Opal Discoverable Features

Opal makes extensive use of the case of discoverable features. These are pieces of composable functionality that can be implemented by any plugin or application, simply by declaring a class with the appropriate fields and methods implemented. (Details of exactly which fields and methods vary by feature.)

Defining Discoverable Features

The only thing required for a discoverable feature, is that it defines the name of the module in which it should expect to find instances.

from opal.core import discoverable

class MyFeature(discoverable.DiscoverableFeature):
    module_name = 'myfeature'

This now gives us access to various interfaces which come with the library. We can access all subclasses of MyFeature by calling MyFeature.list() - which will look through all Django apps in our application, and look for a module named myfeature.py. The list() method then returns any subclasses of MyFeature.

Slugs and Display Names

We often want to define human and machine readable strings to name our features. Display names are intended for people - feel free to include spaces, numbers and punctuation. Slugs are intended for machines - so no spaces, numbers, hyphens, periods or other punctuation. By default, if there is a display name and no explicit slug, we will 'slugify' the display name.

class MyNameFeature(MyFeature):
    display_name = 'Hello World'

print MyNameFeature.get_slug()
# 'hello_world'

class MySlugFeature(MyFeature):
    slug = 'this_is_a_slug'

print MySlugFeature.get_slug()
# 'this_is_a_slug'

Retrieving Subclasses

Once we define a display_name or slug for subclasses of our feature, we can then fetch them via the get() api.

class RedFeature(MyFeature):
    slug = 'red_feature'

MyFeature.get('red_feature') # -> RedFeature

Sortable Features

We can make our feature sortable via an order property by including discoverable.SortableFeature as a parent class. This will ensure that MyFeature.list() respects the .order number of any subclass.

class MyFeature(discoverable.DiscoverableFeature,
                discoverable.SortableFeature):
    module_name = 'myfeature'

class ThirdFeature(MyFeature):
    order = 3

class FirstFeature(MyFeature):
    order = 1

class SecondFeature(MyFeature):
    order = 2

for f in MyFeature.list():
    print f, f.order

# <class '*.*.FirstFeature'>, 1
# <class '*.*.SecondFeature'>, 2
# <class '*.*.ThirdFeature'>, 3

Restrictable Features

We can ensure that only particular users can access a feature by including discoverable.RestrictableFeature as a parent class, and implementing the visible_to classmethod on any restricted subclasses.

 class MyFeature(discoverable.DiscoverableFeature, discoverable.RestrictableFeature):
     module_name = 'myfeature'

For instance, a feature that was only visible to superusers could be implemented like this:

 class SuperuserFeature(MyFeature):

     @classmethod
     def visible_to(klass, user):
         return user.is_superuser

Validating Features

Sometimes we wish to validate features so that we don't cause unintended consequences when we implement subclasses of them. This is available via the is_valid classmethod. For instance, if we wanted to implement a "Bomb" feature, which blew up every time the blow_up attribute was true, we could to this as follows:

class BombFeature(discoverable.DiscoverableFeature):
    module_name = 'bombs'
    blow_up = False

    @classmethod
    def is_valid(klass):
        if klass.blow_up == True:
            from opal.core.exceptions import InvalidDiscoverableFeatureError
            raise InvalidDiscoverableFeatureError('BLOWING UP')


class Threat(BombFeature): pass
# That's fine.

class Detonate(BombFeature):
    blow_up = True
# InvalidDiscoverableFeatureError: BLOWING UP

Abstract Features

Sometimes we want to declare an abstract feature - something with reusable functionality that isn't itself a new feature. We would rather not have our abstract feature show up when we list() our base feature - because it's simply a programming convenience. This is made simple by using opal.utils.AbstractBase.

    class A(discoverable.DiscoverableFeature):
        module_name = 'a'

    class AA(A, AbstractBase): pass

    class B(A): pass

    class C(AA): pass

    class D(AA): pass


    A.list()
    # [B, C, D]

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