from inspect import getmembers
from django.conf import settings

class AppSettings(object):
    """
    An app setting object to be used for handling app setting defaults
    gracefully and providing a nice API for them. Say you have an app
    called ``myapp`` and want to define a few defaults, and refer to the
    defaults easily in the apps code. Add a ``settings.py`` to your app::

        from path.to.utils import AppSettings

        class MyAppSettings(AppSettings):
            SETTING_1 = "one"
            SETTING_2 = (
                "two",
            )

    Then initialize the setting with the correct prefix in the location of
    of your choice, e.g. ``conf.py`` of the app module::

        settings = MyAppSettings(prefix="MYAPP")

    The ``MyAppSettings`` instance will automatically look at Django's
    global setting to determine each of the settings and respect the
    provided ``prefix``. E.g. adding this to your site's ``settings.py``
    will set the ``SETTING_1`` setting accordingly::

        MYAPP_SETTING_1 = "uno"

    Usage
    -----

    Instead of using ``from django.conf import settings`` as you would
    usually do, you can switch to using your apps own settings module
    to access the app settings::

        from myapp.conf import settings

        print myapp_settings.MYAPP_SETTING_1

    ``AppSettings`` instances also work as pass-throughs for other
    global settings that aren't related to the app. For example the
    following code is perfectly valid::

        from myapp.conf import settings

        if "myapp" in settings.INSTALLED_APPS:
            print "yay, myapp is installed!"

    Custom handling
    ---------------

    Each of the settings can be individually configured with callbacks.
    For example, in case a value of a setting depends on other settings
    or other dependencies. The following example sets one setting to a
    different value depending on a global setting::

        from django.conf import settings

        class MyCustomAppSettings(AppSettings):
            ENABLED = True

            def configure_enabled(self, value):
                return value and not self.DEBUG

        custom_settings = MyCustomAppSettings("MYAPP")

    The value of ``custom_settings.MYAPP_ENABLED`` will vary depending on the
    value of the global ``DEBUG`` setting.

    Each of the app settings can be customized by providing
    a method ``configure_<lower_setting_name>`` that takes the default
    value as defined in the class attributes as the only parameter.
    The method needs to return the value to be use for the setting in
    question.
    """
    def __dir__(self):
        return sorted(list(set(self.__dict__.keys() + dir(settings))))

    __members__ = lambda self: self.__dir__()

    def __getattr__(self, name):
        if name.startswith(self._prefix):
            raise AttributeError("%r object has no attribute %r" %
                                 (self.__class__.__name__, name))
        return getattr(settings, name)

    def __setattr__(self, name, value):
        super(AppSettings, self).__setattr__(name, value)
        if name in dir(settings):
            setattr(settings, name, value)

    def __init__(self, prefix):
        super(AppSettings, self).__setattr__('_prefix', prefix)
        for name, value in filter(self.issetting, getmembers(self.__class__)):
            prefixed_name = "%s_%s" % (prefix.upper(), name.upper())
            value = getattr(settings, prefixed_name, value)
            callback = getattr(self, "configure_%s" % name.lower(), None)
            if callable(callback):
                value = callback(value)
            delattr(self.__class__, name)
            setattr(self, prefixed_name, value)

    def issetting(self, (name, value)):
        return name == name.upper()