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Easily add hooks and plugins to your applications

Project Description

The peak.util.plugins module provides some simple utilities to make it easy to create pluggable applications:

  • Hook objects let you easily register and invoke or access extensions that have been manually registered or automatically discovered via setuptools entry points.
  • Extensible objects can automatically find, load, and activate associated add-ons or modifier hooks. This is most useful in conjunction with AddOn classes (from the AddOns package), but can be used with any callables.
  • The PluginManager service manages plugin eggs, and can be subclassed or replaced to support alternative means of locating hook implementations. (That is, in addition to or in replacement of setuptools.)

These utilities work well together but can also be used independently. For example, you can use Extensible objects without using Hook or PluginManager, and you can use PluginManager without Hook or Extensible. (Using Hook objects, however, does require a PluginManager.)

Using Hooks

A hook is a place in an application where plugins can add functionality, by registering implementations of the hook. For example, a hook can be used to notify plugins of some application event, or to request plugins’ participation in some process. Hooks can also just be used to register and find objects that provide some application-specific interface.

Hooks are created using a “group name”, and an optional “implementation name”. (These names correspond to setuptools “entry point group” names and “entry point” names, respectively.)

The group name should consist of one or more Python identifiers, separated by dots. It does not need to be a valid package or module name, but it should be a globally unique name. That is, it should include an application or library name, so that it can’t clash with names in use by other apps, libraries, or plugins.

Let’s create an example hook:

>>> from peak.util import plugins

>>> hook1 = plugins.Hook('plugins.demo.hook1')

>>> hook1
Hook('plugins.demo.hook1', None)

>>> list(hook1)

Iterating over a hook yields any implementations that have been registered under the hook’s group name, but our example hook doesn’t have anything registered for it yet. So let’s register one, using the register method:

>>> hook1.register('Hello world')
>>> list(hook1)
['Hello world']

Note that we can pass absolutely any object to register(); hooks do not know or care what sort of objects they operate on. There is also no duplicate detection: if you register an object more than once, it will be listed more than once when you iterate over the hook.

Also note that hook registration is global and permanent. You cannot remove a registration once it is added. Also, registration works strictly by group name, rather than by hook instance. If two or more Hook objects share the same group name, they will also share the implementations registered for that name:

>>> list(plugins.Hook('plugins.demo.hook1'))
['Hello world']

The way this works is that Hook objects actually delegate registration and retrieval operations to the PluginManager service. This allows modules to register extensions for other modules to use, without needing the other modules to be imported at registration time.

(See also the section below on The PluginManager Service for more information.)

Querying and Notifying

In addition to simply providing iteration over registered implementations, hooks have two convenience methods for querying or notifying plugins. Specifically, you can use Hook.query() and Hook.notify() to invoke hooks whose implementations are functions or other callable objects. For example:

>>> def echo(*args, **kw):
...     print "called with", args, kw

>>> def compute(*args, **kw):
...     return 42, args, kw

>>> demo = plugins.Hook('plugins.demo.hook2')
>>> demo.register(echo)
>>> demo.register(compute)

>>> demo.notify()
called with () {}

>>> demo.notify(57, x=3)
called with (57,) {'x': 3}

>>> for result in demo.query(): print result
called with () {}
(42, (), {})

>>> for result in demo.query(99): print result
called with (99,) {}
(42, (99,), {})

As you can see, the .notify() method does not return a value, but simply calls each implementation of the hook with the given arguments. .query(), however, yields the result of each call.

Named Implementations

By default, the name of a registered implementation is not significant; most of the time, you just want all registered implementations for a specified group name, regardless of the individual implementations’ names. However, it is sometimes useful to subdivide a hook’s implementations using another level of names.

For example, suppose you are writing a blogging application that processes various input formats, and you’d like plugins to be able to register hook implementations for specific file extensions. You can do this by registering implementations with an implementation name:

>>> def rst_formatter(filename):
...     print "formatting",filename,"using reST"

>>> def txt_formatter(filename):
...     print "formatting",filename,"as plain text"

>>> formatters = plugins.Hook('blogtool.formatters')
>>> formatters.register(rst_formatter, '.rst')
>>> formatters.register(txt_formatter, '.txt')

>>> list(formatters)
[<function rst_formatter...>, <function txt_formatter...>]

And then retriveing them using a Hook that has both a group name and an implementation name:

>>> plugins.Hook('blogtool.formatters', '.rst').notify("foo.rst")
formatting foo.rst using reST

>>> plugins.Hook('blogtool.formatters', '.txt').notify("bar.txt")
formatting bar.txt as plain text

The second argument to the Hook() constructor (and the Hook.register() method) is an implementation name. If supplied, it must be a non-empty ASCII string that does not start with a [, contain any control characters or = signs. (It must also not begin or end with whitespace.)

