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Extending Pyramid Configuration

Pyramid allows you to extend its Configurator with custom directives. Custom directives can use other directives, they can add a custom action, they can participate in conflict resolution, and they can provide some number of introspectable objects.

Adding Methods to the Configurator via add_directive

Framework extension writers can add arbitrary methods to a Configurator by using the pyramid.config.Configurator.add_directive() method of the configurator. Using add_directive() makes it possible to extend a Pyramid configurator in arbitrary ways, and allows it to perform application-specific tasks more succinctly.

The add_directive() method accepts two positional arguments: a method name and a callable object. The callable object is usually a function that takes the configurator instance as its first argument and accepts other arbitrary positional and keyword arguments. For example:

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from pyramid.events import NewRequest
from pyramid.config import Configurator

def add_newrequest_subscriber(config, subscriber):
    config.add_subscriber(subscriber, NewRequest)

if __name__ == '__main__':
    config = Configurator()
    config.add_directive('add_newrequest_subscriber',
                         add_newrequest_subscriber)

Once add_directive() is called, a user can then call the added directive by its given name as if it were a built-in method of the Configurator:

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def mysubscriber(event):
    print event.request

config.add_newrequest_subscriber(mysubscriber)

A call to add_directive() is often “hidden” within an includeme function within a “frameworky” package meant to be included as per Including Configuration from External Sources via include(). For example, if you put this code in a package named pyramid_subscriberhelpers:

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def includeme(config):
    config.add_directive('add_newrequest_subscriber',
                         add_newrequest_subscriber)

The user of the add-on package pyramid_subscriberhelpers would then be able to install it and subsequently do:

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def mysubscriber(event):
    print event.request

from pyramid.config import Configurator
config = Configurator()
config.include('pyramid_subscriberhelpers')
config.add_newrequest_subscriber(mysubscriber)

Using config.action in a Directive

If a custom directive can’t do its work exclusively in terms of existing configurator methods (such as pyramid.config.Configurator.add_subscriber(), as above), the directive may need to make use of the pyramid.config.Configurator.action() method. This method adds an entry to the list of “actions” that Pyramid will attempt to process when pyramid.config.Configurator.commit() is called. An action is simply a dictionary that includes a discriminator, possibly a callback function, and possibly other metadata used by Pyramid’s action system.

Here’s an example directive which uses the “action” method:

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def add_jammyjam(config, jammyjam):
    def register():
        config.registry.jammyjam = jammyjam
    config.action('jammyjam', register)

if __name__ == '__main__':
    config = Configurator()
    config.add_directive('add_jammyjam', add_jammyjam)

Fancy, but what does it do? The action method accepts a number of arguments. In the above directive named add_jammyjam, we call action() with two arguments: the string jammyjam is passed as the first argument named discriminator, and the closure function named register is passed as the second argument named callable.

When the action() method is called, it appends an action to the list of pending configuration actions. All pending actions with the same discriminator value are potentially in conflict with one another (see Conflict Detection). When the commit() method of the Configurator is called (either explicitly or as the result of calling make_wsgi_app()), conflicting actions are potentially automatically resolved as per Automatic Conflict Resolution. If a conflict cannot be automatically resolved, a ConfigurationConflictError is raised and application startup is prevented.

In our above example, therefore, if a consumer of our add_jammyjam directive did this:

config.add_jammyjam('first')
config.add_jammyjam('second')

When the action list was committed resulting from the set of calls above, our user’s application would not start, because the discriminators of the actions generated by the two calls are in direct conflict. Automatic conflict resolution cannot resolve the conflict (because no config.include is involved), and the user provided no intermediate pyramid.config.Configurator.commit() call between the calls to add_jammyjam to ensure that the successive calls did not conflict with each other.

This demonstrates the purpose of the discriminator argument to the action method: it’s used to indicate a uniqueness constraint for an action. Two actions with the same discriminator will conflict unless the conflict is automatically or manually resolved. A discriminator can be any hashable object, but it is generally a string or a tuple. You use a discriminator to declaratively ensure that the user doesn’t provide ambiguous configuration statements.

But let’s imagine that a consumer of add_jammyjam used it in such a way that no configuration conflicts are generated.

config.add_jammyjam('first')

What happens now? When the add_jammyjam method is called, an action is appended to the pending actions list. When the pending configuration actions are processed during commit(), and no conflicts occur, the callable provided as the second argument to the action() method within add_jammyjam is called with no arguments. The callable in add_jammyjam is the register closure function. It simply sets the value config.registry.jammyjam to whatever the user passed in as the jammyjam argument to the add_jammyjam function. Therefore, the result of the user’s call to our directive will set the jammyjam attribute of the registry to the string first. A callable is used by a directive to defer the result of a user’s call to the directive until conflict detection has had a chance to do its job.

