Installation and use

Put the python file into scripts/addons/ directory, press F8 to refresh the blender, open the user preferences window add-ons tab, in Add curve categories turn on the Bezier curve fillet tool. It's easy to use, in curve edit mode,  select the corners you want to be a rounding, and press W-key -> Bezier curve fillet  or Ctrl+F. In modal status, moving mouse to adjust the radius, press ctrl and move mouse to change the ratio value, the value depends on  the distance from the initial mouse position.