Python execution problem

[mailpitches at email.com]

Using OSX 10.4.5

This is more of a unix/tcsh question than a python question.
Somehow I got to the point where I have two files 'a.py' and 'b.py'
which have identical contents and permissions, but one refuses to
execute:

[blah:/Library/WebServer/CGI-Executables] me% a.py
tcsh: a.py: Command not found.
[blah:/Library/WebServer/CGI-Executables] me% b.py
okay
[blah:/Library/WebServer/CGI-Executables] me%

Could someone enlighten me about how the shell knows to execute a
script? I always assumed it was just the extension, but this seems to
prove me wrong.

tnx>mp