Pecan Hooks

Although it is easy to use WSGI middleware with Pecan, it can be hard (sometimes impossible) to have access to Pecan’s internals from within middleware. Pecan Hooks are a way to interact with the framework, without having to write separate middleware.

Hooks allow you to execute code at key points throughout the life cycle of your request:

  • on_route(): called before Pecan attempts to route a request to a controller
  • before(): called after routing, but before controller code is run
  • after(): called after controller code has been run
  • on_error(): called when a request generates an exception

Implementating a Pecan Hook

In the below example, a simple hook will gather some information about the request and print it to stdout.

Your hook implementation needs to import PecanHook so it can be used as a base class. From there, you’ll want to override the on_route(), before(), after(), or on_error() methods to define behavior.

from pecan.hooks import PecanHook

class SimpleHook(PecanHook):

    def before(self, state):
        print "\nabout to enter the controller..."

    def after(self, state):
        print "\nmethod: \t %s" % state.request.method
        print "\nresponse: \t %s" % state.response.status

on_route(), before(), and after() are each passed a shared state object which includes useful information, such as the request and response objects, and which controller was selected by Pecan’s routing:

class SimpleHook(PecanHook):

    def on_route(self, state):
        print "\nabout to map the URL to a Python method (controller)..."
        assert state.controller is None  # Routing hasn't occurred yet
        assert isinstance(state.request, webob.Request)
        assert isinstance(state.response, webob.Response)
        assert isinstance(state.hooks, list)  # A list of hooks to apply

    def before(self, state):
        print "\nabout to enter the controller..."
        if state.request.path == '/':
            # `state.controller` is a reference to the actual
            # `@pecan.expose()`-ed controller that will be routed to
            # and used to generate the response body
            assert state.controller.__func__ is RootController.index.__func__
            assert isinstance(state.arguments, inspect.Arguments)
            print state.arguments.args
            print state.arguments.varargs
            print state.arguments.keywords
        assert isinstance(state.request, webob.Request)
        assert isinstance(state.response, webob.Response)
        assert isinstance(state.hooks, list)

on_error() is passed a shared state object and the original exception. If an on_error() handler returns a Response object, this response will be returned to the end user and no furthur on_error() hooks will be executed:

class CustomErrorHook(PecanHook):

    def on_error(self, state, exc):
        if isinstance(exc, SomeExceptionType):
            return webob.Response('Custom Error!', status=500)

Attaching Hooks

Hooks can be attached in a project-wide manner by specifying a list of hooks in your project’s configuration file.

app = {
    'root' : '...'
    # ...
    'hooks': lambda: [SimpleHook()]

Hooks can also be applied selectively to controllers and their sub-controllers using the __hooks__ attribute on one or more controllers and subclassing HookController.

from pecan import expose
from pecan.hooks import HookController
from my_hooks import SimpleHook

class SimpleController(HookController):

    __hooks__ = [SimpleHook()]

    def index(self):
        print "DO SOMETHING!"
        return dict()

Now that SimpleHook is included, let’s see what happens when we run the app and browse the application from our web browser.

pecan serve
serving on view at

about to enter the controller...
method:      GET
response:    200 OK

Hooks can be inherited from parent class or mixins. Just make sure to subclass from HookController.

from pecan import expose
from pecan.hooks import PecanHook, HookController

class ParentHook(PecanHook):

    priority = 1

    def before(self, state):
        print "\nabout to enter the parent controller..."

class CommonHook(PecanHook):

    priority = 2

    def before(self, state):
        print "\njust a common hook..."

class SubHook(PecanHook):

    def before(self, state):
        print "\nabout to enter the subcontroller..."

class SubMixin(object):
    __hooks__ = [SubHook()]

# We'll use the same instance for both controllers,
# to avoid double calls
common = CommonHook()

class SubController(HookController, SubMixin):

    __hooks__ = [common]

    def index(self):
        print "\nI AM THE SUB!"
        return dict()

class RootController(HookController):

    __hooks__ = [common, ParentHook()]

    def index(self):
        print "\nI AM THE ROOT!"
        return dict()

    sub = SubController()

Let’s see what happens when we run the app. First loading the root controller:

pecan serve
serving on view at

GET / HTTP/1.1" 200

about to enter the parent controller...

just a common hook


Then loading the sub controller:

pecan serve
serving on view at

GET /sub HTTP/1.1" 200

about to enter the parent controller...

just a common hook

about to enter the subcontroller...



