.. _extending_chapter: Extending An Existing :mod:`repoze.bfg` Application =================================================== If the developer of a :mod:`repoze.bfg` has obeyed certain constraints while building the application, a third party should be able to change the behavior of that application without touching the actual source code that makes up the application. The behavior of a :mod:`repoze.bfg` application that obeys these constraints can be *extended* or *changed* without modification. Rules for Building An Extensible Application -------------------------------------------- There's only one rule you need to obey if you want to build an application that is extensible using :mod:`repoze.bfg`: you must not use the ``@bfg_view`` decorator (or any other decorator meant to be detected via the ZCML ```` directive) to associate a view with a context. Instead, you must use :term:`ZCML` for this purpose. :term:`ZCML` statements that belong to an application can be "overridden" by integrators as necessary, but decorators cannot. It's also often helpful for third party application "extenders" (aka "integrators") if you break your ZCML into separate files which do very specific things, and include them in the application's main ``configure.zcml`` via ```` statements. This way, an integrator can avoid including any ZCML he does not want by including only the ZCML files which contain the registrations he needs. Extending an Existing Application --------------------------------- If you've inherited a :mod:`repoze.bfg` application that you'd like to extend which uses ``@bfg_view`` decorators, you'll unfortunately need to change the source code of the original application, moving the view declarations out of the decorators and into :term:`ZCML`. Once this is done, you should be able to extend or modify the application like any other. To extend or change the behavior of an existing application, the only hard-and-fast rule is that you need to write some :term:`ZCML`, and perhaps some implementations of the types of things you'd like to override (such as views), which is referred to within that ZCML. The general pattern for extending an application looks something like this: - Create a new Python package. The easiest way to do this is to create a new :mod:`repoze.bfg` application using the "paster" template mechanism. See :ref:`creating_a_project` for more information. - Install the new package into the same Python environment as the original application (e.g. ``python setup.py develop`` or ``python setup.py install``). - Change the ``configure.zcml`` in the new package to include the original :mod:`repoze.bfg` application's ``configure.zcml`` via an include statement, e.g. ````. - On a line in the new package's ``configure.zcml`` file that falls after (XML-ordering-wise) the ``include`` of the original package, put an ``includeOverride`` statement that includes another ZCML file within the new package (for example ````. - Create an ``overrides.zcml`` file within the new package. The statements in the ``overrides.zcml`` file will override any ZCML statements made within the original application (such as views). - Create Python files containing views (and other overridden elements, such as templates) as necessary, and wire these up, either extending or overriding the original view registrations from the base application within the ``overrides.zcml`` file. The ZCML ```` statements you make which *modify* behavior will usually have the same ``for`` and ``name`` (and ``request_type`` if used) as the original, and will be in a form like:: Views which modify the behavior of an application will usually be cutnpaste jobs from the original application with slight tweaks. A similar pattern can be used to extend the application. Just register a new view against some existing model type. - Change the Paste ``.ini`` file that starts up the original application. Add a ``configure_zcml`` statement within the application's section in the file which points at your *new* package's ``configure.zcml`` file. See :ref:`environment_chapter` for more information about this setting. Dealing With ZCML Inclusions ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Sometimes it's possible to include only certain ZCML files from an application that contain only the registrations you really need, omitting others. But sometimes it's not. For brute force purposes, when you're getting ``view`` or ``route`` registrations that you don't actually want in your overridden application, it's always appropriate to just *not include* any ZCML file from the overridden application. Instead, just cut and paste the entire contents of the ``configure.zcml`` (and any ZCML file included by the overridden application's ``configure.zcml``) into your own package and omit the ```` ZCML statement in the overriding package's ``configure.zcml``.