Other arguments exist to the action() method, including args, kw, order, and introspectables.

args and kw exist as values, which, if passed, will be used as arguments to the callable function when it is called back. For example our directive might use them like so:

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def add_jammyjam(config, jammyjam):
    def register(*arg, **kw):
        config.registry.jammyjam_args = arg
        config.registry.jammyjam_kw = kw
        config.registry.jammyjam = jammyjam
    config.action('jammyjam', register, args=('one',), kw={'two':'two'})

In the above example, when this directive is used to generate an action, and that action is committed, config.registry.jammyjam_args will be set to ('one',) and config.registry.jammyjam_kw will be set to {'two':'two'}. args and kw are honestly not very useful when your callable is a closure function, because you already usually have access to every local in the directive without needing them to be passed back. They can be useful, however, if you don’t use a closure as a callable.

order is a crude order control mechanism. order defaults to the integer 0; it can be set to any other integer. All actions that share an order will be called before other actions that share a higher order. This makes it possible to write a directive with callable logic that relies on the execution of the callable of another directive being done first. For example, Pyramid’s pyramid.config.Configurator.add_view() directive registers an action with a higher order than the pyramid.config.Configurator.add_route() method. Due to this, the add_view method’s callable can assume that, if a route_name was passed to it, that a route by this name was already registered by add_route, and if such a route has not already been registered, it’s a configuration error (a view that names a nonexistent route via its route_name parameter will never be called).

introspectables is a sequence of introspectable objects. You can pass a sequence of introspectables to the action() method, which allows you to augment Pyramid’s configuration introspection system.

Adding Configuration Introspection

Note

The introspection subsystem is new in Pyramid 1.3.

Pyramid provides a configuration introspection system that can be used by debugging tools to provide visibility into the configuration of a running application.

All built-in Pyramid directives (such as pyramid.config.Configurator.add_view() and pyramid.config.Configurator.add_route()) register a set of introspectables when called. For example, when you register a view via add_view, the directive registers at least one introspectable: an introspectable about the view registration itself, providing human-consumable values for the arguments it was passed. You can later use the introspection query system to determine whether a particular view uses a renderer, or whether a particular view is limited to a particular request method, or which routes a particular view is registered against. The Pyramid “debug toolbar” makes use of the introspection system in various ways to display information to Pyramid developers.

Introspection values are set when a sequence of introspectable objects is passed to the action() method. Here’s an example of a directive which uses introspectables:

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def add_jammyjam(config, value):
    def register():
        config.registry.jammyjam = value
    intr = config.introspectable(category_name='jammyjams',
                                 discriminator='jammyjam',
                                 title='a jammyjam',
                                 type_name=None)
    intr['value'] = value
    config.action('jammyjam', register, introspectables=(intr,))

if __name__ == '__main__':
    config = Configurator()
    config.add_directive('add_jammyjam', add_jammyjam)

If you notice, the above directive uses the introspectable attribute of a Configurator (pyramid.config.Configurator.introspectable) to create an introspectable object. The introspectable object’s constructor requires at least four arguments: the category_name, the discriminator, the title, and the type_name.

The category_name is a string representing the logical category for this introspectable. Usually the category_name is a pluralization of the type of object being added via the action.

The discriminator is a value unique within the category (unlike the action discriminator, which must be unique within the entire set of actions). It is typically a string or tuple representing the values unique to this introspectable within the category. It is used to generate links and as part of a relationship-forming target for other introspectables.

The title is a human-consumable string that can be used by introspection system frontends to show a friendly summary of this introspectable.

The type_name is a value that can be used to subtype this introspectable within its category for sorting and presentation purposes. It can be any value.

An introspectable is also dictionary-like. It can contain any set of key/value pairs, typically related to the arguments passed to its related directive. While the category_name, discriminator, title and type_name are metadata about the introspectable, the values provided as key/value pairs are the actual data provided by the introspectable. In the above example, we set the value key to the value of the value argument passed to the directive.

Our directive above mutates the introspectable, and passes it in to the action method as the first element of a tuple as the value of the introspectable keyword argument. This associates this introspectable with the action. Introspection tools will then display this introspectable in their index.

Introspectable Relationships

Two introspectables may have relationships between each other.

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def add_jammyjam(config, value, template):
    def register():
        config.registry.jammyjam = (value, template)
    intr = config.introspectable(category_name='jammyjams',
                                 discriminator='jammyjam',
                                 title='a jammyjam',
                                 type_name=None)
    intr['value'] = value
    tmpl_intr = config.introspectable(category_name='jammyjam templates',
                                      discriminator=template,
                                      title=template,
                                      type_name=None)
    tmpl_intr['value'] = template
    intr.relate('jammyjam templates', template)
    config.action('jammyjam', register, introspectables=(intr, tmpl_intr))

if __name__ == '__main__':
    config = Configurator()
    config.add_directive('add_jammyjam', add_jammyjam)

In the above example, the add_jammyjam directive registers two introspectables. The first is related to the value passed to the directive; the second is related to the template passed to the directive. If you believe a concept within a directive is important enough to have its own introspectable, you can cause the same directive to register more than one introspectable, registering one introspectable for the “main idea” and another for a related concept.

The call to intr.relate above (pyramid.interfaces.IIntrospectable.relate()) is passed two arguments: a category name and a directive. The example above effectively indicates that the directive wishes to form a relationship between the intr introspectable and the tmpl_intr introspectable; the arguments passed to relate are the category name and discriminator of the tmpl_intr introspectable.

Relationships need not be made between two introspectables created by the same directive. Instead, a relationship can be formed between an introspectable created in one directive and another introspectable created in another by calling relate on either side with the other directive’s category name and discriminator. An error will be raised at configuration commit time if you attempt to relate an introspectable with another nonexistent introspectable, however.

Introspectable relationships will show up in frontend system renderings of introspection values. For example, if a view registration names a route name, the introspectable related to the view callable will show a reference to the route to which it relates to and vice versa.