STRAIGHTPLUGIN(1)

NAME
straightplugin - straight.plugin Documentation

GETTING STARTED

  Install

            pip install straight.plugin

That was super easy.

  Decide on a Namespace

You’ll want to decide on a namespace within your package where you’ll keep your own plugins and where other developers can add more plugins for your package to use.

For example, if you’re writing a log filtering library named logfilter you may choose logfilter.plugins as a package to hold your plugins, so you’ll create the empty package as you would any other python package. However, the only contents of logfilter/plugins/__init__.py will be a little bit of special code telling python this is a namespace package <#term-namespace-package>.

            # This file will not be needed in Python 3.3
            from pkgutil import extend_path
            __path__ = extend_path(__path__, __name__)

Now, any modules you place in this package are plugin modules able to be loaded by straight.plugin.

            from straight.plugin import load

            plugins = load("logfilter.plugins")

If you’ll be using more plugins than writing them, you should read more <> about the loaders available and how they work.

  Write a Plugin

Writing a plugin is even easier than loading them. There are two important plugin types to learn: Module plugins and class Plugins. Every module in your namespace package <#term-namespace-package> is a module plugin. Every class they define is a class plugin.

When you load module plugins, you get all of them.

When you load class plugins, you filter them by a common base and only get those class plugins which inherit it.

Module plugins are simple and usually define a few functions with names expected by whoever is loading and using the plugins.

            # This is a module plugin

            def add_extra(data):
                if 'x' in data and 'y' in data:
                    data['z'] = x * y

            # This was a fairly useless plugin

Class plugins are only a little longer, but can be a bit more controlled to work with. They depend on a common class the plugins inherit, and this would be defined by the project loading and using the plugins.

            # This is a class plugin

            class RstContentParser(ContentPlugin):
                """Parses any .rst files in a bundle."""

                extensions = ('.rst',)

                def parse(self, content_file):
                    src = content_file.read()
                    return self.parse_string(src)

                def parse_string(self, src):
                    parts = publish_parts(source=src, writer_name='html')
                    return parts['html_body']

You can fit as many class plugins inside a module plugin as you want, and to load them instead of the modules you simply pass a subclasses parameter to load().

            from straight.plugin import load

            plugins = load("jules.plugins", subclasses=ContentPlugin)

The resulting set of plugins are all the classes found which inherit from ContentPlugin. You can do whatever you want with these, but there are some helpful tools to make it easier to work with Class plugins.

You can easily create instances of all the classes, which gives you a set of Instance plugins.

            instances = plugins.produce()

You can even pass initialization parameters to produce() and they’ll be used when creating instances of all the classes. You can see the API docs <#api-plugin-manager> for the PluginManager to see the other ways you can work with groups of plugins.

WRITING PLUGINS

Plugins can exist inside your existing packages or in special namespace packages, which exist only to house plugins.

The only requirement is that any package containing plugins be designated a “namespace package”, which is currently performed in Python via the pkgutil.extend_path utility, seen below. This allows the namespace to be provided in multiple places on the python sys.path, where import looks, and all the contents will be combined.

Use a namespace package <#term-namespace-package>

This allows multiple packages installed on your system to share this name, so they may come from different installed projects and all combine to provide a larger set of plugins.

  Example

            # logfilter/__init__.py

            from pkgutil import extend_path
            __path__ = extend_path(__path__, __name__)

            # logfilter/hide_extra.py

            from logfilter import Skip

            def filter(log_entry):
                level = log_entry.split(':', 1)[0]
                if level != 'EXTRA':
                    return log_entry
                else:
                    raise Skip()

  Using the plugin

In our log tool, we might load all the modules in the logfilter namespace, and then use them all to process each entry in our logs. We don’t need to know all the filters ahead of time, and other packages can be installed on a user’s system providing additional modules in the namespace, which we never even knew about.

            from straight.plugin import load

            class Skip(Exception):
                pass

            plugins = load('logfilter')

            def filter_entry(log_entry):
                for plugin in plugins:
                    try:
                        log_entry = plugin.filter(log_entry)
                    except Skip:
                        pass
                return log_entry

  Distributing Plugins

If you are writing plugins inside your own project to use, they’ll be distributed like any other modules in your package. There is no extra work to do here.

However, if you want to release and distribute plugins on their own, you’ll need to tell your setup.py about your namespace package <#term-namespace-package>.

            setup(
                # ...
                namespace_packages = ['logfilter.plugins']
            )

This will make sure when your plugins are installed alongside the original project, both are importable, even though they came from their own distributions.

You can read more about this at the Distribute documentation on namespace packages <http://packages.python.org/distribute/setuptools.html#namespace-packages>.

PLUGIN LOADERS

Currently, three simple loaders are provided.

    •   The ModuleLoader simply loads the modules found

    •   The ClassLoader loads the subclasses of a given type

    •   The ObjectLoader loads arbitrary objects from the modules

  ClassLoader

The recommended loader is the ClassLoader, used to load all the classes from all of the modules in the namespace given. Optionally, you can pass a subclasses parameter to load(), which will filter the loaded classes to those which are a sub-class of any given type.

