Plugin API

This documents the general plugin API.

Please note, at this point OUR PLUGIN HOOKS MAY AND WILL CHANGE. Authors are encouraged to develop plugins and work with the MediaGoblin community to keep them up to date, but this API will be a moving target for a few releases.

Please check the Release Notes for updates!

How are hooks added? Where do I find them?

Much of this document talks about hooks, both as in terms of regular hooks and template hooks. But where do they come from, and how can you find a list of them?

For the moment, the best way to find available hooks is to check the source code itself. (Yes, we should start a more official hook listing with descriptions soon.) But many hooks you may need do not exist yet: what to do then?

The plan at present is that we are adding hooks as people need them, with community discussion. If you find that you need a hook and MediaGoblin at present doesn’t provide it at present, please talk to us! We’ll evaluate what to do from there.

pluginapi Module

Configuration

Your plugin may define its own configuration defaults.

Simply add to the directory of your plugin a config_spec.ini file. An example might look like:

[plugin_spec]
some_string = string(default="blork")
some_int = integer(default=50)

This means that when people enable your plugin in their config you’ll be able to provide defaults as well as type validation.

You can access this via the app_config variables in mg_globals, or you can use a shortcut to get your plugin’s config section:

>>> from mediagoblin.tools import pluginapi
# Replace with the path to your plugin.
# (If an external package,  it won't be part of mediagoblin.plugins)
>>> floobie_config = pluginapi.get_config('mediagoblin.plugins.floobifier')
>>> floobie_dir = floobie_config['floobie_dir']
# This is the same as the above
>>> from mediagoblin import mg_globals
>>> config = mg_globals.global_config['plugins']['mediagoblin.plugins.floobifier']
>>> floobie_dir = floobie_config['floobie_dir']

A tip: you have access to the %(here)s variable in your config, which is the directory that the user’s MediaGoblin config is running out of. So for example, your plugin may need a “floobie” directory to store floobs in. You could give them a reasonable default that makes use of the default user_dev location, but allow users to override it, like so:

[plugin_spec]
floobie_dir = string(default="%(here)s/user_dev/floobs/")

Note, this is relative to the user’s MediaGoblin config directory, not your plugin directory!

Context Hooks

View specific hooks

You can hook up to almost any template called by any specific view fairly easily. As long as the view directly or indirectly uses the method render_to_response you can access the context via a hook that has a key in the format of the tuple:

(view_symbolic_name, view_template_path)

Where the “view symbolic name” is the same parameter used in request.urlgen() to look up the view. So say we’re wanting to add something to the context of the user’s homepage. We look in mediagoblin/user_pages/routing.py and see:

add_route('mediagoblin.user_pages.user_home',
          '/u/<string:user>/',
          'mediagoblin.user_pages.views:user_home')

Aha! That means that the name is mediagoblin.user_pages.user_home. Okay, so then we look at the view at the mediagoblin.user_pages.user_home method:

@uses_pagination
def user_home(request, page):
    # [...] whole bunch of stuff here
    return render_to_response(
        request,
        'mediagoblin/user_pages/user.html',
        {'user': user,
         'user_gallery_url': user_gallery_url,
         'media_entries': media_entries,
         'pagination': pagination})

Nice! So the template appears to be mediagoblin/user_pages/user.html. Cool, that means that the key is:

("mediagoblin.user_pages.user_home",
 "mediagoblin/user_pages/user.html")

The context hook uses hook_transform() so that means that if we’re hooking into it, our hook will both accept one argument, context, and should return that modified object, like so:

def add_to_user_home_context(context):
    context['foo'] = 'bar'
    return context

hooks = {
    ("mediagoblin.user_pages.user_home",
     "mediagoblin/user_pages/user.html"): add_to_user_home_context}

Global context hooks

If you need to add something to the context of every view, it is not hard; there are two hooks hook that also uses hook_transform (like the above) but make available what you are providing to every view.

Note that there is a slight, but critical, difference between the two.

The most general one is the 'template_global_context' hook. This one is run only once, and is read into the global context… all views will get access to what are in this dict.

The slightly more expensive but more powerful one is 'template_context_prerender'. This one is not added to the global context… it is added to the actual context of each individual template render right before it is run! Because of this you also can do some powerful and crazy things, such as checking the request object or other parts of the context before passing them on.

Adding static resources

It’s possible to add static resources for your plugin. Say your plugin needs some special JavaScript and images… how to provide them? Then how to access them? MediaGoblin has a way!

Attaching to the hook

First, you need to register your plugin’s resources with the hook. This is pretty easy actually: you just need to provide a function that passes back a PluginStatic object.

Using staticdirect

Once you have this, you will want to be able to of course link to your assets! MediaGoblin has a “staticdirect” tool; you want to use this like so in your templates:

staticdirect("css/monkeys.css", "mystaticname")

Replace “mystaticname” with the name you passed to PluginStatic. The staticdirect method is, for convenience, attached to the request object, so you can access this in your templates like:

<img alt="A funny bunny"
     src="{{ request.staticdirect('images/funnybunny.png', 'mystaticname') }}" />

Additional hook tips

This section aims to explain some tips in regards to adding hooks to the MediaGoblin repository.

WTForms hooks

We haven’t totally settled on a way to transform wtforms form objects, but here’s one way. In your view:

from mediagoblin.foo.forms import SomeForm

def some_view(request)
    form_class = hook_transform('some_form_transform', SomeForm)
    form = form_class(request.form)

Then to hook into this form, do something in your plugin like:

import wtforms

class SomeFormAdditions(wtforms.Form):
    new_datefield = wtforms.DateField()

def transform_some_form(orig_form):
    class ModifiedForm(orig_form, SomeFormAdditions)
    return ModifiedForm

hooks = {
    'some_form_transform': transform_some_form}

Interfaces

If you want to add a pseudo-interface, it’s not difficult to do so. Just write the interface like so:

class FrobInterface(object):
    """
    Interface for Frobbing.

    Classes implementing this interface should provide defrob and frob.
    They may also implement double_frob, but it is not required; if
    not provided, we will use a general technique.
    """

    def defrob(self, frobbed_obj):
        """
        Take a frobbed_obj and defrob it.  Returns the defrobbed object.
        """
        raise NotImplementedError()

    def frob(self, normal_obj):
        """
        Take a normal object and frob it.  Returns the frobbed object.
        """
        raise NotImplementedError()

    def double_frob(self, normal_obj):
        """
        Frob this object and return it multiplied by two.
        """
        return self.frob(normal_obj) * 2


def some_frob_using_method():
    # something something something
    frobber = hook_handle(FrobInterface)
    frobber.frob(blah)

    # alternately you could have a default
    frobber = hook_handle(FrobInterface) or DefaultFrobber
    frobber.defrob(foo)

It’s fine to use your interface as the key instead of a string if you like. (Usually this is messy, but since interfaces are public and since you need to import them into your plugin anyway, interfaces might as well be keys.)

Then a plugin providing your interface can be like:

from mediagoblin.foo.frobfrogs import FrobInterface
from frogfrobber import utils

class FrogFrobber(FrobInterface):
    """
    Takes a frogputer science approach to frobbing.
    """
    def defrob(self, frobbed_obj):
        return utils.frog_defrob(frobbed_obj)

    def frob(self, normal_obj):
        return utils.frog_frob(normal_obj)

 hooks = {
     FrobInterface: lambda: return FrogFrobber}