The Mac port of Python contains a binary named "PythonLauncher" which 
can be used as the association of ".py" and ".pyw" files. This program 
will launch the script (either in a Terminal window or without a 
window).

In hindsight adding this tool was not a good idea: this makes it way to 
easy to accidently launch python script downloaded from the Internet. 
Removing PythonLauncher would still allow running the scripts through 
IDLE (which also has an association to ".py" and ".pyw" files in it 
Info.plist).

In my experience most ".py" files are modules instead of standalone 
scripts, opening the file in an editor would therefore be a better 
action in the majority of use-cases.