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:

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:

@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:

@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

Note

When decorating an entire resource class, all method names that resemble responders, including suffixed (see also 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:

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:

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 middleware components can also be used to insert logic before and after requests. However, unlike hooks, middleware components 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.

Before Hooks

falcon.before(action, *args, is_async=False, **kwargs)[source]

Execute the given action function before the responder.

The params argument that is passed to the hook contains only the fields from the URI template path; it does not include query string values.

Hooks may inject extra params as needed. For example:

def do_something(req, resp, resource, params):
    try:
        params['id'] = int(params['id'])
    except ValueError:
        raise falcon.HTTPBadRequest(title='Invalid ID',
                                    description='ID was not valid.')

    params['answer'] = 42
Parameters:

  • action (callable) – A function of the form func(req, resp, resource, params), where resource is a reference to the resource class instance associated with the request and params is a dict of URI template field names, if any, that will be passed into the resource responder as kwargs.

  • *args – Any additional arguments will be passed to action in the order given, immediately following the req, resp, resource, and params arguments.

Keyword Arguments:

  • is_async (bool) –

    Set to True for ASGI apps to provide a hint that the decorated responder is a coroutine function (i.e., that it is defined with async def) or that it returns an awaitable coroutine object.

    Normally, when the function source is declared using async def, the resulting function object is flagged to indicate it returns a coroutine when invoked, and this can be automatically detected. However, it is possible to use a regular function to return an awaitable coroutine object, in which case a hint is required to let the framework know what to expect. Also, a hint is always required when using a cythonized coroutine function, since Cython does not flag them in a way that can be detected in advance, even when the function is declared using async def.

  • **kwargs – Any additional keyword arguments will be passed through to action.

After Hooks

falcon.after(action, *args, is_async=False, **kwargs)[source]

Execute the given action function after the responder.

Parameters:

  • action (callable) – A function of the form func(req, resp, resource), where resource is a reference to the resource class instance associated with the request

  • *args – Any additional arguments will be passed to action in the order given, immediately following the req, resp and resource arguments.

Keyword Arguments:

  • is_async (bool) –

    Set to True for ASGI apps to provide a hint that the decorated responder is a coroutine function (i.e., that it is defined with async def) or that it returns an awaitable coroutine object.

    Normally, when the function source is declared using async def, the resulting function object is flagged to indicate it returns a coroutine when invoked, and this can be automatically detected. However, it is possible to use a regular function to return an awaitable coroutine object, in which case a hint is required to let the framework know what to expect. Also, a hint is always required when using a cythonized coroutine function, since Cython does not flag them in a way that can be detected in advance, even when the function is declared using async def.

  • **kwargs – Any additional keyword arguments will be passed through to action.