.. _extending_chapter: Extending an Existing :app:`Pyramid` Application ================================================ If a :app:`Pyramid` developer has obeyed certain constraints while building an application, a third party should be able to change the application's behavior without needing to modify its source code. The behavior of a :app:`Pyramid` application that obeys certain constraints can be *overridden* or *extended* without modification. We'll define some jargon here for the benefit of identifying the parties involved in such an effort. Developer The original application developer. Integrator Another developer who wishes to reuse the application written by the original application developer in an unanticipated context. They may also wish to modify the original application without changing the original application's source code. The Difference Between "Extensible" and "Pluggable" Applications ---------------------------------------------------------------- Other web frameworks, such as :term:`Django`, advertise that they allow developers to create "pluggable applications". They claim that if you create an application in a certain way, it will be integratable in a sensible, structured way into another arbitrarily-written application or project created by a third-party developer. :app:`Pyramid`, as a platform, does not claim to provide such a feature. The platform provides no guarantee that you can create an application and package it up such that an arbitrary integrator can use it as a subcomponent in a larger Pyramid application or project. Pyramid does not mandate the constraints necessary for such a pattern to work satisfactorily. Because Pyramid is not very "opinionated", developers are able to use wildly different patterns and technologies to build an application. A given Pyramid application may happen to be reusable by a particular third party integrator because the integrator and the original developer may share similar base technology choices (such as the use of a particular relational database or ORM). But the same application may not be reusable by a different developer, because they have made different technology choices which are incompatible with the original developer's. As a result, the concept of a "pluggable application" is left to layers built above Pyramid, such as a "CMS" layer or "application server" layer. Such layers are apt to provide the necessary "opinions" (such as mandating a storage layer, a templating system, and a structured, well-documented pattern of registering that certain URLs map to certain bits of code) which makes the concept of a "pluggable application" possible. "Pluggable applications", thus, should not plug into Pyramid itself but should instead plug into a system written atop Pyramid. Although it does not provide for "pluggable applications", Pyramid *does* provide a rich set of mechanisms which allows for the extension of a single existing application. Such features can be used by frameworks built using Pyramid as a base. All Pyramid applications may not be *pluggable*, but all Pyramid applications are *extensible*. .. index:: single: extensible application .. _building_an_extensible_app: Rules for Building an Extensible Application -------------------------------------------- There is only one rule you need to obey if you want to build a maximally extensible :app:`Pyramid` application: as a developer, you should factor any overridable :term:`imperative configuration` you've created into functions which can be used via :meth:`pyramid.config.Configurator.include`, rather than inlined as calls to methods of a :term:`Configurator` within the ``main`` function in your application's ``__init__.py``. For example, rather than: .. code-block:: python :linenos: from pyramid.config import Configurator if __name__ == '__main__': config = Configurator() config.add_view('myapp.views.view1', name='view1') config.add_view('myapp.views.view2', name='view2') You should move the calls to ``add_view`` outside of the (non-reusable) ``if __name__ == '__main__'`` block, and into a reusable function: .. code-block:: python :linenos: from pyramid.config import Configurator if __name__ == '__main__': config = Configurator() config.include(add_views) def add_views(config): config.add_view('myapp.views.view1', name='view1') config.add_view('myapp.views.view2', name='view2') Doing this allows an integrator to maximally reuse the configuration statements that relate to your application by allowing them to selectively include or exclude the configuration functions you've created from an "override package". Alternatively you can use :term:`ZCML` for the purpose of making configuration extensible and overridable. :term:`ZCML` declarations that belong to an application can be overridden and extended by integrators as necessary in a similar fashion. If you use only :term:`ZCML` to configure your application, it will automatically be maximally extensible without any manual effort. See :term:`pyramid_zcml` for information about using ZCML. Fundamental Plugpoints ~~~~~~~~~~~~~~~~~~~~~~ The fundamental "plug points" of an application developed using :app:`Pyramid` are *routes*, *views*, and *assets*. Routes are declarations made using the :meth:`pyramid.config.Configurator.add_route` method. Views are declarations made using the :meth:`pyramid.config.Configurator.add_view` method. Assets are files that are accessed by :app:`Pyramid` using the :term:`pkg_resources` API such as static files and templates via a :term:`asset specification`. Other directives and configurator methods also deal in routes, views, and assets. For example, the ``add_handler`` directive of the ``pyramid_handlers`` package adds a single route and some number of views. .. index:: single: extending an existing application Extending an Existing Application --------------------------------- The steps for extending an existing application depend largely on whether the application does or does not use configuration decorators or imperative code. If the Application Has