Python Forum

Context: Elegant solution needed to monitor and save database changes during runtime. To minimze IO, the solution should be based on signals rather than polling and comparing against file content.

Solution: Use a callback to start a save timer following database alteration. Thus, I subclassed a dict (Observer) and overridden __setitem__ method to handle the addition of new keys. As the dict content is loaded from a JSON, all nested dict object must also be converted after update() is called.

Edit: it works! can it be further improved?

#!/usr/bin/python3
from typing import Callable


class Observer(dict):
    def __init__(self, *args, callback: Callable):
        super().__init__(*args)
        self.cb = callback

    def __setitem__(self, item: any, value: any):
        """ Fires a callback function when a value is changed """
        value = Observer(value, callback=self.cb) if isinstance(value, dict) else value
        super().__setitem__(item, value)
        if isinstance(value, dict):
            self._convert(value)
        self.cb()

    def update(self, *args):
        """ Converts dict objects after update """
        super().update(*args)
        self._convert(self)

    def _convert(self, d: dict):
        """ Recursively converts nested dict to Observer subclass """
        for key, value in d.items():
            if isinstance(value, dict):
                d[key] = Observer(value, callback=self.cb)
                self._convert(value)


class Database(Observer):
    def __init__(self, d):
        super().__init__(self, callback=lambda: None)  # ##
        self.update(d)


def demo():
    db = Database({1: {"foo": ["bar"]}, 2: {3: {"bubu": 0}}})
    db[4] = {5: {6: {7: {}}}}

    print(f"{type(db)}\n")
    print("update")
    print(f"{type(db[1])=}")
    print(f"{type(db[2])=}")
    print(f"{type(db[2][3])=}")
    print("\nsetitem")
    print(f"{type(db[4])=}")
    print(f"{type(db[4][5])=}")
    print(f"{type(db[4][5][6])=}")
    print(f"{type(db[4][5][6][7])=}")
    print()
    print(db)


demo()

Output:

Quote:<class '__main__.Database'>

update
type(db[1])=<class '__main__.Observer'>
type(db[2])=<class '__main__.Observer'>
type(db[2][3])=<class '__main__.Observer'>

setitem
type(db[4])=<class '__main__.Observer'>
type(db[4][5])=<class '__main__.Observer'>
type(db[4][5][6])=<class '__main__.Observer'>
type(db[4][5][6][7])=<class '__main__.Observer'>

{1: {'foo': ['bar']}, 2: {3: {'bubu': 0}}, 4: {5: {6: {7: {}}}}}

(Jan-22-2021, 04:34 AM)Alfalfa Wrote: [ -> ]As the dict content is loaded from a JSON, all nested dict object must also be converted after update() is called.

I think you can define custom JSONDecoder and set object_hook=Observer and override the default dict.

Alfalfa

buran