'Restore (and return) default value for the specified key.
This method will only work for a ConfigObj that was created
with a configspec and has been validated.
If there is no default value for this key, ``KeyError`` is raised.'

def restore_default(self, key):

    default = self.default_values[key]
    dict.__setitem__(self, key, default)
    if (key not in self.defaults):
        self.defaults.append(key)
    return default
