learning python

Learning PythonPart 2 | Winter 2014 | Koosha Zarei | QIAU

Part 2Chapter 6 – Classes & Objects

3February 2014Learning Python | Part 2- Chapter 5 | Koosha Zarei

Objects and Classes

In Python, every piece of data you see or come into contact with is represented by an object.

Each of these objects has three components: an identity, a type, and a value.


Objects and Classes

The identity represents the location of the object being held in your memory.

While its type tells us what types of data and values it can have.

The value, meanwhile, can be changed in an object, but only if it is set as a mutable type.

if it is set as immutable, then it may not change.


Integers, strings, lists, and so forth are all nothing more than objects.

A class allows you to define and encapsulate a group of objects into one convenient space.

Class will enable you to think of entire objects that contain both data and functions.

Objects and Classes


>>> omelet_type = “Cheese”

>>> omelet_type.lower()‘cheese’>>> omelet_type.upper()‘CHEESE’

Objects and Classes


Also available are methods built into tuple, list, and dictionary objects, like the keys method of Dictionaries.

>>> fridge = {“cheese”:1, “tomato”:2, “milk”:4}>>> for x in fridge.keys():

print(x)

[‘tomato’, ‘cheese’, ‘milk’]

Objects and Classes


When you want to find out more about what is available in an object, Python exposes everything that exists in an object when you use the dir function

dir(fridge)[‘__class__’, ‘__contains__’, ‘__delattr__’, ‘__delitem__’, ‘__doc__’,‘__eq__’, ‘__format__’, ‘__ge__’, ‘__getattribute__’, ‘__getitem__’,‘__gt__’, ‘__hash__’, ‘__init__’, ‘__iter__’, ‘__le__’, ‘__len__’,‘__lt__’, ‘__ne__’, ‘__new__’, ‘__reduce__’, ‘__reduce_ex__’, ‘__repr__’‘__setattr__’, ‘__setitem__’, ‘__sizeof__’, ‘__str__’, ‘__subclasshook__’,‘clear’, ‘copy’, ‘fromkeys’, ‘get’, ‘items’, ‘keys’, ‘pop’, ‘popitem’,‘setdefault’, ‘update’, ‘values’]

Objects and Classes


Objects and Classes

When you have an object, you want to be able to use it naturally. For instance, once you’ve defined it, the Testclass class could produce objects that behave in a way that would feel natural when you read the source code.


Objects and Classes

In other words, when you create an object that models something from the real world, you can form your program’s objects and classes so they help the pieces of the program work in a way that someone familiar with the real-life object will recognize and be able to understand.


Objects and Classes

Python employs the concept of creating an entire class of code that acts as a placeholder.

The definition of a class is simple and mainly involves the use of the special word class along with a name.


Objects and Classes

class Fridge:“””This class implements a fridge where ingredients can be added and removed individually, or ingroups.”””


Objects and Classes

>>> f.items = {}>>> f.items[“mystery meat”] = 1


Objects and Classes

class Fridge:

def __init__(self, items={}):“””Optionally pass in an initial dictionary of items”””

if type(items) != type({}):raise TypeError(“Fridge requires a dictionary but was given %s” % type(items))

self.items = itemsreturn


Objects and Classes

When Python creates your object, the __init__ method is what passes the object its first parameter. The (self) portion is actually a variable used to represent the instance of the object.


Objects and Classes

For the Fridge class, it’s common to have a method that can operate on a group of data, and another method that works with just a single element.


Objects and Classes

Whenever you have this situation, you can save your effort by mak invoke the method that works on any number of items. In fact, sometimes it’s useful to have this method be considered private, or not a part of the interface.

This way it can be used or not used and changed without affecting how the class is used, because any changes you make will not be seen outside an object, only inside.


Objects and Classes

class Fridge:

# the docstringdef __add_multi(self, food_name, quantity):

# the docstring

if (not food_name in self.items):self.items[food_name] = 0

self.items[food_name] = self.items[food_name] + quantity


Objects and Classes

Now you have a way of adding any number of single food items to a Fridge object.

Now the add_one and the add_many methods can both be written to use it instead of you having to write similar functions two times.


Objects and Classes

def add_one(self, food_name):

#docstring

if type(food_name) != type(“”):raise TypeError, “add_one requires a string, given a %s” % type(food_name)

else:self.__add_multi(food_name, 1)

return True


Objects and Classes

def add_many(self, food_dict):

if type(food_dict) != type({}):raise TypeError(“add_many requires a dictionary, got a %s” %food_dict)

for item in food_dict.keys():self.__add_multi(item, food_dict[item])

return


Objects and Classes

add_one and add_many each serve similar purposes, and each one has the code to ensure that it is being used appropriately. At the same time, they both use __add_multi to actually do the heavy lifting. If anything changes regarding how your class works inside of __add_multi, you will save time because it will change how both of these methods behave.


Objects and Classes

>>> f = Fridge({“eggs”:6, “milk”:4, “cheese”:3})>>> f.items{‘cheese’: 3, ‘eggs’: 6, ‘milk’: 4}>>> f.add_one(“grape”)True>>> f.items{‘cheese’: 3, ‘eggs’: 6, ‘grape’: 1, ‘milk’: 4}>>> f.add_many({“mushroom”:5, “tomato”:3})>>> f.items{‘tomato’: 3, ‘cheese’: 3, ‘grape’: 1, ‘mushroom’: 5, ‘eggs’: 6, ‘milk’: 4}


Objects and Classes

We will want to add are the methods that enable you to determine whether something is in the Fridge.


Objects and Classes

def has_various(self, foods):

try:for food in foods.keys():

if self.items[food] < foods[food]:return False

return True

except KeyError:return False


Objects and Classes

def has(self, food_name, quantity=1):

return self.has_various( {food_name:quantity} )


Objects and Classes

>>> f = Fridge({“eggs”:6, “milk”:4, “cheese”:3})>>> if f.has(“cheese”, 2):… print(“It’s time to make an omelet!”)...

It’s time to make an omelet!


Objects and Classesdef __get_multi(self, food_name, quantity):

try:if (self.items[food_name] is None):

return False;

if (quantity > self.items[food_name]):return False;

self.items[food_name] = self.items[food_name] - quantityexcept KeyError:

return False

return quantity


Objects and Classes

def get_one(self, food_name):

if type(food_name) != type(“”):raise TypeError(“get_one requires a string, given a %s” %type(food_name))

else:result = self.__get_multi(food_name, 1)

return result


Objects and Classes

def get_many(self, food_dict):

if self.has_various(food_dict):foods_removed = {}

for item in food_dict.keys():foods_removed[item] = self.__get_multi(item, food_dict[item])

return foods_removed

learning python - part 2 - chapter 6

Software

classes objects

classes python

koosha zarei objects

dictionary objects

values objects

milk objects

group of objects