I can open IDLE.
After opening IDLE, I can do File > Open > some_file.py just fine.
When IDLE is open, I can also just double-click on any .py files in Finder and they will open in IDLE.
From the Terminal, whether IDLE is already open or not, I can type idle -e some_file.py and open a .py file just fine that way.
However, when IDLE is not already open, if I double-click on .py files in Finder, nothing happens. If I right-click > Open with > IDLE in Finder, nothing happens. For some reason, it appears I cannot launch IDLE by opening .py files in Finder.
I'm using OSX 10.8.2, Python 2.7.3. Any ideas?