For "synchronous" (contrary to "asynchronous") they commonly also use the term "sequential", because the computer / program follows all the steps in sequence before switching to a different task, even if those steps involve waiting.

### Concurrency and Burgers

This idea of **asynchronous** code described above is also sometimes called **"concurrency"**. It is different from **"parallelism"**.

If we want to call asynchronous functions in our tests, our test functions have to be asynchronous. AnyIO provides a neat plugin for this, that allows us to specify that some test functions are to be called asynchronously.

## HTTPX

Even if your **FastAPI** application uses normal `def` functions instead of `async def`, it is still an `async` application underneath.

        By asking questions in a structured way (following this) it will be much easier to help you.

        And there's a high chance that you will find the solution along the way and you won't even have to submit it and wait for an answer. 😎

                the client doesn't have to provide it.

                But when using `Item` for output, for a response body, `tags` is always
                available because it has a default value, even if it's just an empty
                list. So, the client should be able to always expect it.

                In this case, there would be two different schemas, one for input and
                another one for output.

### Context variables

<img src="/img/tutorial/custom-response/image01.png">

## Available responses

Here are some of the available responses.

Keep in mind that you can use `Response` to return anything else, or even create a custom sub-class.

!!! note "Technical Details"
    You could also use `from starlette.responses import HTMLResponse`.

...and with that single declaration you get:

* Editor support, including:
    * Completion.
    * Type checks.
* Validation of data:
    * Automatic and clear errors when the data is invalid.
    * Validation even for deeply nested JSON objects.
* <abbr title="also known as: serialization, parsing, marshalling">Conversion</abbr> of input data: coming from the network to Python data and types. Reading from:
    * JSON.
    * Path parameters.

!!! tip
    If you manually call `app.openapi()`, you should update the `operationId`s before that.

!!! warning
    If you do this, you have to make sure each one of your *path operation functions* has a unique name.

    Even if they are in different modules (Python files).

## Exclude from OpenAPI

class DefaultPlaceholder:
    """
    You shouldn't use this class directly.

    It's used internally to recognize when a default value has been overwritten, even
    if the overridden default value was truthy.
    """

    def __init__(self, value: Any):
        self.value = value

    def __bool__(self) -> bool:
        return bool(self.value)

This new `examples` field in JSON Schema is **just a `list`** of examples, not a dict with extra metadata as in the other places in OpenAPI (described above).

!!! info
    Even after OpenAPI 3.1.0 was released with this new simpler integration with JSON Schema, for a while, Swagger UI, the tool that provides the automatic docs, didn't support OpenAPI 3.1.0 (it does since version 5.0.0 🎉).