<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def set_instance(self, key, instance, default_scope=GlobalScope):
        '''
        Sets instance under specified provider key. If a provider for specified key does not exist, one is created
        without a factory using the given scope.

        :param key: Provider key
        :type key: object
        :param instance: Instance
        :type instance: object
        :param default_scope: Scope key, factory, or instance
        :type default_scope: object or callable
        '''
        if key not in self._providers:
            # We don't know how to create this kind of instance at this time, so add it without a factory.
            factory = None
            self.register_factory(key, factory, default_scope)
        self._providers[key].set_instance(instance)