Python Singleton Class with Decorators

singleton-with-decorator.ipynb

{
  "nbformat": 4,
  "nbformat_minor": 0,
  "metadata": {
    "colab": {
      "name": "Python Singleton Class with Decorators",
      "provenance": [],
      "collapsed_sections": [],
      "authorship_tag": "ABX9TyPUKNA9PvBgIaSGqHTcPQTn",
      "include_colab_link": true
    },
    "kernelspec": {
      "name": "python3",
      "display_name": "Python 3"
    }
  },
  "cells": [

    {
      "cell_type": "markdown",
      "metadata": {
        "id": "fUYU5-0ZdtiH",
        "colab_type": "text"
      },
      "source": [
        "## Define the decorator"
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "ItFzu5Y6dfUN",
        "colab_type": "code",
        "colab": {}
      },
      "source": [
        "from functools import wraps\n",
        "\n",
        "def singleton(orig_cls):\n",
        "    orig_new = orig_cls.__new__\n",
        "    instance = None\n",
        "\n",
        "    @wraps(orig_cls.__new__)\n",
        "    def __new__(cls, *args, **kwargs):\n",
        "        nonlocal instance\n",
        "        if instance is None:\n",
        "            instance = orig_new(cls, *args, **kwargs)\n",
        "        return instance\n",
        "    orig_cls.__new__ = __new__\n",
        "    return orig_cls"
      ],
      "execution_count": 1,
      "outputs": []
    },
    {
      "cell_type": "markdown",
      "metadata": {
        "id": "MY1TVedgdy0O",
        "colab_type": "text"
      },
      "source": [
        "## Define a decorated class"
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "ggiY3_1sdxwr",
        "colab_type": "code",
        "colab": {}
      },
      "source": [
        "@singleton\n",
        "class Logger:\n",
        "    def log(msg):\n",
        "        print(msg)"
      ],
      "execution_count": 3,
      "outputs": []
    },
    {
      "cell_type": "markdown",
      "metadata": {
        "id": "2uzA8Pokd8gn",
        "colab_type": "text"
      },
      "source": [
        "## Assert that only a single instance is created\n",
        "\n",
        "Following assert statement makes sure two variables are referencing the same object.\n",
        "\n",
        "```python\n",
        "assert var1 is var2\n",
        "```\n",
        "\n",
        "If `var1` is referring to a different object than `var2`, than the `assert` operator will raise `AssertionError`.\n",
        "\n",
        "In our case assertion is successful. No error is raised."
      ]
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "pZvEqaZEd5Wa",
        "colab_type": "code",
        "colab": {}
      },
      "source": [
        "logger1 = Logger()\n",
        "logger2 = Logger()\n",
        "assert logger1 is logger2"
      ],
      "execution_count": 4,
      "outputs": []
    },
    {
      "cell_type": "code",
      "metadata": {
        "id": "-xW9DdiJeBob",
        "colab_type": "code",
        "colab": {}
      },
      "source": [
        ""
      ],
      "execution_count": null,
      "outputs": []
    }
  ]
}