* As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
* Processing data:
    * For example, let's say you receive a file that must go through a slow process, you can return a response of "Accepted" (HTTP 202) and process the file in the background.

## Using `BackgroundTasks` { #using-backgroundtasks }

#### Existing language

Let's say you want to translate a page for a language that already has translations for some pages, like Spanish.

In the case of Spanish, the 2-letter code is `es`. So, the directory for Spanish translations is located at `docs/es/`.

{* ../../docs_src/python_types/tutorial008_py39.py hl[1] *}

This means:

* The variable `prices` is a `dict`:
    * The keys of this `dict` are of type `str` (let's say, the name of each item).
    * The values of this `dict` are of type `float` (let's say, the price of each item).

#### Union { #union }

You can declare that a variable can be any of **several types**, for example, an `int` or a `str`.

But it's signed. So, when you receive a token that you emitted, you can verify that you actually emitted it.

That way, you can create a token with an expiration of, let's say, 1 week. And then when the user comes back the next day with the token, you know that user is still logged in to your system.

/// info

`Body` also has all the same extra validation and metadata parameters as `Query`, `Path` and others you will see later.

///

## Embed a single body parameter { #embed-a-single-body-parameter }

Let's say you only have a single `item` body parameter from a Pydantic model `Item`.

By default, **FastAPI** will then expect its body directly.

     * because we know that newClassMap() is implemented using ConstrainedMap which is itself
     * well-tested. A purist would object to this, but what can I say, we're dirty cheaters.
     */
    map.put(Integer.class, new Integer(5));
    assertThrows(ClassCastException.class, () -> map.put(Double.class, new Long(42)));
    // Won't compile: map.put(String.class, "x");
  }

But then the attackers try with username `stanleyjobsox` and password `love123`.

And your application code does something like:

```Python

 * <p>The {@link #get}, {@link #removeAll}, and {@link #replaceValues} methods each return a {@link
 * SortedSet} of values, while {@link Multimap#entries()} returns a {@link Set} of map entries.
 * Though the method signature doesn't say so explicitly, the map returned by {@link #asMap} has
 * {@code SortedSet} values.
 *
 * <p><b>Warning:</b> As in all {@link SetMultimap}s, do not modify either a key <i>or a value</i>

    return (Set<V>) super.replaceValues(key, values);
  }

  /**
   * {@inheritDoc}
   *
   * <p>Though the method signature doesn't say so explicitly, the returned map has {@link Set}
   * values.
   */
  @Override
  public Map<K, Collection<V>> asMap() {
    return super.asMap();
  }

  /**
   * Stores a key-value pair in the multimap.

type string string

// int is a signed integer type that is at least 32 bits in size. It is a
// distinct type, however, and not an alias for, say, int32.
type int int

// uint is an unsigned integer type that is at least 32 bits in size. It is a
// distinct type, however, and not an alias for, say, uint32.
type uint uint

// uintptr is an integer type that is large enough to hold the bit pattern of
// any pointer.
type uintptr uintptr