Configuration Decorations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You've inherited a :app:`Pyramid` application which you'd like to extend or override that uses :class:`pyramid.view.view_config` decorators or other :term:`configuration decoration` decorators. If you just want to *extend* the application, you can run a :term:`scan` against the application's package, then add additional configuration that registers more views or routes. .. code-block:: python :linenos: if __name__ == '__main__': config.scan('someotherpackage') config.add_view('mypackage.views.myview', name='myview') If you want to *override* configuration in the application, you *may* need to run :meth:`pyramid.config.Configurator.commit` after performing the scan of the original package, then add additional configuration that registers more views or routes which perform overrides. .. code-block:: python :linenos: if __name__ == '__main__': config.scan('someotherpackage') config.commit() config.add_view('mypackage.views.myview', name='myview') Once this is done, you should be able to extend or override the application like any other (see :ref:`extending_the_application`). You can alternatively just prevent a :term:`scan` from happening by omitting any call to the :meth:`pyramid.config.Configurator.scan` method. This will cause the decorators attached to objects in the target application to do nothing. At this point, you will need to convert all the configuration done in decorators into equivalent imperative configuration or ZCML, and add that configuration or ZCML to a separate Python package as described in :ref:`extending_the_application`. .. _extending_the_application: Extending the Application ~~~~~~~~~~~~~~~~~~~~~~~~~ To extend or override the behavior of an existing application, you will need to create a new package which includes the configuration of the old package, and you'll perhaps need to create implementations of the types of things you'd like to override (such as views), to which they are referred within the original package. The general pattern for extending an existing application looks something like this: - Create a new Python package. The easiest way to do this is to create a new :app:`Pyramid` application using our :term:`cookiecutter`. See :ref:`creating_a_project` for more information. - In the new package, create Python files containing views and other overridden elements, such as templates and static assets as necessary. - Install the new package into the same Python environment as the original application (e.g., ``$VENV/bin/pip install -e .`` or ``$VENV/bin/pip install .``). - Change the ``main`` function in the new package's ``__init__.py`` to include the original :app:`Pyramid` application's configuration functions via :meth:`pyramid.config.Configurator.include` statements or a :term:`scan`. - Wire the new views and assets created in the new package up using imperative registrations within the ``main`` function of the ``__init__.py`` file of the new application. This wiring should happen *after* including the configuration functions of the old application. These registrations will extend or override any registrations performed by the original application. See :ref:`overriding_views`, :ref:`overriding_routes`, and :ref:`overriding_resources`. .. index:: pair: overriding; views .. _overriding_views: Overriding Views ~~~~~~~~~~~~~~~~ The :term:`view configuration` declarations that you make which *override* application behavior will usually have the same :term:`view predicate` attributes as the original that you wish to override. These ```` declarations will point at "new" view code in the override package that you've created. The new view code itself will usually be copy-and-paste copies of view callables from the original application with slight tweaks. For example, if the original application has the following ``configure_views`` configuration method: .. code-block:: python :linenos: def configure_views(config): config.add_view('theoriginalapp.views.theview', name='theview') You can override the first view configuration statement made by ``configure_views`` within the override package, after loading the original configuration function: .. code-block:: python :linenos: from pyramid.config import Configurator from originalapp import configure_views if __name == '__main__': config = Configurator() config.include(configure_views) config.add_view('theoverrideapp.views.theview', name='theview') In this case, the ``theoriginalapp.views.theview`` view will never be executed. Instead, a new view, ``theoverrideapp.views.theview`` will be executed when request circumstances dictate. A similar pattern can be used to *extend* the application with ``add_view`` declarations. Just register a new view against some other set of predicates to make sure the URLs it implies are available on some other page rendering. .. index:: pair: overriding; routes .. _overriding_routes: Overriding Routes ~~~~~~~~~~~~~~~~~ Route setup is currently typically performed in a sequence of ordered calls to :meth:`~pyramid.config.Configurator.add_route`. Because these calls are ordered relative to each other, and because this ordering is typically important, you should retain their relative ordering when performing an override. Typically this means *copying* all the ``add_route`` statements into the override package's file and changing them as necessary. Then exclude any ``add_route`` statements from the original application. .. index:: pair: overriding; assets .. _overriding_resources: Overriding Assets ~~~~~~~~~~~~~~~~~ Assets are files on the filesystem that are accessible within a Python *package*. An entire chapter is devoted to assets: :ref:`assets_chapter`. Within this chapter is a section named :ref:`overriding_assets_section`. This section of that chapter describes in detail how to override package assets with other assets by using the :meth:`pyramid.config.Configurator.override_asset` method. Add such ``override_asset`` calls to your override package's ``__init__.py`` to perform overrides.