///

An environment variable (also known as "**env var**") is a variable that lives **outside** of the Python code, in the **operating system**, and could be read by your Python code (or by other programs as well).

Environment variables could be useful for handling application **settings**, as part of the **installation** of Python, etc.

## Create and Use Env Vars

   *   mutation, which is not an actual risk because we use `synchronized`.
   * - Make `remaining` require a non-null `seq` argument. But this is a bit weird because the
   *   method, while it would avoid the instance field `seq` would still access the instance field
   *   `pos`.
   */

  @Override
  public synchronized int read(CharBuffer target) throws IOException {
    checkNotNull(target);
    checkOpen();

   *   mutation, which is not an actual risk because we use `synchronized`.
   * - Make `remaining` require a non-null `seq` argument. But this is a bit weird because the
   *   method, while it would avoid the instance field `seq` would still access the instance field
   *   `pos`.
   */

  @Override
  public synchronized int read(CharBuffer target) throws IOException {
    checkNotNull(target);
    checkOpen();

Now, let's see how you could use that to return a custom response.

Let's say that you want to return an <a href="https://en.wikipedia.org/wiki/XML" class="external-link" target="_blank">XML</a> response.

If you use a `requirements.txt` file you could specify the version with:

```txt
fastapi[standard]==0.112.0
```

that would mean that you would use exactly the version `0.112.0`.

Or you could also pin it with:

```txt
fastapi[standard]>=0.112.0,<0.113.0
```

    if (finalizableReferenceClass == null) {
      /*
       * FinalizableReference's class loader was reclaimed. While there's a chance that other
       * finalizable references could be enqueued subsequently (at which point the class loader
       * would be resurrected by virtue of us having a strong reference to it), we should pretty
       * much just shut down and make sure we don't keep it alive any longer than necessary.
       */

# OpenAPI Webhooks

There are cases where you want to tell your API **users** that your app could call *their* app (sending a request) with some data, normally to **notify** of some type of **event**.

This means that instead of the normal process of your users sending requests to your API, it's **your API** (or your app) that could **send requests to their system** (to their API, their app).

This is normally called a **webhook**.

## Webhooks steps

# OpenAPI Callbacks

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

 *       suite()} method with {@code Suppress}. Would {@code FooTest} itself be suppressed, too?
 *   <li>In at least one case, a use of {@code sun.misc.FpUtils}, the test will not even
 *       <i>compile</i> against Android. Now, this might be an artifact of our build system, one
 *       that we could probably work around. Or we could manually strip the test from open-source

    # For coverage
    # TODO: in theory this would allow declaring types that could be lists of bytes
    # to be read from files and other types, but I'm not even sure it's a good idea
    # to support it as a first class "feature"
    assert is_bytes_sequence_annotation(Union[List[str], List[bytes]])


def test_is_uploadfile_sequence_annotation():
    # For coverage