This defines which file to load when a function is called, plus doc-string, plus it indicates whether the function is interactive or not.
(autoload 'python-mode "python-mode" "Python Mode." t)
This checks for python-mode.elc or python-mode.el on the LoadPath.
This works well with AutoModeAlist:
(add-to-list 'auto-mode-alist '("\\.py\\'" . python-mode))
The autoload will make sure that the correct source file is loaded when
‘python-mode’ is requested, and the
‘auto-mode-alist’ setting will make sure that
‘python-mode’ is requested when a
.py file is loaded.