But then we get a `dict` in the parameter `commons` of the *path operation function*.

And we know that editors can't provide a lot of support (like completion) for `dict`s, because they can't know their keys and value types.

We can do better...

## What makes a dependency { #what-makes-a-dependency }

Up to now you have seen dependencies declared as functions.

When using a proxy to handle HTTPS, your **application server** (for example Uvicorn via FastAPI CLI) doesn't known anything about the HTTPS process, it communicates with plain HTTP with the **TLS Termination Proxy**.

This **proxy** would normally set some HTTP headers on the fly before transmitting the request to the **application server**, to let the application server know that the request is being **forwarded** by the proxy.

/// note | Technical Details

     *   <li>If this field's value is some other thread object, we know that it's not our thread.
     *   <li>If this field's value == null because it originally belonged to another thread and that
     *       thread cleared it, we still know that it's not associated with our thread
     *   <li>If this field's value == null because it was associated with our thread and was
     *       cleared, we know that we're not executing inline any more
     * </ul>

**FastAPI** will know that it can use the class `OAuth2PasswordBearer` (declared in a dependency) to define the security scheme in OpenAPI because it inherits from `fastapi.security.oauth2.OAuth2`, which in turn inherits from `fastapi.security.base.SecurityBase`.

All the security utilities that integrate with OpenAPI (and the automatic API docs) inherit from `SecurityBase`, that's how **FastAPI** can know how to integrate them in OpenAPI.

{* ../../docs_src/dependencies/tutorial005_an_py310.py hl[23] *}

/// info

Notice that we are only declaring one dependency in the *path operation function*, the `query_or_cookie_extractor`.

But **FastAPI** will know that it has to solve `query_extractor` first, to pass the results of that to `query_or_cookie_extractor` while calling it.

///

```mermaid
graph TB

query_extractor(["query_extractor"])

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

### Pydantic v1 `regex` instead of `pattern` { #pydantic-v1-regex-instead-of-pattern }

Before Pydantic version 2 and before FastAPI 0.100.0, the parameter was called `regex` instead of `pattern`, but it's now deprecated.

You could still see some code using it:

//// tab | Pydantic v1

    if (mapEntryBelowToTruncate != null) {
      // we know ( [
      RangeMapEntry<K, V> rangeMapEntry = mapEntryBelowToTruncate.getValue();
      if (rangeMapEntry.getUpperBound().compareTo(rangeToRemove.lowerBound) > 0) {
        // we know ( [ )
        if (rangeMapEntry.getUpperBound().compareTo(rangeToRemove.upperBound) > 0) {
          // we know ( [ ] ), so insert the range ] ) back into the map --

With **FastAPI** you get all of **Pydantic**'s features (as FastAPI is based on Pydantic for all the data handling):

* **No brainfuck**:
    * No new schema definition micro-language to learn.
    * If you know Python types you know how to use Pydantic.
* Plays nicely with your **<abbr title="Integrated Development Environment, similar to a code editor">IDE</abbr>/<abbr title="A program that checks for code errors">linter</abbr>/brain**:

/// note

Some response codes (see the next section) indicate that the response does not have a body.

FastAPI knows this, and will produce OpenAPI docs that state there is no response body.

///

## About HTTP status codes { #about-http-status-codes }

/// note

If you already know what HTTP status codes are, skip to the next section.

///

<a href="https://x.com/compose/tweet?text=I'm loving @fastapi because... https://github.com/fastapi/fastapi" class="external-link" target="_blank">Tweet about **FastAPI**</a> and let me and others know why you like it. 🎉

I love to hear about how **FastAPI** is being used, what you have liked in it, in which project/company are you using it, etc.

## Vote for FastAPI { #vote-for-fastapi }