For an example, look at the code in: ./addons/base_status/base_state.py

def _get_default_user(self, cr, uid, context=None):

    """ Gives current user id

        :param context: if portal not in context returns False

    """

    if context is None:

        context = {}

    if not context or not context.get('portal'):

        return False

    return uid