005_Python_Property.ipynb

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  {
   "cell_type": "markdown",
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   "source": [
    "<small><small><i>\n",
    "All the IPython Notebooks in **Python Advanced Topics** lecture series by Dr. Milaan Parmar are available @ **[GitHub](https://github.com/milaan9/07_Python_Advanced_Topics)**\n",
    "</i></small></small>"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Python `@property` decorator\n",
    "\n",
    "In this class, you will learn about Python **`@property`** decorator; a pythonic way to use getters and setters in object-oriented programming.\n",
    "\n",
    "Python programming provides us with a built-in **`@property`** decorator which makes usage of getter and setters much easier in Object-Oriented Programming.\n",
    "\n",
    "Before going into details on what **`@property`** decorator is, let us first build an intuition on why it would be needed in the first place."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Class Without Getters and Setters\n",
    "\n",
    "Let us assume that we decide to make a **[class](https://github.com/milaan9/06_Python_Object_Class/blob/main/002_Python_Classes_and_Objects.ipynb)** that stores the temperature in degrees Celsius. It would also implement a method to convert the temperature into degrees Fahrenheit. One way of doing this is as follows:"
   ]
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   "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"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We can make objects out of this class and manipulate the **`temperature`** attribute as we wish:"
   ]
  },
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   "execution_count": 2,
   "metadata": {
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "37\n",
      "98.60000000000001\n"
     ]
    }
   ],
   "source": [
    "# Basic method of setting and getting attributes in Python\n",
    "\n",
    "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",
    "\n",
    "# Create a new object\n",
    "human = Celsius()\n",
    "\n",
    "# Set the temperature\n",
    "human.temperature = 37\n",
    "\n",
    "# Get the temperature attribute\n",
    "print(human.temperature)\n",
    "\n",
    "# Get the to_fahrenheit method\n",
    "print(human.to_fahrenheit())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The extra decimal places when converting into Fahrenheit is due to the floating point arithmetic error. To learn more, visit **[Python Floating Point Arithmetic Error](https://github.com/milaan9/02_Python_Datatypes/blob/main/001_Python_Numbers.ipynb)**.\n",
    "\n",
    "Whenever we assign or retrieve any object attribute like **`temperature`** as shown above, Python searches it in the object's built-in **`__dict__`** dictionary attribute."
   ]
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    {
     "data": {
      "text/plain": [
       "{'temperature': 37}"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "human.__dict__"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Therefore, **`man.temperature`** internally becomes **`man.__dict__['temperature']`**."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Using Getters and Setters\n",
    "\n",
    "Suppose we want to extend the usability of the **`Celsius`** class defined above. We know that the temperature of any object cannot reach below -273.15 degrees Celsius (Absolute Zero in Thermodynamics)\n",
    "\n",
    "Let's update our code to implement this value constraint.\n",
    "\n",
    "An obvious solution to the above restriction will be to hide the attribute **`temperature`** (make it private) and define new getter and setter methods to manipulate it. This can be done as follows:"
   ]
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   "source": [
    "# Making Getters and Setter methods\n",
    "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",
    "    # getter method\n",
    "    def get_temperature(self):\n",
    "        return self._temperature\n",
    "\n",
    "    # setter method\n",
    "    def set_temperature(self, value):\n",
    "        if value < -273.15:\n",
    "            raise ValueError(\"Temperature below -273.15 is not possible.\")\n",
    "        self._temperature = value"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "As we can see, the above method introduces two new **`get_temperature()`** and **`set_temperature()`** methods.\n",
    "\n",
    "Furthermore, **`temperature`** was replaced with **`_temperature`**. An underscore **`_`** at the beginning is used to denote private variables in Python."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now, let's use this implementation:"
   ]
  },
  {
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "37\n",
      "98.60000000000001\n"
     ]
    },
    {
     "ename": "ValueError",
     "evalue": "Temperature below -273.15 is not possible.",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-5-f7d40ce0c112>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m     28\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     29\u001b[0m \u001b[1;31m# new constraint implementation\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 30\u001b[1;33m \u001b[0mhuman\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mset_temperature\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m-\u001b[0m\u001b[1;36m300\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m     31\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     32\u001b[0m \u001b[1;31m# Get the to_fahreheit method\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;32m<ipython-input-5-f7d40ce0c112>\u001b[0m in \u001b[0;36mset_temperature\u001b[1;34m(self, value)\u001b[0m\n\u001b[0;32m     14\u001b[0m     \u001b[1;32mdef\u001b[0m \u001b[0mset_temperature\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mself\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0mvalue\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     15\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mvalue\u001b[0m \u001b[1;33m<\u001b[0m \u001b[1;33m-\u001b[0m\u001b[1;36m273.15\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 16\u001b[1;33m             \u001b[1;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Temperature below -273.15 is not possible.\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m     17\u001b[0m         \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_temperature\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mvalue\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     18\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mValueError\u001b[0m: Temperature below -273.15 is not possible."
