human
Can you provide a the Python program you requested given the following:
Complete the below
def interface(cls):

    ''''A decorator to create plugin interfaces::
    @interface
    class ISomething (object):
    def contract(self):
    pass
    Following class methods are injected:
    .. function:: .get(context=<current context>)
    :returns: any existing instance or creates a new one
    .. function:: .get_all(context=<current context>)
    :returns: list of instances for each implementation
    .. function:: .get_class()
    :returns: any implementation class
    .. function:: .get_classes()
    :returns: list of implementation classes
    .. function:: .get_instances(context=<current context>)
    :returns: list of all existing instances
    :type cls: class
    :rtype: class'
    '''
assistant
#Here is the Python program you requested.
    def get(cls, context=None):
        if (not context):
            context = extract_context()
        impls = manager.get_implementations(cls)
        if (len(impls) == 0):
            raise NoImplementationsError(('Implementations    for    %s    not    found' % cls.__name__))
        return context.get_instance(impls[0])
    cls.get = get.__get__(cls)
    def get_all(cls, context=None):
        if (not context):
            context = extract_context()
        return [context.get_instance(x) for x in manager.get_implementations(cls)]
    cls.get_all = get_all.__get__(cls)
    def get_class(cls):
        return manager.get_implementations(cls)[0]
    cls.get_class = get_class.__get__(cls)
    def get_classes(cls):
        return manager.get_implementations(cls)
    cls.get_classes = get_classes.__get__(cls)
    def get_instances(cls, context=None):
        if (not context):
            context = extract_context()
        return context.get_instances(cls)
    cls.get_instances = get_instances.__get__(cls)
    cls._interface = True
    manager.register_interface(cls)
    return cls