# Request Body { #request-body }

When you need to send data from a client (let's say, a browser) to your API, you send it as a **request body**.

A **request** body is data sent by the client to your API. A **response** body is the data your API sends to the client.

# Body - Fields { #body-fields }

The same way you can declare additional validation and metadata in *path operation function* parameters with `Query`, `Path` and `Body`, you can declare validation and metadata inside of Pydantic models using Pydantic's `Field`.

## Import `Field` { #import-field }

First, you have to import it:

{* ../../docs_src/body_fields/tutorial001_an_py310.py hl[4] *}


/// warning

# Body - Updates { #body-updates }

## Update replacing with `PUT` { #update-replacing-with-put }

To update an item you can use the <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/PUT" class="external-link" target="_blank">HTTP `PUT`</a> operation.

You can use the `jsonable_encoder` to convert the input data to data that can be stored as JSON (e.g. with a NoSQL database). For example, converting `datetime` to `str`.

# Body - Nested Models { #body-nested-models }

With **FastAPI**, you can define, validate, document, and use arbitrarily deeply nested models (thanks to Pydantic).

## List fields { #list-fields }

You can define an attribute to be a subtype. For example, a Python `list`:

{* ../../docs_src/body_nested_models/tutorial001_py310.py hl[12] *}

This will make `tags` be a list, although it doesn't declare the type of the elements of the list.

# Body - Multiple Parameters { #body-multiple-parameters }

Now that we have seen how to use `Path` and `Query`, let's see more advanced uses of request body declarations.

## Mix `Path`, `Query` and body parameters { #mix-path-query-and-body-parameters }

First, of course, you can mix `Path`, `Query` and request body parameter declarations freely and **FastAPI** will know what to do.

          logger.log("--> END ${request.method} (encoded body omitted)")
        } else if (requestBody.isDuplex()) {
          logger.log("--> END ${request.method} (duplex request body omitted)")
        } else if (requestBody.isOneShot()) {
          logger.log("--> END ${request.method} (one-shot body omitted)")
        } else {
          var buffer = Buffer()
          requestBody.writeTo(buffer)

Download a file, print its headers, and print its response body as a string.

The `string()` method on response body is convenient and efficient for small documents. But if the response body is large (greater than 1 MiB), avoid `string()` because it will load the entire document into memory. In that case, prefer to process the body as a stream.

=== ":material-language-kotlin: Kotlin"
    ```kotlin

<img src="/img/tutorial/response-status-code/image01.png">

/// note

Some response codes (see the next section) indicate that the response does not have a body.

FastAPI knows this, and will produce OpenAPI docs that state there is no response body.

///

## About HTTP status codes { #about-http-status-codes }

/// note

If you already know what HTTP status codes are, skip to the next section.

///

    val same = brotliInterceptor.decompress(response)

    val responseString = same.body.string()
    assertThat(responseString).isEmpty()
  }

  private fun response(
    url: String,
    bodyHex: ByteString,
    fn: Response.Builder.() -> Unit = {},
  ): Response =
    Response
      .Builder()
      .body(bodyHex.toResponseBody("text/plain".toMediaType()))
      .code(200)
      .message("OK")

/// info

`Form` is a class that inherits directly from `Body`.

///

/// tip

To declare form bodies, you need to use `Form` explicitly, because without it the parameters would be interpreted as query parameters or body (JSON) parameters.

///