Python IDLE can't be opened by double-clicking .py files in Finder
You can right-click on a python file and select get info. Near the bottom of the get info window is a section that reads Open With. You can select a version of Idle to use. There is a button below that reads change all. If you click this, then all files with .py extension will be opened with idle. Just tested this and it works.  Below is an image indicating the relevant points.