     ]
    }
   ],
   "source": [
    "# Making Getters and Setter methods\n",
    "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",
    "    # getter method\n",
    "    def get_temperature(self):\n",
    "        return self._temperature\n",
    "\n",
    "    # setter method\n",
    "    def set_temperature(self, value):\n",
    "        if value < -273.15:\n",
    "            raise ValueError(\"Temperature below -273.15 is not possible.\")\n",
    "        self._temperature = value\n",
    "\n",
    "\n",
    "# Create a new object, set_temperature() internally called by __init__\n",
    "human = Celsius(37)\n",
    "\n",
    "# Get the temperature attribute via a getter\n",
    "print(human.get_temperature())\n",
    "\n",
    "# Get the to_fahrenheit method, get_temperature() called by the method itself\n",
    "print(human.to_fahrenheit())\n",
    "\n",
    "# new constraint implementation\n",
    "human.set_temperature(-300)\n",
    "\n",
    "# Get the to_fahreheit method\n",
    "print(human.to_fahrenheit())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This update successfully implemented the new restriction. We are no longer allowed to set the temperature below -273.15 degrees Celsius.\n",
    "\n",
    ">**Note**: The private variables don't actually exist in Python. There are simply norms to be followed. The language itself doesn't apply any restrictions.\n",
    "```python\n",
    ">>> human._temperature = -300\n",
    ">>> human.get_temperature()\n",
    "-300\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "However, the bigger problem with the above update is that all the programs that implemented our previous class have to modify their code from **`obj.temperature`** to **`obj.get_temperature()`** and all expressions like **`obj.temperature = val`** to **`obj.set_temperature(val)`**.\n",
    "\n",
    "This refactoring can cause problems while dealing with hundreds of thousands of lines of codes.\n",
    "\n",
    "All in all, our new update was not backwards compatible. This is where **`@property`** comes to rescue."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## The property Class\n",
    "\n",
    "A pythonic way to deal with the above problem is to use the property class. Here is how we can update our code:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2021-06-22T06:24:15.200609Z",
     "start_time": "2021-06-22T06:24:15.184983Z"
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   },
   "outputs": [],
   "source": [
    "# using property class\n",
    "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",
    "    # getter\n",
    "    def get_temperature(self):\n",
    "        print(\"Getting value...\")\n",
    "        return self._temperature\n",
    "\n",
    "    # setter\n",
    "    def set_temperature(self, value):\n",
    "        print(\"Setting value...\")\n",
    "        if value < -273.15:\n",
    "            raise ValueError(\"Temperature below -273.15 is not possible\")\n",
    "        self._temperature = value\n",
    "\n",
    "    # creating a property object\n",
    "    temperature = property(get_temperature, set_temperature)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "We added a **`print()`** function inside **`get_temperature()`** and **`set_temperature()`** to clearly observe that they are being executed.\n",
    "\n",
    "The last line of the code makes a property object `temperature`. Simply put, property attaches some code (**`get_temperature`** and **`set_temperature`**) to the member attribute accesses (**`temperature`**)."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Let's use this update code:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2021-06-22T06:24:17.212379Z",
     "start_time": "2021-06-22T06:24:17.186011Z"
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Setting value...\n",
      "Getting value...\n",
      "37\n",
      "Getting value...\n",
      "98.60000000000001\n",
      "Setting value...\n"
     ]
    },
    {
     "ename": "ValueError",
     "evalue": "Temperature below -273.15 is not possible",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-7-dd6f749658a4>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m     29\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mhuman\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mto_fahrenheit\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     30\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 31\u001b[1;33m \u001b[0mhuman\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mtemperature\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m-\u001b[0m\u001b[1;36m300\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-7-dd6f749658a4>\u001b[0m in \u001b[0;36mset_temperature\u001b[1;34m(self, value)\u001b[0m\n\u001b[0;32m     16\u001b[0m         \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Setting value...\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     17\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mvalue\u001b[0m \u001b[1;33m<\u001b[0m \u001b[1;33m-\u001b[0m\u001b[1;36m273.15\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 18\u001b[1;33m             \u001b[1;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Temperature below -273.15 is not possible\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m     19\u001b[0m         \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_temperature\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mvalue\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     20\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mValueError\u001b[0m: Temperature below -273.15 is not possible"
