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283 lines
7.7 KiB
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283 lines
7.7 KiB
Plaintext
{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"![](http://www.upm.es/sfs/Rectorado/Gabinete%20del%20Rector/Logos/UPM/EscPolitecnica/EscUpmPolit_p.gif \"UPM\")"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Course Notes for Learning Intelligent Systems"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Department of Telematic Engineering Systems, Universidad Politécnica de Madrid, © 2016 Carlos A. Iglesias"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Classes\n",
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"\n",
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"In Python everthing is an object. Classes allow programmers to define their own object types.\n",
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"\n",
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"Classes are defined with the **class** reserved keyword."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Instance attributes and methods\n",
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"\n",
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"The first argument of instance class method is self, that refers to the current instance of the class.\n",
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"There is a special method, __init__ that initializes the object. It is like a constructor, but the object is already created when __init__ is called.\n",
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"\n",
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"Instance attributes are define as self.variables. (self is the same than this in Java)."
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"#Example class declaration\n",
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"class TV_Set:\n",
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" def __init__(self, name):\n",
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" self.name = name\n",
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" self.status = 'off'\n",
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"\n",
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" def on(self):\n",
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" self.status = 'on'\n",
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"\n",
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" def off(self):\n",
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" self.status = 'off'"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"#Example object instantiation\n",
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"\n",
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"my_tv = TV_Set('Samsung')\n",
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"print(my_tv, my_tv.status)\n",
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"type(my_tv)"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"# Call on method\n",
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"my_tv.on()\n",
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"print(my_tv.name, my_tv.status)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Class attributes\n",
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"We can also define class variables and class methods. \n",
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"\n",
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"Class variables are shared across all class instances. Class methods are called directly in the class. (In Java, this was defined with the keyword static)."
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"#Example class declaration\n",
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"class TV_Set:\n",
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" num_tvs = 0\n",
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" def __init__(self, name):\n",
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" self.name = name\n",
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" self.status = 'off'\n",
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" TV_Set.num_tvs += 1\n",
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"\n",
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" def on(self):\n",
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" self.status = 'on'\n",
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"\n",
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" def off(self):\n",
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" self.status = 'off'\n",
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"\n",
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"print(TV_Set.num_tvs)\n",
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"\n",
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"tv_1 = TV_Set('LG')\n",
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"print(TV_Set.num_tvs)\n",
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"\n",
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"tv_2 = TV_Set('Samsung')\n",
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"print(TV_Set.num_tvs)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Special or Magic methods\n",
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"\n",
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"[Special methods](https://docs.python.org/2/reference/datamodel.html#specialnames) allow programmers to customize a class. The convention for special methods is \\__method\\__.\n",
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"\n",
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"We have already seen a special method: \\__init\\__. Other special methods are \\__str\\__ (self) for printing) or \\__eq\\__(self, other) for comparing if two objects are equal."
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"class Person:\n",
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" def __init__(self, name, age):\n",
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" self.name = name\n",
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" self.age = age\n",
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" \n",
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"p = Person('Pedro', 10)\n",
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"p.age = 0\n",
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"print(p)"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"# Example __str(self)__\n",
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"\n",
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"class Person:\n",
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" def __init__(self, name, age):\n",
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" self.name = name\n",
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" self.age = age\n",
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" \n",
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" def __str__(self):\n",
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" return 'name: ' + self.name + ', ' + 'age: ' + str(self.age) # str() for converting int into string\n",
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" \n",
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"p = Person('Pedro', 10)\n",
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"p.age = 0\n",
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"print(p)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Private attributes and methods\n",
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"\n",
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"In Python, all methods and attributes are public. However, Python uses the following conventions\n",
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"\n",
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"* **Names prefixed by a single leading underscore**: (e.g. self._a, _\\_calculate()): private use. The statement import does not import objects whose name starts with an underscore. In fact, for class inheritance it has the semantics of *protected* (the subclass would have access to the method / attribute).\n",
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"\n",
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"* **Names prefixed by a double leading underscore** (e.g. \\_\\_calculate(), self.\\_\\_boo): used to avoid a method to be overridden by a subclass. The interpreter invokes name mangling, (inside class FooBar, \\_\\_boo becomes _FooBar\\__boo). This is not used as widely as a single leading underscore."
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Instead of Getters and Setters, Python uses Properties"
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"# Now we could change the age of Pedro to a negative value\n",
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"p.age = -1\n",
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"print(p)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"In Java, we would define age as private, and getters and setters. In Python, we can use **properties** when we want to control how they are accessed, without changing the interface."
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]
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},
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{
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"cell_type": "code",
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"execution_count": null,
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"metadata": {},
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"outputs": [],
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"source": [
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"class Person:\n",
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" def __init__(self, name, age):\n",
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" self.name = name\n",
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" self.age = age\n",
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" \n",
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" def __str__(self):\n",
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" return 'name: ' + self.name + ', ' + 'age: ' + str(self.age) # str() for converting int into string\n",
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" \n",
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" @property # property for getter, in this case it is not needed\n",
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" def age(self):\n",
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" return self._age \n",
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" \n",
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" @age.setter\n",
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" def age(self, val):\n",
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" if (val < 0):\n",
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" self._age = 0 # A better alternative would be to throw an exception: raise ValueError(\"Age < 0\")\n",
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" else:\n",
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" self._age = val\n",
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" \n",
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"p = Person('Pedro', -1)\n",
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"print(p)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Licence"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"The notebook is freely licensed under under the [Creative Commons Attribution Share-Alike license](https://creativecommons.org/licenses/by/2.0/). \n",
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"\n",
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"© 2016 Carlos A. Iglesias, Universidad Politécnica de Madrid."
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 3",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.5.1"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 0
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}
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