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 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pure Python Metaclass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Initializing singleton: 21aac41a-ee95-40c0-88f1-f55eaf81ac60\n",
      "MySingleton(uuid=UUID('21aac41a-ee95-40c0-88f1-f55eaf81ac60'))\n",
      "MySingleton(uuid=UUID('21aac41a-ee95-40c0-88f1-f55eaf81ac60'))\n",
      "MySingleton(uuid=UUID('21aac41a-ee95-40c0-88f1-f55eaf81ac60'))\n",
      "MySingleton(uuid=UUID('21aac41a-ee95-40c0-88f1-f55eaf81ac60'))\n",
      "MySingleton(uuid=UUID('21aac41a-ee95-40c0-88f1-f55eaf81ac60'))\n"
     ]
    }
   ],
   "source": [
    "from dataclasses import dataclass, field\n",
    "from uuid import UUID, uuid4\n",
    "\n",
    "\n",
    "# Metaclass definition\n",
    "class Singleton(type):\n",
    "    _instances = {}\n",
    "\n",
    "    def __call__(cls, *args, **kwargs):\n",
    "        if cls not in cls._instances:\n",
    "            cls._instances[cls] = super().__call__(*args, **kwargs)\n",
    "        return cls._instances[cls]\n",
    "\n",
    "\n",
    "# gives an example of inheritance\n",
    "@dataclass\n",
    "class ParentClass:\n",
    "    uuid: UUID = field(init=False, default_factory=uuid4)\n",
    "\n",
    "\n",
    "# Main singleton class\n",
    "@dataclass\n",
    "class MySingleton(ParentClass, metaclass=Singleton):\n",
    "    def __post_init__(self):\n",
    "        print(f'Initializing singleton: {self.uuid}')\n",
    "\n",
    "\n",
    "# Initialize the singleton\n",
    "MySingleton()\n",
    "\n",
    "# \"Initialize\" a few more times\n",
    "for _ in range(5):\n",
    "    print(MySingleton())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Pydantic Metaclass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "from typing import Any, Optional, Self\n",
    "\n",
    "from pydantic import BaseModel, Field\n",
    "from pydantic._internal._model_construction import ModelMetaclass\n",
    "\n",
    "\n",
    "class PydanticSingleton(ModelMetaclass):\n",
    "    _instances = {}\n",
    "\n",
    "    def __call__(cls, *args, **kwargs):\n",
    "        if model := cls._instances.get(cls):\n",
    "            model_dict = model.model_dump()\n",
    "            model_dict.update(kwargs)\n",
    "            model = model.model_validate(model_dict)\n",
    "            cls._instances[cls] = model\n",
    "        else:\n",
    "            cls._instances[cls] = super(PydanticSingleton, cls).__call__(*args, **kwargs)\n",
    "\n",
    "        return cls._instances[cls]\n",
    "\n",
    "\n",
    "class MyPydanticSingleton(BaseModel, metaclass=PydanticSingleton):\n",
    "    id: UUID = Field(default_factory=uuid4)\n",
    "    required_field: str\n",
    "    foo: Optional[str] = None"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "id=UUID('9965c864-20b7-4809-b73a-c16c352fea20') required_field='Iteration 0' foo=None\n",
      "id=UUID('9965c864-20b7-4809-b73a-c16c352fea20') required_field='Iteration 1' foo=None\n",
      "id=UUID('9965c864-20b7-4809-b73a-c16c352fea20') required_field='Iteration 2' foo=None\n",
      "id=UUID('9965c864-20b7-4809-b73a-c16c352fea20') required_field='Iteration 3' foo=None\n",
      "id=UUID('9965c864-20b7-4809-b73a-c16c352fea20') required_field='Iteration 4' foo=None\n"
     ]
    }
   ],
   "source": [
    "s = [MyPydanticSingleton(required_field=f'Iteration {i}') for i in range(5)]\n",
    "print('\\n'.join(map(str, s)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "MyPydanticSingleton(id=UUID('9965c864-20b7-4809-b73a-c16c352fea20'), required_field='Iteration 4', foo='bar')"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "MyPydanticSingleton(foo='bar')"
   ]
  }
 ],
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