{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"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:"
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:07.534892Z",
"start_time": "2021-06-22T06:24:07.529034Z"
}
},
"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"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"We can make objects out of this class and manipulate the **`temperature`** attribute as we wish:"
]
},
{
"cell_type": "code",
"execution_count": 2,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:08.889417Z",
"start_time": "2021-06-22T06:24:08.873792Z"
}
},
"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."
]
},
{
"cell_type": "code",
"execution_count": 3,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:10.170211Z",
"start_time": "2021-06-22T06:24:10.144824Z"
},
"scrolled": true
},
"outputs": [
{
"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:"
]
},
{
"cell_type": "code",
"execution_count": 4,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:11.473954Z",
"start_time": "2021-06-22T06:24:11.465168Z"
}
},
"outputs": [],
"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:"
]
},
{
"cell_type": "code",
"execution_count": 5,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:13.263058Z",
"start_time": "2021-06-22T06:24:12.928094Z"
}
},
"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"
}
},
"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": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:18.774427Z",
"start_time": "2021-06-22T06:24:18.768568Z"
},
"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",
"execution_count": 9,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:19.924845Z",
"start_time": "2021-06-22T06:24:19.904340Z"
}
},
"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,
"metadata": {
"ExecuteTime": {
"end_time": "2021-06-22T06:24:21.069408Z",
"start_time": "2021-06-22T06:24:21.056712Z"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Setting value...\n"
]
}
],
"source": [
"human.temperature = 37"
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {
"ExecuteTime": {
"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"
}
],
"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": []
}
],
"metadata": {
"hide_input": false,
"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
"version": 3
},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython3",
"version": "3.8.8"
},
"toc": {
"base_numbering": 1,
"nav_menu": {},
"number_sections": true,
"sideBar": true,
"skip_h1_title": false,
"title_cell": "Table of Contents",
"title_sidebar": "Contents",
"toc_cell": false,
"toc_position": {},
"toc_section_display": true,
"toc_window_display": false
},
"varInspector": {
"cols": {
"lenName": 16,
"lenType": 16,
"lenVar": 40
},
"kernels_config": {
"python": {
"delete_cmd_postfix": "",
"delete_cmd_prefix": "del ",
"library": "var_list.py",
"varRefreshCmd": "print(var_dic_list())"
},
"r": {
"delete_cmd_postfix": ") ",
"delete_cmd_prefix": "rm(",
"library": "var_list.r",
"varRefreshCmd": "cat(var_dic_list()) "
}
},
"types_to_exclude": [
"module",
"function",
"builtin_function_or_method",
"instance",
"_Feature"
],
"window_display": false
}
},
"nbformat": 4,
"nbformat_minor": 4
}