To support application extensibility, the :app:`Pyramid` :term:`Configurator`, by default, detects configuration conflicts and allows you to include configuration imperatively from other packages or modules. It also, by default, performs configuration in two separate phases. This allows you to ignore relative configuration statement ordering in some circumstances.
Here's a familiar example of one of the simplest :app:`Pyramid` applications, configured imperatively:
When you start this application, all will be OK. However, what happens if we try to add another view to the configuration with the same set of :term:`predicate` arguments as one we've already added?
The application now has two conflicting view configuration statements. When we try to start it again, it won't start. Instead, we'll receive a traceback that ends something like this:
This traceback is trying to tell us:
- We've got conflicting information for a set of view configuration
- There are two statements which conflict, shown beneath the
config.add_view(hello_world. 'hello')on line 14 of
config.add_view(goodbye_world, 'hello')on line 17 of
These two configuration statements are in conflict because we've tried to
tell the system that the set of :term:`predicate` values for both view
configurations are exactly the same. Both the
goodbye_world views are configured to respond under the same set of
circumstances. This circumstance: the :term:`view name` (represented by the
name= predicate) is
This presents an ambiguity that :app:`Pyramid` cannot resolve. Rather than allowing the circumstance to go unreported, by default Pyramid raises a :exc:`ConfigurationConflictError` error and prevents the application from running.
Conflict detection happens for any kind of configuration: imperative configuration or configuration that results from the execution of a :term:`scan`.
Manually Resolving Conflicts
There are a number of ways to manually resolve conflicts: by changing registrations to not conflict, by strategically using :meth:`pyramid.config.Configurator.commit`, or by using an "autocommitting" configurator.
The Right Thing
The most correct way to resolve conflicts is to "do the needful": change your configuration code to not have conflicting configuration statements. The details of how this is done depends entirely on the configuration statements made by your application. Use the detail provided in the :exc:`ConfigurationConflictError` to track down the offending conflicts and modify your configuration code accordingly.
If you're getting a conflict while trying to extend an existing application, and that application has a function which performs configuration like this one:
Don't call this function directly with
config as an argument. Instead,
Using :meth:`~pyramid.config.Configuration.include` instead of calling the function directly provides a modicum of automated conflict resolution, with the configuration statements you define in the calling code overriding those of the included function. See also :ref:`automatic_conflict_resolution` and :ref:`including_configuration`.
You can manually commit a configuration by using the
:meth:`~pyramid.config.Configurator.commit` method between configuration
calls. For example, we prevent conflicts from occurring in the application
we examined previously as the result of adding a
commit. Here's the
application that generates conflicts:
We can prevent the two
add_view calls from conflicting by issuing a call
to :meth:`~pyramid.config.Configurator.commit` between them:
In the above example we've issued a call to
:meth:`~pyramid.config.Configurator.commit` between the two
calls. :meth:`~pyramid.config.Configurator.commit` will execute any pending
Calling :meth:`~pyramid.config.Configurator.commit` is safe at any time. It executes all pending configuration actions and leaves the configuration action list "clean".
Using An Autocommitting Configurator
You can also use a heavy hammer to circumvent conflict detection by using a
configurator constructor parameter:
autocommit=True. For example:
autocommit parameter passed to the Configurator is
conflict detection (and :ref:`twophase_config`) is disabled. Configuration
statements will be executed immediately, and succeeding statements will
override preceding ones.
:meth:`~pyramid.config.Configurator.commit` has no effect when
If you use a Configurator in code that performs unit testing, it's usually a good idea to use an autocommitting Configurator, because you are usually unconcerned about conflict detection or two-phase configuration in test code.
Automatic Conflict Resolution
If your code uses the :meth:`~pyramid.config.Configurator.include` method to include external configuration, some conflicts are automatically resolved. Configuration statements that are made as the result of an "include" will be overridden by configuration statements that happen within the caller of the "include" method.
Automatic conflict resolution supports this goal: if a user wants to reuse a Pyramid application, and they want to customize the configuration of this application without hacking its code "from outside", they can "include" a configuration function from the package and override only some of its configuration statements within the code that does the include. No conflicts will be generated by configuration statements within the code which does the including, even if configuration statements in the included code would conflict if it was moved "up" to the calling code.
Methods Which Provide Conflict Detection
These are the methods of the configurator which provide conflict detection:
:meth:`~pyramid.config.Configurator.add_view`, :meth:`~pyramid.config.Configurator.add_route`, :meth:`~pyramid.config.Configurator.add_renderer`, :meth:`~pyramid.config.Configurator.set_request_factory`, :meth:`~pyramid.config.Configurator.set_session_factory`, :meth:`~pyramid.config.Configurator.set_request_property`, :meth:`~pyramid.config.Configurator.set_root_factory`, :meth:`~pyramid.config.Configurator.set_view_mapper`, :meth:`~pyramid.config.Configurator.set_authentication_policy`, :meth:`~pyramid.config.Configurator.set_authorization_policy`, :meth:`~pyramid.config.Configurator.set_renderer_globals_factory`, :meth:`~pyramid.config.Configurator.set_locale_negotiator`, :meth:`~pyramid.config.Configurator.set_default_permission`, :meth:`~pyramid.config.Configurator.add_traverser`, :meth:`~pyramid.config.Configurator.add_resource_url_adapter`, and :meth:`~pyramid.config.Configurator.add_response_adapter`.
:meth:`~pyramid.config.Configurator.add_static_view` also indirectly
provides conflict detection, because it's implemented in terms of the
Including Configuration from External Sources
Some application programmers will factor their configuration code in such a way that it is easy to reuse and override configuration statements. For example, such a developer might factor out a function used to add routes to his application:
Rather than calling this function directly with
config as an argument.
Instead, use :meth:`pyramid.config.Configuration.include`:
include rather than calling the function directly will allow
:ref:`automatic_conflict_resolution` to work.
For this to work properly, the
myapp module must contain a callable with
the special name
includeme, which should perform configuration (like the
add_routes callable we showed above as an example).
When a non-autocommitting :term:`Configurator` is used to do configuration (the default), configuration execution happens in two phases. In the first phase, "eager" configuration actions (actions that must happen before all others, such as registering a renderer) are executed, and discriminators are computed for each of the actions that depend on the result of the eager actions. In the second phase, the discriminators of all actions are compared to do conflict detection.
Due to this, for configuration methods that have no internal ordering constraints, execution order of configuration method calls is not important. For example, the relative ordering of :meth:`~pyramid.config.Configurator.add_view` and :meth:`~pyramid.config.Configurator.add_renderer` is unimportant when a non-autocommitting configurator is used. This code snippet:
Has the same result as:
Even though the view statement depends on the registration of a custom
renderer, due to two-phase configuration, the order in which the
configuration statements are issued is not important.
add_view will be
able to find the
.rn renderer even if
add_renderer is called after
The same is untrue when you use an autocommitting configurator (see :ref:`autocommitting_configurator`). When an autocommitting configurator is used, two-phase configuration is disabled, and configuration statements must be ordered in dependency order.
Some configuration methods, such as :meth:`~pyramid.config.Configurator.add_route` have internal ordering constraints: the routes they imply require relative ordering. Such ordering constraints are not absolved by two-phase configuration. Routes are still added in configuration execution order.