Reactive event sourcing in 45 lines

Complaint.py

event_reaction_map = {}


def apply_effects_of(event):
        if event in event_reaction_map:
            for reaction in event_reaction_map[event]:
                reaction()


def react_to(event):
    def reaction_subscription(reaction):
        subscribe_reaction(reaction)
        return reaction

    def subscribe_reaction(reaction):
        if event in event_reaction_map:
            event_reaction_map[event].append(reaction)
        else:
            event_reaction_map[event] = [reaction]

    return reaction_subscription


@react_to("complaint")  # Decorators are just syntactic sugar. Comment this line and uncomment the line below for proof.
def complaint_reaction():
    print("That's outrageous !")
    events.happening("reaction")
# complaint_reaction=react_to("complaint")(complaint_reaction)


@react_to("reaction")
def reaction_followup():
    print("You said it, mate !")


class Events:
    def __init__(self):
        self.events = []

    def happening(self, event):
        self.events.append(event)
        apply_effects_of(event)


events = Events()
events.happening("complaint")