extensions 0.4

Simple plugin system

extensions is a simple plugin system inspired from setuptools entry points [1]. It allows an application to define and/or use plugins.

• Code base : http://bitbucket.org/tarek/extensions

• Website : http://pypi.python.org/pypi/extensions

• Feedback : [e-mail]

Contents

• How to define a plugin

  • Using a configuration file

• How to use a plugin

  • Iterate over registered plugins

  • The Plugin object

• Distribute your plugins

• Compatibility with setuptools entry points

• References

How to define a plugin

A plugin can be any callable object . It has to be registered through the extensions registry.

For example, let’s take a simple function that calculates the average of some numbers, and let’s save it into a file called extensions.py in a package called myapp:

def average(*args):
     return sum(args) / len(args)

This function can be registered in the plugin system using the register function. Plugins have a name and belong to a group. For our example, the group can be myapp.operator and the name average:

from extensions import register

# usage : register(group, name, location)
register('myapp.operator', 'average', 'myapp.extensions:average')

The third parameter gives the location of the callable, with the form module_name:attrs, where module_name is the full name of the module, and attrs the attributes in the module.

Notice that the group name includes the name of the package, which is a good practice to avoid collisions since the group names are global to all applications that use extensions.

Using a configuration file

There’s another way to register your plugins by using a configuration file. You can write a ini-like file, using groups for sections title, and name = location for the values.

For example, if you create a operators.cfg file with this content:

[myapp.operator]

average = myapp.extensions:average

You will be able to load it using register_file:

from extensions import register_file

register_file('operators.cfg')

This function will scan your file and register the plugins for you.

How to use a plugin

Iterate over registered plugins

extensions provides a get function that allows you to iterate over all registered plugins for a given group:

from extensions import get

for plugin in get(group='myapp.operator'):
    print plugin.name

You can also give the name to the function:

for plugin in get(group='myapp.operator', name='average'):
     print plugin.name

Or even iterate over all plugins:

from itertools import islice

for plugin in islice(get(), 5):
    print plugin.name

The Plugin object

The objects returned by the get function are Plugin class instances.

The Plugin class provides one method called load, that returns the registered object, so you can use it

# let's get the plugin `average` of the group `myapp.operator`
plugin = get(group='myapp.operator', name='average').next()

# let's load it
func = plugin.load()

# let's use it now
average = func(1, 2, 3)

Plugin also provides a name and a group attribute, that corresponds to the name of the registered plugin, and to its group.

Distribute your plugins

If you want to distribute your plugins, you just have to import the module that registers the plugins into your setup.py file:

from distutils.core import setup
from myapp import plugins  # registers the plugins

setup(name='myapp', version='1.0'
      packages=['myapp'])

This will register the plugins when the package is installed by creating a special file called PLUGINS into the .egg-info directory created when your package is installed.

Compatibility with setuptools entry points

extensions is fully compatible with setuptools entry points. So you can iterate and use entry points defined in third-party applications that are installed in your Python.

If you want to iterate through setuptools entry points, use the consume_entry_points option when you call the get function:

plugins = get(consume_entry_points=True)

This will iterate over extensions plugins and setuptools entry points.

References

[1]

http://peak.telecommunity.com/DevCenter/setuptools#extensible-applications-and-frameworks