For example,

            import os
            from straight.plugin.loaders import ClassLoader
            from myapp import FileHandler

            plugins = ClassLoader().load('myplugins', subclasses=FileHandler)

            for filename in os.listdir('.'):
                for handler_cls in plugins:
                    handler = handler_cls(filename)
                    if handler.valid():
                        handler.process()

However, it is preferred that you use the load() helper provided.

            from straight.plugin import load

            plugins = load('myplugins', subclasses=FileHandler)

This will automatically use the ClassLoader when given a subclasses argument.

  ModuleLoader

Before anything else, straight.plugin loads modules. The ModuleLoader is used to do this.

            from straight.plugin.loaders import ModuleLoader

            plugins = ModuleLoader().load('myplugins')

A note about PEP-420 <http://www.python.org/dev/peps/pep-0420/>:

Python 3.3 will support a new type of package, the Namespace Package. This allows language-level support for the namespaces that make straight.plugin work and when 3.3 lands, you can create addition plugins to be found in a namespace. For now, continue to use the pkgutil boilerplate, but when 3.3 is released, straight.plugin already supports both forms of namespace package!

  ObjectLoader

If you need to combine multiple plugins inside each module, you can load all the objects from the modules, rather than the modules themselves.

            from straight.plugin.loaders import ObjectLoader

            plugins = ObjectLoader().load('myplugins')

STRAIGHT PLUGIN API

  Loaders

    straight.plugin.loaders.unified_load(namespace, subclasses=None, recurse=False)
        Provides a unified interface to both the module and class loaders, finding modules by default or classes if given a subclasses parameter.

    class straight.plugin.loaders.Loader(*args, **kwargs)
        Base loader class. Only used as a base-class for other loaders.

    class straight.plugin.loaders.ModuleLoader(recurse=False)
        Performs the work of locating and loading straight plugins.

        This looks for plugins in every location in the import path.

    class straight.plugin.loaders.ObjectLoader(recurse=False)
        Loads classes or objects out of modules in a namespace, based on a provided criteria.

        The load() method returns all objects exported by the module.

    class straight.plugin.loaders.ClassLoader(recurse=False)
        Loads classes out of plugin modules which are subclasses of a single given base class.

  PluginManager

    class straight.plugin.manager.PluginManager(plugins)

        call(methodname, *args, **kwargs)
            Call a common method on all the plugins, if it exists.

        first(methodname, *args, **kwargs)
            Call a common method on all the plugins, if it exists. Return the first result (the first non-None)

        pipe(methodname, first_arg, *args, **kwargs)
            Call a common method on all the plugins, if it exists. The return value of each call becomes the replaces the first argument in the given argument list to pass to the next.

            Useful to utilize plugins as sets of filters.

        produce(*args, **kwargs)
            Produce a new set of plugins, treating the current set as plugin factories.

GLOSSARY

    distribution
        Separately installable sets of Python modules as stored in the Python package index, and installed by distutils or setuptools.

        definition taken from PEP 382 <http://www.python.org/dev/peps/pep-0382/> text

    module
        An importable python namespace defined in a single file.

    namespace package
        Mechanism for splitting a single Python package across multiple directories on disk. One or more distributions (see distribution) may provide modules which exist inside the same namespace package.

        definition taken from PEP 382 <http://www.python.org/dev/peps/pep-0382/> text

    package
        A Python package is a module defined by a directory, containing a __init__.py file, and can contain other modules or other packages within it.

                package/
                    __init__.py
                    subpackage/
                        __init__.py
                    submodule.py

    see also, namespace package

    vendor package
        Groups of files installed by an operating system’s packaging mechanism (e.g. Debian or Redhat packages install on Linux systems).

        definition taken from PEP 382 <http://www.python.org/dev/peps/pep-0382/> text

Straight Plugin is very easy.

Straight Plugin provides a type of plugin you can create from almost any existing Python modules, and an easy way for outside developers to add functionality and customization to your projects with their own plugins.

Using any available plugins is a snap.

            from straight.plugin import load

            plugins = load('theproject.plugins', subclasses=FileHandler)

            handlers = plugins.produce()
            for line in open(filename):
                print handlers.pipe(line)

And, writing plugins is just as easy.

            from theproject import FileHandler

            class LineNumbers(FileHandler):
                def __init__(self):
                    self.lineno = 0
                def pipe(line):
                    self.lineno += 1
                    return "%04d %s" % (self.lineno, line)

Plugins are found from a namespace, which means the above example would find any FileHandler classes defined in modules you might import as theproject.plugins.default or theproject.plugins.extra. Through the magic of namespace packages <#term-namespace-package>, we can even split these up into separate installations, even managed by different teams. This means you can ship a project with a set of default plugins implementing its behavior, and allow other projects to hook in new functionality simply by shipping their own plugins under the same namespace.

Get started and learn more, today <>

MORE RESOURCES

    •   Full Documentation: <http://readthedocs.org/docs/straightplugin/>

    •   Mailing List: <https://groups.google.com/forum/#!forum/straight.plugin>

    •   Index <>

    •   Module Index <>

    •   Search Page <>

Author
Calvin Spealman

Copyright
2012, Calvin Spealman