Class variables and __init__ function details

Class_variables_and_init.ipynb

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  {
   "cells": [
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Test:\n",
      "    \"\"\"demonstrating option initialization\"\"\"\n",
      "    def __init__(self, data=0):\n",
      "        self.data = data\n",
      "    \n",
      "    def __repr__(self):\n",
      "        \"\"\"overloading default print and repr function\"\"\"\n",
      "        return \"<Class Test, data={0}>\".format(self.data)\n",
      "\n",
      "t = Test()   # uninitialized\n",
      "t2 = Test(10)\n",
      "\n",
      "print('t ->', t)\n",
      "print('t2 ->', t2)\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "('t ->', <Class Test, data=0>)\n",
        "('t2 ->', <Class Test, data=10>)\n"
       ]
      }
     ],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Above is the output of the program\n",
      "\n",
      "The next example demonstrates the difference between `class` and `instance` variables"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Demo:\n",
      "    classvar = 10\n",
      "    \n",
      "    def __init__(self):\n",
      "        self.data = 10\n",
      "\n",
      "    def __repr__(self):\n",
      "        return \"<Class Demo, classvar={0}, data={1}>\".format(Demo.classvar, self.data)\n",
      "\n",
      "a = Demo()\n",
      "b = Demo()\n",
      "\n",
      "print(a)\n",
      "print(b)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "<Class Demo, classvar=10, data=10>\n",
        "<Class Demo, classvar=10, data=10>\n"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "now we change the `classvar` in and try to print both `a` and `b` again"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "Demo.classvar = 50\n",
      "\n",
      "print(a)\n",
      "print(b)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "<Class Demo, classvar=50, data=10>\n",
        "<Class Demo, classvar=50, data=10>\n"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "we see that **class variable** change reflects in __both instances__ `a` and `b`\n",
      "\n",
      "Next we try to change `data` which is and __instance variable__ from `a` and then print again"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "a.data = 100\n",
      "\n",
      "print(a)\n",
      "print(b)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "<Class Demo, classvar=50, data=100>\n",
        "<Class Demo, classvar=50, data=10>\n"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "as we see, instance variables reflect changes in **only** that particular instance\n",
      "\n",
      "whereas the changes to *class variables* are reflected across __all__ the **instance variables**\n",
      "\n",
      "next we try to change classvar from __init__ using optional arguments"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Demo:\n",
      "    \"\"\"\n",
      "        This will override (redefine) the previous Demo class\n",
      "    \"\"\"\n",
      "    \n",
      "    def __init__(self, classvar_value=50, instance_value = 10):\n",
      "        Demo.classvar = classvar_value\n",
      "        # classvar only comes into existence after the above line is executed\n",
      "        # i.e. after at least 1 instance is created\n",
      "        \n",
      "        self.data = instance_value\n",
      "    \n",
      "    def __repr__(self):\n",
      "        return \"classvar={0}, data={1}\".format(Demo.classvar, self.data)\n",
      "\n",
      "a = Demo()\n",
      "b = Demo(100)\n",
      "c = Demo(100, 100)\n",
      "\n",
      "print(a, b, c, sep='\\n')\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "classvar=100, data=10\n",
        "classvar=100, data=10\n",
        "classvar=100, data=100\n"
       ]
      }
     ],
     "prompt_number": 2
    }
   ],
   "metadata": {}
  }
 ]
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