Why is the . needed for adding suffixes to the 'auto-mode-alist?

Question

In https://www.emacswiki.org/emacs/AutoModeAlist one needs to use this command in order to add .py for python-mode (add-to-list 'auto-mode-alist '("\\.py\\'" . python-mode))
Why do we need the . between '"\\.py\\'" and the python-mode ?
Thanks.

Answer 1

This is lisp syntax for a dotted pair:

(cons 'a 'b) -> (a . b)

Here's a link.

Briefly, dotted pairs are the fundamental building blocks for lists: lists are special dotted pairs, defined recursively. Most of the time, you work with lists so the dotted pairs are hidden in the background, but once in a while (as in this case), they poke through.

Answer 2

That's because auto-mode-alist is an "association list" (a list of pairs).