{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Python @property\n",
    "\n",
    "https://www.programiz.com/python-programming/property"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "> **Purpose**: To implement constaints to attributes\n",
    "\n",
    "Example: Temperatures cannot go below -273 degree Celsius"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Using Getters and Setters"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Celsius:\n",
    "    def __init__(self, temperature = 0):\n",
    "        self.set_temperature(temperature)\n",
    "\n",
    "    def to_fahrenheit(self):\n",
    "        return (self.get_temperature() * 1.8) + 32\n",
    "\n",
    "    # new update\n",
    "    def get_temperature(self):\n",
    "        return self._temperature\n",
    "\n",
    "    def set_temperature(self, value):\n",
    "        if value < -273:\n",
    "            raise ValueError(\"Temperature below -273 is not possible\")\n",
    "        self._temperature = value"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "ename": "ValueError",
     "evalue": "Temperature below -273 is not possible",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-2-5cf2215ec01b>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mc\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mCelsius\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m-\u001b[0m\u001b[0;36m277\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;32m<ipython-input-1-9df6ff2be14f>\u001b[0m in \u001b[0;36m__init__\u001b[0;34m(self, temperature)\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0;32mclass\u001b[0m \u001b[0mCelsius\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      2\u001b[0m     \u001b[0;32mdef\u001b[0m \u001b[0m__init__\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mtemperature\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;36m0\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 3\u001b[0;31m         \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mset_temperature\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mtemperature\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      4\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      5\u001b[0m     \u001b[0;32mdef\u001b[0m \u001b[0mto_fahrenheit\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;32m<ipython-input-1-9df6ff2be14f>\u001b[0m in \u001b[0;36mset_temperature\u001b[0;34m(self, value)\u001b[0m\n\u001b[1;32m     12\u001b[0m     \u001b[0;32mdef\u001b[0m \u001b[0mset_temperature\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     13\u001b[0m         \u001b[0;32mif\u001b[0m \u001b[0mvalue\u001b[0m \u001b[0;34m<\u001b[0m \u001b[0;34m-\u001b[0m\u001b[0;36m273\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 14\u001b[0;31m             \u001b[0;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"Temperature below -273 is not possible\"\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m     15\u001b[0m         \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0m_temperature\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;31mValueError\u001b[0m: Temperature below -273 is not possible"
     ]
    }
   ],
   "source": [
    "c = Celsius(-277)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "c = Celsius(37)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "37"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "c.get_temperature()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "ename": "ValueError",
     "evalue": "Temperature below -273 is not possible",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-5-8fd371f36b30>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0mc\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mset_temperature\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m10\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mc\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mset_temperature\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m-\u001b[0m\u001b[0;36m300\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;32m<ipython-input-1-9df6ff2be14f>\u001b[0m in \u001b[0;36mset_temperature\u001b[0;34m(self, value)\u001b[0m\n\u001b[1;32m     12\u001b[0m     \u001b[0;32mdef\u001b[0m \u001b[0mset_temperature\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mself\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     13\u001b[0m         \u001b[0;32mif\u001b[0m \u001b[0mvalue\u001b[0m \u001b[0;34m<\u001b[0m \u001b[0;34m-\u001b[0m\u001b[0;36m273\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 14\u001b[0;31m             \u001b[0;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"Temperature below -273 is not possible\"\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m     15\u001b[0m         \u001b[0mself\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0m_temperature\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mvalue\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;31mValueError\u001b[0m: Temperature below -273 is not possible"
     ]
    }
   ],
   "source": [
    "c.set_temperature(10)\n",
    "c.set_temperature(-300)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "> Note that private variables don't exist in Python"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "-300"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "c._temperature = -300\n",
    "c.get_temperature()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## The Power of @property"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Celsius:\n",
    "    def __init__(self, temperature = 0):\n",
    "        self.temperature = temperature\n",
    "\n",
    "    def to_fahrenheit(self):\n",
    "        return (self.temperature * 1.8) + 32\n",
    "\n",
    "    def get_temperature(self):\n",
    "        print(\"Getting value\")\n",
    "        return self._temperature\n",
    "\n",
    "    def set_temperature(self, value):\n",
    "        if value < -273:\n",
    "            raise ValueError(\"Temperature below -273 is not possible\")\n",
    "        print(\"Setting value\")\n",
    "        self._temperature = value\n",
    "\n",
    "    temperature = property(get_temperature,set_temperature)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "In Python, `property()` is a built-in function that creates and returns a property object. The signature of this function is\n",
    "\n",
    "```py\n",
    "property(fget=None, fset=None, fdel=None, doc=None)\n",
    "```\n",
    "\n",
    "A property object has three methods, `getter()`, `setter()`, and `delete()` to specify `fget`, `fset` and `fdel` at a later point. This means, the line\n",
    "\n",
    "```py\n",
    "temperature = property(get_temperature,set_temperature)\n",
    "```\n",
    "\n",
    "could have been broken down as\n",
    "\n",
    "```py\n",
    "# make empty property\n",
    "temperature = property()\n",
    "# assign fget\n",
    "temperature = temperature.getter(get_temperature)\n",
    "# assign fset\n",
    "temperature = temperature.setter(set_temperature)\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Programmers familiar with **decorators** in Python can recognize that the above construct can be implemented as decorators."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Celsius:\n",
    "    def __init__(self, temperature = 0):\n",
    "        self._temperature = temperature\n",
    "\n",
    "    def to_fahrenheit(self):\n",
    "        return (self.temperature * 1.8) + 32\n",
    "\n",
    "    @property\n",
    "    def temperature(self):\n",
    "        print(\"Getting value\")\n",
    "        return self._temperature\n",
    "\n",
    "    @temperature.setter\n",
    "    def temperature(self, value):\n",
    "        if value < -273:\n",
    "            raise ValueError(\"Temperature below -273 is not possible\")\n",
    "        print(\"Setting value\")\n",
    "        self._temperature = value"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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