Note, by the way, that these implementation names are not necessarily unique. For example, multiple plugins could register a .rst formatter, and it is up to the code using the Hook to decide how to handle that! Also notice that the blogtool.formatters hook above lists all registered formatters, no matter what implementation name they are registered under. It’s also possible for the same implementation to be registered under more than one name, in which case it will be listed more than once in the overall hook, e.g.:

>>> formatters.register(rst_formatter, '.txt')
>>> list(formatters)
[<function rst_formatter...>, <function txt_formatter...>,
 <function rst_formatter...>]

>>> list(plugins.Hook('blogtool.formatters', '.txt'))
[<function txt_formatter...>, <function rst_formatter...>]

Finally, note that hooks tied to a specific implementation name can also be used for registration, e.g.:

>>> foo = plugins.Hook('blogtool.formatters', '.foo')
>>> foo
Hook('blogtool.formatters', '.foo')

>>> foo.register(42)
>>> list(foo)

>>> list(formatters)
[..., 42]

But you must either omit the implementation name from the .register() call (as shown above), or else it must match the implementation name the hook was created with:

>>> foo.register(21, '.foo')    # ok - same name

>>> foo.register(99, 'blue!')   # not ok!
Traceback (most recent call last):
ValueError: Can only register .foo implementations

Automatic Discovery Using Entry Points and Eggs

So far, we have only seen manually-registered hook implementations. This is fine for demonstration, but in a real application with plugins, it would be necessary to first find and load the plugins’ code in order to do such registrations.

To address this issue, setuptools provides a feature called “entry points”, which can be used to include registration data in plugins’ eggs. This allows hook implementations to be imported on demand.

To register a hook implementation for auto-discovery, the plugin’s must use setuptools, and define entry points like this:

    entry_points = """
    .rst = some.module:some_object
    .txt = other.module:other_object

    any old name here = whatever:SomeClass
    something without an equals sign = foo:bar

When the plugin is active on sys.path (i.e., it’s importable), iterating over an appropriate Hook object will automatically import the specified object(s) and yield them.

In the example above, some_object will be imported from some.module and yielded whenever iterating over Hook('blogtool.formatters') or Hook('blogtool.formatters', '.rst'). (Note that you must always name the implementations listed in, even if the application does not look up implementations by name!)

Through this automatic, on-demand import process, it is not necessary to find and import the plugins in order to register hook implementations. Instead, merely installing the plugin is sufficient to make its hook implementations available. This also speeds up application startup, because implementations are not imported until they are used. (And performance can often be further improved by putting less frequently-used hook implementations into separate modules from those that are used more often.)

Of course, most applications will want to have one or more special directories for installing and using egg plugins; you can use the PluginManager service to locate and selectively activate the plugins found in such directories.

See also:

Extensible Objects

The Extensible mixin class is a convenient way to activate add-on hooks for an object. To implement an extensible object, you subclass or mix in Extensible, add an extend_with attribute, and call load_extensions() at an appropriate time:

>>> AppExtensions = plugins.Hook('my_app.App.extensions')

>>> class App(plugins.Extensible):
...     extend_with = AppExtensions

>>> def hello(app):
...     print "Hi, I'm extending", app
>>> AppExtensions.register(hello)

>>> a = App()
>>> a.load_extensions()
Hi, I'm extending <App object at...>

The extend_with attribute must be a one-argument callable, a sequence of callables, or a sequence of sequences of callables (recursively). Since Hook objects are iterable, they can be used as long as their implementations are either one-argument callables or nested sequences thereof.

When an Extensible object’s load_extensions() method is called, the extend_with sequence is recursively iterated, and all callables found are invoked with the extensible object as the sole argument. Here’s an example using a mixed and nested sequence of callables:

>>> class App(plugins.Extensible):
...     extend_with = (AppExtensions, (hello, AppExtensions), hello)

>>> a = App()
>>> a.load_extensions()
Hi, I'm extending <App object at...>
Hi, I'm extending <App object at...>
Hi, I'm extending <App object at...>
Hi, I'm extending <App object at...>

The callables can be any 1-argument callable, but you will usually want them to be AddOn classes. Add-ons let you attach additional state and methods to an object, in a private namespace that doesn’t interfere with the object’s existing attributes and methods. (See the AddOns package for more info.)

The PluginManager Service

  • document addEntryPoint(), iterHooks()
  • actually implement some plugin directory services, cached working_set and environment, etc.

Replacing the PluginManager

  • example of subclassing PluginManager and activating it using a with: block

Threading Concerns

By default, a separate PluginManager is created for each thread, and they will share a single working set but use different environments. This is probably NOT what you want in a threaded environment!

This issue will be addressed in future releases, but for now you should avoid using PluginManager configuration methods from multiple threads.

It is, however, safe to use Hook objects from multiple threads, as this is a read-only operation. In principle, accessing a Hook from one thread while configuring the PluginManager in another thread could cause a hook to be skipped or doubled, although this is very unlikely. It would be best to do all of your PluginManager configuration before starting threads that use Hook objects, at least with the current version of Plugins.

Mailing List

Questions, discussion, and bug reports for this software should be directed to the PEAK mailing list; see for details.

Implementation Status

While the PluginManager features are still in development (and remain undocumented), this package is only available via SVN checkout. However, all documented features are tested and usable, and I don’t expect any significant changes to the APIs currently documented here.

Release History

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