Make sure to set proper priority values for nested hooks in order to get them executed in the desired order.


Two hooks of the same type will be added/executed twice, if passed as different instances to a parent and a child controller. If passed as one instance variable - will be invoked once for both controllers.

Hooks That Come with Pecan

Pecan includes some hooks in its core. This section will describe their different uses, how to configure them, and examples of common scenarios.


This hook is useful for debugging purposes. It has access to every attribute the response object has plus a few others that are specific to the framework.

There are two main ways that this hook can provide information about a request:

  1. Terminal or logging output (via an file-like stream like stdout)
  2. Custom header keys in the actual response.

By default, both outputs are enabled.

Configuring RequestViewerHook

There are a few ways to get this hook properly configured and running. However, it is useful to know that no actual configuration is needed to have it up and running.

By default it will output information about these items:

  • path : Displays the url that was used to generate this response
  • status : The response from the server (e.g. ‘200 OK’)
  • method : The method for the request (e.g. ‘GET’, ‘POST’, ‘PUT or ‘DELETE’)
  • controller : The actual controller method in Pecan responsible for the response
  • params : A list of tuples for the params passed in at request time
  • hooks : Any hooks that are used in the app will be listed here.

The default configuration will show those values in the terminal via stdout and it will also add them to the response headers (in the form of X-Pecan-item_name).

This is how the terminal output might look for a /favicon.ico request:

path         - /favicon.ico
status       - 404 Not Found
method       - GET
controller   - The resource could not be found.
params       - []
hooks        - ['RequestViewerHook']

In the above case, the file was not found, and the information was printed to stdout. Additionally, the following headers would be present in the HTTP response:

X-Pecan-path        /favicon.ico
X-Pecan-status      404 Not Found
X-Pecan-method      GET
X-Pecan-controller  The resource could not be found.
X-Pecan-params      []
X-Pecan-hooks       ['RequestViewerHook']

The configuration dictionary is flexible (none of the keys are required) and can hold two keys: items and blacklist.

This is how the hook would look if configured directly (shortened for brevity):

'hooks': lambda: [

Modifying Output Format

The items list specify the information that the hook will return. Sometimes you will need a specific piece of information or a certain bunch of them according to the development need so the defaults will need to be changed and a list of items specified.


When specifying a list of items, this list overrides completely the defaults, so if a single item is listed, only that item will be returned by the hook.

The hook has access to every single attribute the request object has and not only to the default ones that are displayed, so you can fine tune the information displayed.

These is a list containing all the possible attributes the hook has access to (directly from webob):

accept make_tempfile
accept_charset max_forwards
accept_encoding method
accept_language params
application_url path
as_string path_info
authorization path_info_peek
blank path_info_pop
body path_qs
body_file path_url
body_file_raw postvars
body_file_seekable pragma
cache_control query_string
call_application queryvars
charset range
content_length referer
content_type referrer
cookies relative_url
copy remote_addr
copy_body remote_user
copy_get remove_conditional_headers
date request_body_tempfile_limit
decode_param_names scheme
environ script_name
from_file server_name
from_string server_port
get_response str_GET
headers str_POST
host str_cookies
host_url str_params
http_version str_postvars
if_match str_queryvars
if_modified_since unicode_errors
if_none_match upath_info
if_range url
if_unmodified_since urlargs
is_body_readable urlvars
is_body_seekable uscript_name
is_xhr user_agent

And these are the specific ones from Pecan and the hook:

  • controller
  • hooks
  • params (params is actually available from webob but it is parsed by the hook for redability)

Blacklisting Certain Paths

Sometimes it’s annoying to get information about every single request. To limit the output, pass the list of URL paths for which you do not want data as the blacklist.

The matching is done at the start of the URL path, so be careful when using this feature. For example, if you pass a configuration like this one:

{ 'blacklist': ['/f'] }

It would not show any url that starts with f, effectively behaving like a globbing regular expression (but not quite as powerful).

For any number of blocking you may need, just add as many items as wanted:

{ 'blacklist' : ['/favicon.ico', '/javascript', '/images'] }

Again, the blacklist key can be used along with the items key or not (it is not required).