Hi Graeme,You need to edit the source code for IDLE to accomplish this. Go to your idelib folder for the install of IDLE and edit the EditorWindow.py file (you may want to make a copy just in case there is a problem). Go to the following code:
def ispythonsource(self, filename):
if not filename or os.path.isdir(filename):
return True
base, ext = os.path.splitext(os.path.basename(filename))
if os.path.normcase(ext) in (".py", ".pyw")
return True
Add ".pyt" to the list of extension to check and save the changes. This should now highlight the syntax when you open it in IDLE.