* `datetime.timedelta`:
    * A Python `datetime.timedelta`.
    * In requests and responses will be represented as a `float` of total seconds.
    * Pydantic also allows representing it as a "ISO 8601 time diff encoding", <a href="https://docs.pydantic.dev/latest/concepts/serialization/#json_encoders" class="external-link" target="_blank">see the docs for more info</a>.

* `expose_headers` - Indicate any response headers that should be made accessible to the browser. Defaults to `[]`.
* `max_age` - Sets a maximum time in seconds for browsers to cache CORS responses. Defaults to `600`.

The middleware responds to two particular types of HTTP request...

### CORS preflight requests

Then, for the request in the next tab, your app will wait for one second less, and so on.

This means that it will end up finishing some of the last tabs' requests earlier than some of the previous ones.

This includes, for example:

* Email notifications sent after performing an action:
    * 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:

    bytes: lambda o: o.decode(),
    Color: str,
    datetime.date: isoformat,
    datetime.datetime: isoformat,
    datetime.time: isoformat,
    datetime.timedelta: lambda td: td.total_seconds(),
    Decimal: decimal_encoder,
    Enum: lambda o: o.value,
    frozenset: list,
    deque: list,
    GeneratorType: list,
    IPv4Address: str,
    IPv4Interface: str,
    IPv4Network: str,

And also after the `response` is generated, before returning it.

For example, you could add a custom header `X-Process-Time` containing the time in seconds that it took to process the request and generate a response:

```Python hl_lines="10  12-13"
{!../../../docs_src/middleware/tutorial001.py!}
```

## Other middlewares

    github_event = PartialGitHubEvent.parse_raw(contents)

    # Avoid race conditions with multiple labels
    sleep_time = random.random() * 10  # random number between 0 and 10 seconds
    logging.info(
        f"Sleeping for {sleep_time} seconds to avoid "
        "race conditions and multiple comments"
    )
    time.sleep(sleep_time)

    # Get PR
    logging.debug(f"Processing PR: #{github_event.pull_request.number}")

```Python
my_user: User = User(id=3, name="John Doe", joined="2018-07-19")

second_user_data = {
    "id": 4,
    "name": "Mary",
    "joined": "2018-11-30",
}

my_second_user: User = User(**second_user_data)
```

!!! info
    `**second_user_data` şu anlama geliyor:

```Python
old_dict = {
    "old key": "old value",
    "second old key": "second old value",
}
new_dict = {**old_dict, "new key": "new value"}
```

Hier wird `new_dict` alle Schlüssel-Wert-Paare von `old_dict` plus das neue Schlüssel-Wert-Paar enthalten:

```Python
{
    "old key": "old value",
    "second old key": "second old value",
    "new key": "new value",
}
```

```Python
old_dict = {
    "old key": "old value",
    "second old key": "second old value",
}
new_dict = {**old_dict, "new key": "new value"}
```

Here, `new_dict` will contain all the key-value pairs from `old_dict` plus the new key-value pair:

```Python
{
    "old key": "old value",
    "second old key": "second old value",
    "new key": "new value",
}
```