Parse error when using a xacro property loaded through a yaml file.

Environment: Ubuntu 16, ROS Kinetic. I have a robot_parameters.yaml file with robot parameters: wheel_separation : 0.235 PI : 3.142 In a xacro file I have: When I do a --check-order on the xacro, I get the following error: unsupported operand type(s) for /: 'dict' and 'int' when evaluating expression 'PI/2' XacroException(None,) How would I use an imported name(in this case, PI) via yaml and use it for xacro calculations.