Lesson 21 — Signals

Run code when something happens

A signal lets one part of the app say "a post was saved" and other parts react — send an email, clear a cache — without the save code knowing about them. In Django: @receiver(post_save, sender=Post).

The idea is a list of listeners (functions). Sending the signal calls each one:

for listener in listeners:
    listener(obj)

Note: the exercise runs in your browser without Django installed, so it is a small plain-Python simulation of what Django does for you.

Why it matters: side effects stay separate, so adding a new reaction never breaks saving.