If the developer of a Pyramid
application has obeyed certain constraints while building that application, a third party should be able to change its behavior without needing to modify its source code. The behavior of a Pyramid
application that obeys certain constraints can be overridden or extended without modification.
single: extensible application
There's only one rule you need to obey if you want to build a maximally extensible Pyramid
application: you should not use any configuration decoration
or imperative
configuration
. This means the application developer should avoid relying on configuration decoration
meant to be detected via a scan
, and you mustn't configure your Pyramid
application imperatively by using any code which configures the application through methods of the Configurator
(except for the pyramid.configuration.Configurator.load_zcml
method).
Instead, you must always use ZCML
for the equivalent purposes. ZCML
declarations that belong to an application can be "overridden" by integrators as necessary, but decorators and imperative code which perform the same tasks cannot. Use only ZCML
to configure your application if you'd like it to be extensible. See declarative_chapter
for information about using ZCML.
The fundamental "plug points" of an application developed using Pyramid
are routes, views, and resources. Routes are declarations made using the ZCML <route>
directive. Views are declarations made using the ZCML <view>
directive (or the @view_config
decorator). Resources are files that are accessed by Pyramid
using the pkg_resources
API such as static files and templates.
single: ZCML granularity
It's extremely helpful to third party application "extenders" (aka "integrators") if the ZCML
that composes the configuration for an application is broken up into separate files which do very specific things. These more specific ZCML files can be reintegrated within the application's main configure.zcml
via <include file="otherfile.zcml"/>
declarations. When ZCML files contain sets of specific declarations, an integrator can avoid including any ZCML he does not want by including only ZCML files which contain the declarations he needs. He is not forced to "accept everything" or "use nothing".
For example, it's often useful to put all <route>
declarations in a separate ZCML file, as <route>
statements have a relative ordering that is extremely important to the application: if an extender wants to add a route to the "middle" of the routing table, he will always need to disuse all the routes and cut and paste the routing configuration into his own application. It's useful for the extender to be able to disuse just a single ZCML file in this case, accepting the remainder of the configuration from other ZCML
files in the original application.
Granularizing ZCML is not strictly required. An extender can always disuse all your ZCML, choosing instead to copy and paste it into his own package, if necessary. However, doing so is considerate, and allows for the best reusability.
single: extending an existing application
The steps for extending an existing application depend largely on whether the application does or does not use configuration decorators and/or imperative code.
Extending an Application Which Possesses Configuration Decorators Or Which Does Configuration Imperatively
If you've inherited a Pyramid
application which uses pyramid.view.view_config
decorators or which performs configuration imperatively, one of two things may be true:
- If you just want to extend the application, you can write additional ZCML that registers more views or routes, loading any existing ZCML and continuing to use any existing imperative configuration done by the original application.
If you want to override configuration in the application, you may need to change the source code of the original application.
If the only source of trouble is the existence of
pyramid.view.view_config
decorators, you can just prevent ascan
from happening (by omitting the<scan>
declaration from ZCML or omitting any call to thepyramid.configuration.Configurator.scan
method). This will cause the decorators to do nothing. At this point, you will need to convert all the configuration done in decorators into equivalentZCML
and add that ZCML to a separate Python package as described inextending_the_application
.If the source of trouble is configuration done imperatively in a function called during application startup, you'll need to change the code: convert imperative configuration statements into equivalent
ZCML
declarations.
Once this is done, you should be able to extend or override the application like any other (see extending_the_application
).
Extending an Application Which Does Not Possess Configuration Decorators or Imperative Configuration
To extend or override the behavior of an existing application, you will need to write some ZCML
, and perhaps some implementations of the types of things you'd like to override (such as views), which are referred to within that ZCML.
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
Pyramid
application using the "paster" template mechanism. Seecreating_a_project
for more information. - Install the new package into the same Python environment as the original application (e.g.
python setup.py develop
orpython setup.py install
). - Change the
configure.zcml
in the new package to include the originalPyramid
application'sconfigure.zcml
via an include statement, e.g.<include package="theoriginalapp"/>
. Alternately, if the original application writer anticipated overriding some things and not others, instead of including the "main"configure.zcml
of the original application, include only specific ZCML files from the original application using thefile
attribute of the<include>
statement, e.g.<include package="theoriginalapp" file="views.zcml"/>
. - On a line in the new package's
configure.zcml
file that falls after (XML-ordering-wise) all theinclude
statements of the original package ZCML, put anincludeOverrides
statement which identifies another ZCML file within the new package (for example<includeOverrides file="overrides.zcml"/>
. - Create an
overrides.zcml
file within the new package. The statements in theoverrides.zcml
file will override any ZCML statements made within the original application (such as view declarations). - Create Python files containing views and other overridden elements, such as templates and static resources as necessary, and wire these up using ZCML registrations within the
overrides.zcml
file. These registrations may extend or override the original view registrations. Seeoverriding_views
,overriding_routes
andoverriding_resources
. - In the
__init__.py
of the new package, load theconfigure.zcml
file of the new package using thepyramid.configuration.Configurator.load_zcml
method.
pair: overriding; views
The ZCML <view>
declarations you make which override application behavior will usually have the same context
and name
(and predicate
attributes, if used) as the original. These <view>
declarations will point at "new" view code. The new view code itself will usually be cut-n-paste copies of view callables from the original application with slight tweaks. For example:
<view context="theoriginalapplication.models.SomeModel"
name="theview"
view=".views.a_view_that_does_something_slightly_different"
/>
A similar pattern can be used to extend the application with <view>
declarations. Just register a new view against some existing model type and make sure the URLs it implies are available on some other page rendering.
pair: overriding; routes
Route setup is currently typically performed in a sequence of ordered ZCML <route>
declarations. Because these declarations are ordered relative to each other, and because this ordering is typically important, you should retain the relative ordering of these declarations when performing an override. Typically, this means copying all the <route>
declarations into an external ZCML file and changing them as necessary. Then disinclude any ZCML from the original application which contains the original declarations.
pair: overriding; resources
"Resource" files are static files on the filesystem that are accessible within a Python package. An entire chapter is devoted to resources: resources_chapter
. Within this chapter is a section named overriding_resources_section
. This section of that chapter describes in detail how to override package resources with other resources by using ZCML
<resource>
declarations. Add such <resource>
declarations to your override package's configure.zcml
to perform overrides.
single: ZCML inclusion
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 <include package=""/>
ZCML declaration in the overriding package's configure.zcml
.