You can define event handlers (functions) that need to be executed before the application starts up, or when the application is shutting down.

These functions can be declared with `async def` or normal `def`.

### `startup` event

To add a function that should be run before the application starts, declare it with the event `"startup"`:

{* ../../docs_src/events/tutorial001.py hl[8] *}

      incorporateRequirements(requirements, moreRequirements, testerAnnotation);
    }

    return requirements;
  }

  /**
   * Find all the tester annotations declared on a tester class or method.
   *
   * @param classOrMethod a class or method whose tester annotations to find
   * @return an iterable sequence of tester annotations on the class
   */

/// tip

Prefer to use the `Annotated` version if possible.

///

```Python hl_lines="10  15"
{!> ../../docs_src/request_files/tutorial002.py!}
```

////

You will receive, as declared, a `list` of `bytes` or `UploadFile`s.

/// note | "Technical Details"

You could also use `from starlette.responses import HTMLResponse`.

    Set<MethodSignature> missing = ImmutableSortedSet.copyOf(difference(required, found));
    if (!missing.isEmpty()) {
      fail(
          rootLocaleFormat(
              "%s should hide the public static methods declared in %s: %s",
              descendant.getSimpleName(), ancestor.getSimpleName(), missing));
    }
  }

  private static Set<MethodSignature> getAllRequiredToFauxveride(Class<?> ancestor) {

    Set<MethodSignature> missing = ImmutableSortedSet.copyOf(difference(required, found));
    if (!missing.isEmpty()) {
      fail(
          rootLocaleFormat(
              "%s should hide the public static methods declared in %s: %s",
              descendant.getSimpleName(), ancestor.getSimpleName(), missing));
    }
  }

  private static Set<MethodSignature> getAllRequiredToFauxveride(Class<?> ancestor) {

          return IteratorBasedImmutableMap.this;
        }

        @Override
        public UnmodifiableIterator<Entry<K, V>> iterator() {
          return entryIterator();
        }

        // redeclare to help optimizers with b/310253115
        @SuppressWarnings("RedundantOverride")
        @Override
        @J2ktIncompatible // serialization
        @GwtIncompatible // serialization
        Object writeReplace() {

If you go to the API docs, you will see that it has the **schemas** for the data to be sent in requests and received in responses:

<img src="/img/tutorial/generate-clients/image01.png">

You can see those schemas because they were declared with the models in the app.

That information is available in the app's **OpenAPI schema**, and then shown in the API docs (by Swagger UI).

So, the interactive docs will have all the information from these dependencies too:

<img src="/img/tutorial/dependencies/image01.png">

## Simple usage

If you look at it, *path operation functions* are declared to be used whenever a *path* and *operation* matches, and then **FastAPI** takes care of calling the function with the correct parameters, extracting the data from the request.

`OAuth2PasswordBearer` makes **FastAPI** know that it is a security scheme. So it is added that way to OpenAPI.

But `OAuth2PasswordRequestForm` is just a class dependency that you could have written yourself, or you could have declared `Form` parameters directly.

But as it's a common use case, it is provided by **FastAPI** directly, just to make it easier.

///

### Use the form data

/// tip

And if that `Response` has a JSON media type (`application/json`), like is the case with the `JSONResponse` and `UJSONResponse`, the data you return will be automatically converted (and filtered) with any Pydantic `response_model` that you declared in the *path operation decorator*.

/// note

If you use a response class with no media type, FastAPI will expect your response to have no content, so it will not document the response format in its generated OpenAPI docs.