An <a href="https://en.wikipedia.org/wiki/Environment_variable" class="external-link" target="_blank">environment variable</a> (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).

You can create and use environment variables in the shell, without needing Python:

## 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.

### Disable the automatic docs

The first step is to disable the automatic docs, as by default, those use the default CDN.

Then wrap the WSGI (e.g. Flask) app with the middleware.

And then mount that under a path.

```Python hl_lines="2-3  23"
{!../../../docs_src/wsgi/tutorial001.py!}
```

## Check it

Now, every request under the path `/v1/` will be handled by the Flask application.

And the rest will be handled by **FastAPI**.

This provider might be charging you per request, and calling it might take some extra time than if you had a fixed mock user for tests.

You probably want to test the external provider once, but not necessarily call it for every test that runs.

In this case, you can override the dependency that calls that provider, and use a custom dependency that returns a mock user, only for your tests.

### Use the `app.dependency_overrides` attribute

    * Although it's probably not very problematic here with the way `SQLAlchemy` works.
    * But if you added more code to the middleware that had a lot of <abbr title="input and output">I/O</abbr> waiting, it could then be problematic.
* A middleware is run for *every* request.
    * So, a connection will be created for every request.

Let's imagine that loading the model can **take quite some time**, because it has to read a lot of **data from disk**. So you don't want to do it for every request.

labels: [question]
body:
  - type: markdown
    attributes:
      value: |
        Thanks for your interest in FastAPI! 🚀

        Please follow these instructions, fill every question, and do every step. 🙏

        I'm asking this because answering questions and solving problems in GitHub is what consumes most of the time.

```

### Cache the OpenAPI schema

You can use the property `.openapi_schema` as a "cache", to store your generated schema.

That way, your application won't have to generate the schema every time a user opens your API docs.

It will be generated only once, and then the same cached schema will be used for the next requests.

```Python hl_lines="13-14  25-26"
{!../../../docs_src/extending_openapi/tutorial001.py!}

For large responses, returning a `Response` directly is much faster than returning a dictionary.

To see how to achieve this parallelism in production see the section about [Deployment](deployment/index.md){.internal-link target=_blank}.

## `async` and `await`