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('Bad request', 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(object):
    def on_post(self, req, resp, project_id):
        pass

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

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('Bad request', 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(object):
    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.

falcon.before(action, *args, **kwargs)

Decorator to execute the given action function before the responder.

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.

  Note

  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('Invalid ID',
                                      'ID was not valid.')
  
      params['answer'] = 42
  

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

falcon.after(action, *args, **kwargs)

Decorator to 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, resource, and params arguments.
• **kwargs – Any additional keyword arguments will be passed through to action.