human
Complete this python program:
def default_listener(col_attr, default):
assistant
#Here is the Python program you requested.

    """Establish a default-setting listener.
    Given a class_, attrname, and a :class:`.DefaultGenerator` instance.
    The default generator should be a :class:`.ColumnDefault` object with a
    plain Python value or callable default; otherwise, the appropriate behavior
    for SQL functions and defaults should be determined here by the
    user integrating this feature.
    """
    @event.listens_for(col_attr, 'init_scalar', retval=True, propagate=True)
    def init_scalar(target, value, dict_):
        if default.is_callable:
            value = default.arg(None)
        elif default.is_scalar:
            value = default.arg
        else:
            raise NotImplementedError("Can't    invoke    pre-default    for    a    SQL-level    column    default")
        dict_[col_attr.key] = value
        return value