Python course / Applied

Patterns and architecture

Give a growing program a shape: a pure core, a thin shell, and pieces you can swap. Most architecture problems start as one class quietly doing three jobs.

strategies
registries
composition
pure core
repositories
state machines
undo
configuration

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a registry instead of an if chain

COMMANDS = {}


def command(name):
    def register(func):
        COMMANDS[name] = func
        return func

    return register


@command("add")
def add(basket, item):
    return basket + [item]


@command("clear")
def clear(basket, item):
    return []


basket = []
for name, item in [("add", "tea"), ("add", "jam"), ("clear", None)]:
    basket = COMMANDS[name](basket, item)
    print(name, "->", basket)

add -> ['tea']
add -> ['tea', 'jam']
clear -> []

What you will learn

Replace a chain of if and elif choosing a behaviour with a dictionary of functions, so adding one is a single line.
Register command handlers with a decorator into a dictionary, and dispatch text commands through it.
Replace a family of subclasses, one per combination of choices, with one class built from the pieces it is given.
Split a program into pure functions that decide and a thin outer layer that reads and prints, so the decisions can be tested with no input or output at all.
Put storage behind a small set of methods, with an in-memory version and a file version that pass the same tests.
Model something with a fixed set of states and allowed moves as a transition table, refusing moves the table does not allow.

The lessons in this module

A class that should be a function
Strategies as functions
A registry of commands
Composition over inheritance
Events: telling whoever is listening
A pure core and a thin shell
Repositories: storage behind a promise
Wiring it together in one place
State machines
Commands that can be undone
Settings checked at the door
Errors that belong to your code
Refactoring under a safety net
When the pattern is the problem
Capstone: the café
Module review: a basket with undo

Ready when you are

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