def expr(expression, transform=None):
    ''' Convenience function to explicitly return an "expr" specification for
    a Bokeh :class:`~bokeh.core.properties.DataSpec` property.

    Args:
        expression (Expression) : a computed expression for a
            ``DataSpec`` property.

        transform (Transform, optional) : a transform to apply (default: None)

    Returns:
        dict : ``{ "expr": expression }``

    .. note::
        This function is included for completeness. String values for
        property specifications are by default interpreted as field names.

    '''
    if transform:
        return dict(expr=expression, transform=transform)
    return dict(expr=expression)