When you install Python, you might be asked if you want to update the `PATH` environment variable.

//// tab | Linux, macOS

Let's say you install Python and it ends up in a directory `/opt/custompython/bin`.

If you say yes to update the `PATH` environment variable, then the installer will add `/opt/custompython/bin` to the `PATH` environment variable.

It could look like this:

```plaintext

* `http://localhost`
* `https://localhost`
* `http://localhost:8080`

Even if they are all in `localhost`, they use different protocols or ports, so, they are different "origins".

## Steps { #steps }

So, let's say you have a frontend running in your browser at `http://localhost:8080`, and its JavaScript is trying to communicate with a backend running at `http://localhost` (because we don't specify a port, the browser will assume the default port `80`).

In a real application, you probably would have your tests in a different file.

And your **FastAPI** application might also be composed of several files/modules, etc.

### **FastAPI** app file { #fastapi-app-file }

Let's say you have a file structure as described in [Bigger Applications](bigger-applications.md){.internal-link target=_blank}:

```
.
├── app
│   ├── __init__.py
│   └── main.py
```

@SuppressWarnings("JUnit4ClassUsedInJUnit3")
public class CollectionSerializationEqualTester<E> extends AbstractCollectionTester<E> {
  @CollectionFeature.Require(SERIALIZABLE)
  /*
   * As the class docs say, this test only makes sense for collections that define equals().
   * Accordingly, our testing framework adds it only when testing an implementation of List, Set, or
   * Multiset.
   */
  @SuppressWarnings("UndefinedEquals")

@DoNotMock("Use ImmutableRangeMap or TreeRangeMap")
@GwtIncompatible
public interface RangeMap<K extends Comparable, V> {
  /*
   * TODO(cpovirk): These docs sometimes say "map" and sometimes say "range map." Pick one, or at
   * least decide on a policy for when to use which.
   */

  /**
   * Returns the value associated with the specified key, or {@code null} if there is no such value.
   *

  public void clear() {
    synchronized (lock) {
      list.clear();
    }
  }

  /** Returns a snapshot of the logged records. */
  /*
   * TODO(cpovirk): consider higher-level APIs here (say, assertNoRecordsLogged(),
   * getOnlyRecordLogged(), getAndClearLogRecords()...)
   *
   * TODO(cpovirk): consider renaming this method to reflect that it takes a snapshot (and/or return
   * an ImmutableList)
   */

 * or method references instead of classes, leaving your code easier to migrate in the future.
 *
 * <p>To use an existing supplier instance (say, named {@code supplier}) in a context where the
 * <i>other type</i> of supplier is expected, use the method reference {@code supplier::get}. A

But it's possible to customize it, you can set a specific CDN, or serve the files yourself.

## Custom CDN for JavaScript and CSS { #custom-cdn-for-javascript-and-css }

Let's say that you want to use a different <abbr title="Content Delivery Network">CDN</abbr>, for example you want to use `https://unpkg.com/`.

This could be useful if for example you live in a country that restricts some URLs.

 * presented below:
 *
 * <pre>
 * void runManyThings({@literal List<ThingToRun>} thingsToRun) {
 *   for (ThingToRun thingToRun : thingsToRun) {
 *     thingToRun.run(); // say this may throw an exception, but you want to
 *                       // always run all thingsToRun
 *   }
 * }
 * </pre>
 *
 * <p>This is what the code would become:
 *
 * <pre>

The first thing you should do is to "pin" the version of **FastAPI** you are using to the specific latest version that you know works correctly for your application.

For example, let's say you are using version `0.112.0` in your app.

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

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