     ]
    }
   ],
   "source": [
    "# using property class\n",
    "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",
    "    # getter\n",
    "    def get_temperature(self):\n",
    "        print(\"Getting value...\")\n",
    "        return self._temperature\n",
    "\n",
    "    # setter\n",
    "    def set_temperature(self, value):\n",
    "        print(\"Setting value...\")\n",
    "        if value < -273.15:\n",
    "            raise ValueError(\"Temperature below -273.15 is not possible\")\n",
    "        self._temperature = value\n",
    "\n",
    "    # creating a property object\n",
    "    temperature = property(get_temperature, set_temperature)\n",
    "\n",
    "\n",
    "human = Celsius(37)\n",
    "\n",
    "print(human.temperature)\n",
    "\n",
    "print(human.to_fahrenheit())\n",
    "\n",
    "human.temperature = -300"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "As we can see, any code that retrieves the value of **`temperature`** will automatically call **`get_temperature()`** instead of a dictionary (**`__dict__`**) look-up. Similarly, any code that assigns a value to **`temperature`** will automatically call **`set_temperature()`**.\n",
    "\n",
    "We can even see above that **`set_temperature()`** was called even when we created an object."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
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     "end_time": "2021-06-22T06:24:18.774427Z",
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    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Setting value...\n"
     ]
    }
   ],
   "source": [
    "human = Celsius(37)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Can you guess why?**\n",
    "\n",
    "The reason is that when an object is created, the **`__init__()`** method gets called. This method has the line **`self.temperature = temperature`**. This expression automatically calls **`set_temperature()`**.\n",
    "\n",
    "Similarly, any access like **`c.temperature`** automatically calls **`get_temperature()`**. This is what property does. Here are a few more examples."
   ]
  },
  {
   "cell_type": "code",
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   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Getting value...\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "37"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "human.temperature"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
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     "end_time": "2021-06-22T06:24:21.069408Z",
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Setting value...\n"
     ]
    }
   ],
   "source": [
    "human.temperature = 37"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
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     "end_time": "2021-06-22T06:24:21.877044Z",
     "start_time": "2021-06-22T06:24:21.856538Z"
    }
   },
   "outputs": [
    {
     "ename": "NameError",
     "evalue": "name 'c' is not defined",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-11-2087a9c41308>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0mc\u001b[0m\u001b[1;33m.\u001b[0m \u001b[0mto_fahrenheit\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m: name 'c' is not defined"
     ]
    }
   ],
   "source": [
    "c. to_fahrenheit()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "By using property, we can see that no modification is required in the implementation of the value constraint. Thus, our implementation is backward compatible.\n",
    "\n",
    ">**Note**: The actual temperature value is stored in the private **`_temperature`** variable. The **`temperature`** attribute is a property object which provides an interface to this private variable."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## The `@property` Decorator\n",
    "\n",
    "In Python, **`property()`** is a built-in function that creates and returns a **`property`** object. The syntax of this function is:\n",
    "\n",
    "```python\n",
    "property(fget=None, fset=None, fdel=None, doc=None)\n",
    "```\n",
    "\n",
    "where,\n",
    "\n",
    "* **`fget`** is function to get value of the attribute\n",
    "* **`fset`** is function to set value of the attribute\n",
    "* **`fdel`** is function to delete the attribute\n",
    "* **`doc`** is a string (like a comment)\n",
    "\n",
    "As seen from the implementation, these function arguments are optional. So, a property object can simply be created as follows."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2021-06-22T06:24:24.203272Z",
     "start_time": "2021-06-22T06:24:24.192533Z"
