Refactoring toward, and away from, patterns

Python · reach/reach.py

"""Before removing a Singleton, find everything that reaches it, and every factory that only ever makes one class."""
import ast
from pathlib import Path

tree = ast.parse(Path("notify_app.py").read_text())
callers = []
for owner in [None, *[n for n in tree.body if isinstance(n, ast.ClassDef)]]:
    body = tree.body if owner is None else owner.body
    for f in (n for n in body if isinstance(n, ast.FunctionDef)):
        for call in ast.walk(f):
            if isinstance(call, ast.Call) and isinstance(call.func, ast.Attribute) and call.func.attr == "instance":
                callers.append(f"{owner.name}.{f.name}" if owner else f.name)
                break
print(f"Notifications.instance() is called from {len(callers)} places: {', '.join(callers)}")
for cls in (n for n in tree.body if isinstance(n, ast.ClassDef)):
    for f in (n for n in cls.body if isinstance(n, ast.FunctionDef)):
        returns = [r.value for r in ast.walk(f) if isinstance(r, ast.Return) and isinstance(r.value, ast.Call)]
        made = {ast.unparse(r.func) for r in returns}
        if f.name == "create" and len(made) == 1:
            print(f"{cls.name}.create() only ever makes {made.pop()}: a factory with one product")

Notifications.instance() is called from 5 places: welcome, password_reset, order_shipped, Reminders.due, Reminders.overdue
NotifierFactory.create() only ever makes EmailNotifier: a factory with one product

reach/notify_app.py

"""A small app whose messages go through a Singleton made by a factory with one product."""


class EmailNotifier:
    def send(self, to, text):
        return f"email to {to}: {text}"


class NotifierFactory:
    @staticmethod
    def create():
        return EmailNotifier()


class Notifications:
    _instance = None

    @classmethod
    def instance(cls):
        if cls._instance is None:
            cls._instance = cls()
            cls._instance.notifier = NotifierFactory.create()
        return cls._instance

    def send(self, to, text):
        return self.notifier.send(to, text)


def welcome(user):
    return Notifications.instance().send(user, "welcome aboard")


def password_reset(user):
    return Notifications.instance().send(user, "here is your reset link")


def order_shipped(user, order):
    return Notifications.instance().send(user, f"order {order} has shipped")


class Reminders:
    def due(self, user):
        return Notifications.instance().send(user, "your invoice is due")

    def overdue(self, user):
        return Notifications.instance().send(user, "your invoice is overdue")