```Python
say_hi(name=None)  # This works, None is valid 🎉
```

The good news is, once you are on Python 3.10 you won't have to worry about that, as you will be able to simply use `|` to define unions of types:

{* ../../docs_src/python_types/tutorial009c_py310.py hl[1,4] *}

And then you won't have to worry about names like `Optional` and `Union`. 😎

#### Generic types { #generic-types }

<img src="/img/tutorial/body-nested-models/image01.png">

You couldn't get this kind of editor support if you were working directly with `dict` instead of Pydantic models.

But you don't have to worry about them either, incoming dicts are converted automatically and your output is converted automatically to JSON too.

## Bodies of arbitrary `dict`s { #bodies-of-arbitrary-dicts }

* `fixedquery`: has the exact value `fixedquery`.
* `$`: ends there, doesn't have any more characters after `fixedquery`.

If you feel lost with all these **"regular expression"** ideas, don't worry. They are a hard topic for many people. You can still do a lot of stuff without needing regular expressions yet.

Now you know that whenever you need them you can use them in **FastAPI**.

## Default values { #default-values }

### No te preocupes por el estilo { #dont-worry-about-style }

* No te preocupes demasiado por cosas como los estilos de los mensajes de commit, yo haré squash y merge personalizando el commit manualmente.

So, behind the scenes, it will actually work as if everything was the same single app.

///

/// check

You don't have to worry about performance when including routers.

This will take microseconds and will only happen at startup.

So it won't affect performance. ⚡

///

Using `check_same_thread=False` allows FastAPI to use the same SQLite database in different threads. This is necessary as **one single request** could use **more than one thread** (for example in dependencies).

Don't worry, with the way the code is structured, we'll make sure we use **a single SQLModel *session* per request** later, this is actually what the `check_same_thread` is trying to achieve.

### Create the Tables { #create-the-tables }

   *
   * TODO: cpovirk - Is this an issue anywhere anymore? It looks to have been fixed for Java 8
   * (https://bugs.openjdk.org/browse/JDK-7121314) and before Lollipop
   * (https://r.android.com/47508). We *would* need to worry for J2KT, whose own concurrency support
   * is evolving (b/381065164, b/458160722), but this class is @J2ktIncompatible.
   *
   * 2. The superclass toArray() method declares the more general return type `@Nullable Object[]`,