When editing a tool validator class I do as described in the help:
" ... to program a ToolValidator class, right-click your tool, click Properties, click the Validation tab, then click Edit. This opens your installed Python editor, such as IDLE or PythonWin. ... "
In ArcMap, go to the Geoprocessing menu > Geoprocessing Options. Under Script Tool Editor/Debugger set the Editor to the IDLE or PYTHONWIN executable. Ex:
In ArcMap, go to the Geoprocessing menu > Geoprocessing Options. Under Script Tool Editor/Debugger set the Editor to the IDLE or PYTHONWIN executable. Ex: