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.. _hooks_chapter:
Using ZCML Hooks
================
ZCML "hooks" can be used to influence the behavior of the
:mod:`repoze.bfg` framework in various ways. This is an advanced
topic; not many people will want or need to do any of these things.
Changing the request factory
----------------------------
You may change the class used as the "request factory" from within the
:mod:`repoze.bfg` ``Router`` class (the ``Router`` class turns the
WSGI environment into a "request" object which is used ubiquitously
throughout :mod:`repoze.bfg`). The default "request factory" is the
class ``webob.Request``. You may change it by placing the following
ZCML in your ``configure.zcml`` file.
.. code-block:: xml
:linenos:
<utility provides="repoze.bfg.interfaces.IRequestFactory"
component="helloworld.factories.request_factory"/>
Replace ``helloworld.factories.request_factory`` with the Python
dotted name to the request factory you want to use. Here's some
sample code that implements a minimal request factory:
.. code-block:: python
from webob import Request
from repoze.bfg.interfaces import IRequest
class MyRequest(Request):
implements(IRequest)
def request_factory():
return MyRequest
.. warning:: If you register an ``IRequestFactory`` utility in this
way, you *must* be sure that the factory returns an object that
implements *at least* the ``repoze.bfg.interfaces.IRequest``
interface. Otherwise all application view lookups will fail (they
will all return a 404 response code). Likewise, if you want to be
able to use method-related interfaces such as ``IGETRequest``,
``IPOSTRequest``, etc. in your view declarations, the callable
returned by the factory must also do the same introspection of the
environ that the default request factory does and decorate the
returned object to implement one of these interfaces based on the
``HTTP_METHOD`` present in the environ. Note that the above
example does not do this, so lookups for method-related interfaces
will fail.
Changing the response factory
-----------------------------
You may change the class used as the "response factory" from within
the :mod:`repoze.bfg` ``chameleon_zpt``, ``chameleon_genshi``,
``chameleon_text`` (the ``render_template_to_response`` function used
within each) and other various places where a Response object is
constructed by :mod:`repoze.bfg`. The default "response factory" is
the class ``webob.Response``. You may change it by placing the
following ZCML in your ``configure.zcml`` file.
.. code-block:: xml
:linenos:
<utility provides="repoze.bfg.interfaces.IResponseFactory"
component="helloworld.factories.response_factory"/>
Replace ``helloworld.factories.response_factory`` with the Python
dotted name to the response factory you want to use. Here's some
sample code that implements a minimal response factory:
.. code-block:: python
from webob import Response
class MyResponse(Response):
pass
def response_factory():
return MyResponse
Unlike a request factory, a response factory does not need to return
an object that implements any particular interface; it simply needs
have a ``status`` attribute, a ``headerlist`` attribute, and and
``app_iter`` attribute.
Changing the NotFound Application
---------------------------------
When :mod:`repoze.bfg` can't map a URL to code, it creates and invokes
a NotFound WSGI application. The application it invokes can be
customized by placing something like the following ZCML in your
``configure.zcml`` file.
.. code-block:: xml
:linenos:
<utility provides="repoze.bfg.interfaces.INotFoundAppFactory"
component="helloworld.factories.notfound_app_factory"/>
Replace ``helloworld.factories.notfound_app_factory`` with the Python
dotted name to the request factory you want to use. Here's some
sample code that implements a minimal NotFound application factory:
.. code-block:: python
from webob.exc import HTTPNotFound
class MyNotFound(HTTPNotFound):
pass
def notfound_app_factory():
return MyNotFound
.. note:: When a NotFound application factory is invoked, it is passed
the WSGI environ and the WSGI ``start_response`` handler by
:mod:`repoze.bfg`. Within the WSGI environ will be a key named
``repoze.bfg.message`` that has a value explaining why the not
found error was raised. This error will be different when the
``debug_notfound`` environment setting is true than it is when it
is false.
Changing the Forbidden Response
-------------------------------
When :mod:`repoze.bfg` can't authorize execution of a view based on
the authorization policy in use, it invokes a "forbidden response
factory". Usually this forbidden response factory is a default 401
response, but it can be overridden as necessary by placing something
like the following ZCML in your ``configure.zcml`` file.
.. code-block:: xml
:linenos:
<utility provides="repoze.bfg.interfaces.IForbiddenResponseFactory"
component="helloworld.factories.forbidden_response_factory"/>
Replace ``helloworld.factories.forbidden_app_factory`` with the Python
dotted name to the forbidden response factory you want to use. The
response factory must accept two parameters: ``context`` and
``request``. The ``context`` is the context found by the router when
the view invocation was denied. The ``request`` is the current
:term:`request` representing the denied action. Here's some sample
code that implements a minimal forbidden response factory:
.. code-block:: python
from repoze.bfg.chameleon_zpt import render_template_to_response
def forbidden_response_factory(context, request):
return render_template_to_response('templates/login_form.pt')
.. note:: When an forbidden response factory is invoked, it is passed
the request as the second argument. An attribute of the request is
``environ``, which is the WSGI environment. Within the WSGI
environ will be a key named ``repoze.bfg.message`` that has a value
explaining why the current view invocation was forbidden. This
error will be different when the ``debug_authorization``
environment setting is true than it is when it is false.
.. warning:: the default forbidden application factory sends a
response with a ``401 Unauthorized`` status code for backwards
compatibility reasons. You can influence the status code of
Forbidden responses by using an alterate forbidden application
factory. For example, it would make sense to return an forbidden
application with a ``403 Forbidden`` status code.
Changing the Default Routes Context Factory
-------------------------------------------
The default Routes "context factory" (the object used to create
context objects when you use ``<route..>`` statements in your ZCML) is
``repoze.bfg.urldispatch.DefaultRoutesContext``. You may change the
class used as the Routes "context factory" by placing the following
ZCML in your ``configure.zcml`` file.
.. code-block:: xml
:linenos:
<utility provides="repoze.bfg.interfaces.IRoutesContextFactory"
component="helloworld.factories.routes_context_factory"/>
Replace ``helloworld.factories.routes_context_factory`` with the
Python dotted name to the context factory you want to use. Here's
some sample code that implements a minimal context factory:
.. code-block:: python
class RoutesContextFactory(object):
def __init__(self, **kw):
self.__dict__.update(kw)
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