Emacs Autoloading

The autoload facility in Emacs allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along.

(autoload 'pylookup-lookup "pylookup")

When you try and run pylookup-lookup, the system will try and load pylookup.