.. _hooks:

Hooks

Falcon supports before and after hooks. You install a hook simply by applying one of the decorators below, either to an individual responder or to an entire resource.

For example, consider this hook that validates a POST request for an image resource:

.. code:: python

def validate_image_type(req, resp, resource, params):
    if req.content_type not in ALLOWED_IMAGE_TYPES:
        msg = 'Image type not allowed. Must be PNG, JPEG, or GIF'
        raise falcon.HTTPBadRequest(title='Bad request', description=msg)

You would attach this hook to an on_post responder like so:

.. code:: python

@falcon.before(validate_image_type)
def on_post(self, req, resp):
    pass

Or, suppose you had a hook that you would like to apply to all responders for a given resource. In that case, you would simply decorate the resource class:

.. code:: python

def extract_project_id(req, resp, resource, params):
    params['project_id'] = None
    if req.content_type == falcon.MEDIA_JSON:
        params['project_id'] = req.get_media().get('projectId')

@falcon.before(extract_project_id)
class Message:
    def on_post(self, req, resp, project_id):
        pass

    def on_get(self, req, resp, project_id):
        pass

In addition to just running your logic before all responders of this resource, the extract_project_id hook also injects the project_id parameter -- see :func:falcon.before for a more elaborate explanation.

.. note:: When decorating an entire resource class, all method names that resemble responders, including suffix\ed (see also :meth:~falcon.App.add_route) ones, are decorated. If, for instance, a method is called on_get_items, but it is not meant for handling GET requests under a route with the suffix items, the easiest workaround for preventing the hook function from being applied to the method is renaming it not to clash with the responder pattern.

Note also that you can pass additional arguments to your hook function as needed:

.. code:: python

def validate_image_type(req, resp, resource, params, allowed_types):
    if req.content_type not in allowed_types:
        msg = 'Image type not allowed.'
        raise falcon.HTTPBadRequest(title='Bad request', description=msg)

@falcon.before(validate_image_type, ['image/png'])
def on_post(self, req, resp):
    pass

Falcon supports using any callable as a hook. This allows for using a class instead of a function:

.. code:: python

class Authorize:
    def __init__(self, roles):
        self._roles = roles

    def __call__(self, req, resp, resource, params):
        pass

@falcon.before(Authorize(['admin']))
def on_post(self, req, resp):
    pass

Falcon :ref:middleware components <middleware> can also be used to insert logic before and after requests. However, unlike hooks, :ref:middleware components <middleware> are triggered globally for all requests.

.. Tip:: In order to pass data from a hook function to a resource function use the req.context and resp.context objects. These context objects are intended to hold request and response data specific to your app as it passes through the framework.

You can also create your own decorators to wrap responder methods without needing to use hooks.

Here's an example of a custom decorator that logs request/response information:

.. code:: python

def log_access(responder):
    @functools.wraps(responder)
    def wrapper(resource, req, resp, *args, **kwargs) -> None:
        # Call the original responder
        responder(resource, req, resp, *args, **kwargs)

        # Log after the responder has been called
        logging.info(
            f'[{type(resource).__name__}] {req.method} {req.relative_uri}'
            f' => HTTP {resp.status_code}'
        )

    return wrapper

class HelloResource:
    @log_access
    def on_get(self, req, resp) -> None:
        resp.media = {"Hello": "World"}

In this example, the log_access decorator will log information about each request after the responder completes.

.. note:: The @functools.wraps(responder) ensures that the wrapper function preserves the original method's name and attributes.

You can also create parameterized decorators:

.. code:: python

def log_access(level=logging.INFO):
    def decorator(responder):
        @functools.wraps(responder)
        def wrapper(resource, req, resp, *args, **kwargs) -> None:
            responder(resource, req, resp, *args, **kwargs)

            logging.log(
                level,
                f'[{type(resource).__name__}] {req.method} {req.relative_uri}'
                f' => HTTP {resp.status_code}'
            )

        return wrapper

    return decorator

class HelloResource:
    @log_access(level=logging.DEBUG)
    def on_get(self, req, resp) -> None:
        resp.media = {"Hello": "World"}

Before Hooks

.. autofunction:: falcon.before

After Hooks

.. autofunction:: falcon.after

Configuration of Hooks

.. autodata:: falcon.hooks.decorate_on_request