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<property at 0x170b84f24a0>"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
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   "source": [
    "property()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A property object has three methods, `getter()`, `setter()`, and `deleter()` to specify `fget`, `fset` and `fdel` at a later point. This means, the line:\n",
    "\n",
    "```python\n",
    "temperature = property(get_temperature,set_temperature)\n",
    "```\n",
    "\n",
    "can be broken down as:\n",
    "\n",
    "```python\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",
    "```\n",
    "\n",
    "These two pieces of codes are equivalent.\n",
    "\n",
    "Programmers familiar with **[Python Decorators](https://github.com/milaan9/07_Python_Advanced_Topics/blob/main/004_Python_Decorators.ipynb)** can recognize that the above construct can be implemented as decorators.\n",
    "\n",
    "We can even not define the names **`get_temperature`** and **`set_temperature`** as they are unnecessary and pollute the class namespace."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "For this, we reuse the **`temperature`** name while defining our getter and setter functions. Let's look at how to implement this as a decorator:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "ExecuteTime": {
     "end_time": "2021-06-22T06:24:26.264846Z",
     "start_time": "2021-06-22T06:24:26.225786Z"
    },
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Setting value...\n",
      "Getting value...\n",
      "37\n",
      "Getting value...\n",
      "98.60000000000001\n",
      "Setting value...\n"
     ]
    },
    {
     "ename": "ValueError",
     "evalue": "Temperature below -273 is not possible",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-13-57953b0b580c>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m     28\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mhuman\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mto_fahrenheit\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     29\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 30\u001b[1;33m \u001b[0mcoldest_thing\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mCelsius\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m-\u001b[0m\u001b[1;36m300\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-13-57953b0b580c>\u001b[0m in \u001b[0;36m__init__\u001b[1;34m(self, temperature)\u001b[0m\n\u001b[0;32m      3\u001b[0m \u001b[1;32mclass\u001b[0m \u001b[0mCelsius\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      4\u001b[0m     \u001b[1;32mdef\u001b[0m \u001b[0m__init__\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mself\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0mtemperature\u001b[0m\u001b[1;33m=\u001b[0m\u001b[1;36m0\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 5\u001b[1;33m         \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mtemperature\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mtemperature\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      6\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      7\u001b[0m     \u001b[1;32mdef\u001b[0m \u001b[0mto_fahrenheit\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mself\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;32m<ipython-input-13-57953b0b580c>\u001b[0m in \u001b[0;36mtemperature\u001b[1;34m(self, value)\u001b[0m\n\u001b[0;32m     17\u001b[0m         \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Setting value...\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     18\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mvalue\u001b[0m \u001b[1;33m<\u001b[0m \u001b[1;33m-\u001b[0m\u001b[1;36m273.15\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 19\u001b[1;33m             \u001b[1;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Temperature below -273 is not possible\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m     20\u001b[0m         \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_temperature\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mvalue\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     21\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mValueError\u001b[0m: Temperature below -273 is not possible"
     ]
    }
   ],
   "source": [
    "# Using @property decorator\n",
    "\n",
    "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",
    "        print(\"Setting value...\")\n",
    "        if value < -273.15:\n",
    "            raise ValueError(\"Temperature below -273 is not possible\")\n",
    "        self._temperature = value\n",
    "\n",
    "\n",
    "# create an object\n",
    "human = Celsius(37)\n",
    "\n",
    "print(human.temperature)\n",
    "\n",
    "print(human.to_fahrenheit())\n",
    "\n",
    "coldest_thing = Celsius(-300)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The above implementation is simple and efficient. It is the recommended way to use **`property`**."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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