always invoked once, even if the HTTP response is served from the cache.

import okio.BufferedSource
import okio.ByteString
import okio.use

/**
 * A one-shot stream from the origin server to the client application with the raw bytes of the
 * response body. Each response body is supported by an active connection to the webserver. This
 * imposes both obligations and limits on the client application.
 *
 * ### The response body must be closed.
 *

    /**
     * Converts a string into <code>application/x-www-form-urlencoded</code>
     * format using the specified encoding scheme.
     *
     * @param s
     *            The string to be converted. Must not be {@literal null} or empty.
     * @param enc
     *            The encoding scheme. Must not be {@literal null} or empty.
     * @return The string encoded in <code>application/x-www-form-urlencoded</code> format.
     */

                            "404": {
                                "description": "The item was not found",
                                "content": {
                                    "application/json": {
                                        "schema": {
                                            "$ref": "#/components/schemas/Message"
                                        }
                                    }

import org.codelibs.curl.CurlResponse;

try (CurlResponse response = Curl.get("https://example.com")
                                .param("q", "curl4j")
                                .header("Accept", "application/json")
                                .execute()) {
    System.out.println("Status: " + response.getHttpStatusCode());
    System.out.println(response.getContentAsString());
}
```

### Asynchronous request

                                "description": "Return the JSON item or an image.",
                                "content": {
                                    "image/png": {},
                                    "application/json": {
                                        "schema": {"$ref": "#/components/schemas/Item"}
                                    },
                                },
                            },

    assertThat(body!!.contentType().toString()).isEqualTo(
      "application/x-www-form-urlencoded; charset=utf-8",
    )
    assertThat(bodyAsString(body)).isEqualTo("foo")
  }

  @Test
  fun contentTypeHeader() {
    val request =
      fromArgs(
        "-d",
        "foo",
        "-H",
        "Content-Type: application/json",
        "http://example.com",
      ).createRequest()

Dans cette section, vous verrez comment faire.

## Le processus normal { #the-normal-process }

Le processus normal (par défaut) est le suivant.

Une application (instance) `FastAPI` a une méthode `.openapi()` censée retourner le schéma OpenAPI.

Lors de la création de l'objet application, un *chemin d'accès* pour `/openapi.json` (ou pour l'URL que vous avez définie dans votre `openapi_url`) est enregistré.