slides11-objects_and_classes in python.pptx

Copyright © 2015 Pearson Education, Inc.
C H A P T E R 10
Classes and
Object-
Oriented
Programming

Procedural Programming
• Procedures: synonym for functions and
sub-routines
• Procedural programming: writing
programs made of functions that
perform specific tasks
• Functions typically operate on data items that
are separate from the functions
• Data items commonly passed from one
function to another
• Focus: On the algorithm and steps. Create
functions that operate on the program’s data

Object-Oriented Programming
• Object-oriented programming: focused on
creating classes and objects
• Model the problem on the data involved first,
not the big steps.
• Class: A programmer defined data type
• Object: entity that contains data and
functions
• Data is known as data attributes and functions are
known as methods
• Methods perform operations on the data attributes
• Encapsulation: combining data and code into
a single object

Object Oriented Programming
• Recall a CPU only knows how to
perform on the order of 100 operations
• High level languages such as Python
allow us to create new operations by
defining new functions
• Object oriented languages allow
programmers to create new data types
in addition to the ones built into the
language
• int, float, string, list, tuple, file, dictionary, set

5
Object Oriented Design
Example - Monopoly
If we had to start
from scratch what
new data types would
we need to create?
Data Types Needed:

Object Orientation
•The basic idea of object oriented programming (OOP) is
to view your problem as a collection of objects, each of
which has certain state and can perform certain actions.
•Each object has:
• some data that it maintains characterizing its
current state;
• a set of actions (methods) that it can perform.
•A programmer interacts with an object by calling its
methods; this is called method invocation. That should be
the only way that another programmer interacts with an
object.
•Significant object-oriented languages include Python,
Java, C++, C#, Perl, JavaScript, Objective C, and others.

(cont’d.)

(cont’d.)
• Data hiding: object’s data attributes are
hidden from code outside the object
• Access restricted to the object’s methods
• Protects from accidental corruption
• Outside code does not need to know internal
structure of the object
• Object reusability: the same object can
be used in different programs
• Example: 3D image object can be used for
architecture and game programming

An Everyday Example of an
Object
• Data attributes: define the state of an
object
• Example: clock object would have second,
minute, and hour data attributes
• Public methods: allow external code to
manipulate the object
• Example: set_time, set_alarm_time
• Private methods: used for object’s
inner workings

Classes
• Class: code that specifies the data
attributes and methods of a particular
type of object
• Similar to a blueprint of a house or a cookie
cutter
• Instance: an object created from a
class
• Similar to a specific house built according to
the blueprint or a specific cookie
• There can be many instances of one class

Classes

Classes
Class Definition for Playing cards
Playing cards have:
A Rank
A Suit
Define a
PlayingCard
class and then
create objects of
type PlayingCard
to form a deck
or a hand
of cards.

Another Concrete Example
•Imagine that you’re trying to do some
simple arithmetic. You need a Calculator
application, programmed in an OO manner.
It will have:
•Some data: the current value of its
• accumulator (the value stored
and displayed on the screen).
• History of ops?
• Memory?
•Some methods: things that you can
ask of the calculator to do:
• add a number to the accumulator,
subtract a number, multiply by a
number, divide by a number, zero
out the accumulator value, etc.

Calculator Specification
•In Python, you implement a particular type of object (soda
machine, calculator, etc.) with a class.
•Let’s define a class for our simple interactive calculator.
•Data: the current value of the accumulator.
Maybe a history of operations? Memory spots?
• Methods: any of the following.
• clear: zero the accumulator
• print: display the accumulator value
• add k: add k to the accumulator
• sub k: subtract k from the accumulator
• mult k: multiply accumulator by k
• div k: divide accumulator by k

A More Concrete Example
•Example: A soda machine has:
• Data: products inside,
change available, amount
previously deposited, etc.
• Methods: accept a coin,
select a product, dispense a
soda, provide change after
purchase, return money
deposited, etc.
• Assignment 13

Class Definitions
• Class definition: set of statements that
define a class’s methods and data attributes
• Format: begin with class ClassName:
• Class names typically start with uppercase letter and inter
nal words are capitalized, aka CamelCase
• Method definition like other Python
function definitions
• self parameter: required in every method in the class –
references the specific object that the method is working
on - The object the method is working on. The object
that called the method
name = 'Olivia'
name.upper() # name is the argument to self

Class Definitions (cont’d.)
• Initializer method: automatically executed
when an instance of the class is created
• Initializes object’s data attributes and assigns
self parameter to the object that was just
created.
• Format: def __init__ (self):
• That's two underscores before and after init.
• Typically the first method in a class definition.

• To create a new instance of a class call
the initializer method
• Format: my_instance = ClassName()
• To call any of the class methods using
the created instance, use dot notation
• Format: my_instance.method()
• Because the self parameter references the
specific instance of the object, the method will
affect this instance
• Reference to self is passed automatically

Hiding Attributes and Storing
Classes in Modules
• An object’s data attributes (aka the internal
variables) should be difficult to access
• To make sure of this, place two underscores (__) in
front of attribute name
• Example: __current_minute
• Classes can be stored in modules
• Filename for module must end in .py
• Module can be imported to programs that use the
class

The Circle Class - in Circle.py

Client Code of Circle Class
• Recall, variables prefixed with the
double underscore (_ _) are hidden
from clients.
• Careful, easy to create logic errors

Logic Error in Client Code
• Clients can add attributes (internal
data, internal variables) to objects
• Flexible? Yes. Dangerous? You bet!

The BankAccount Class –
More About Classes
• Class methods can have multiple
parameters in addition to self
• For __init__, parameters needed to create
an instance of the class
• Example: a BankAccount object is created with a
balance
• When called, the initializer method receives a value to be
assigned to a __balance attribute
• For other methods, parameters may be
needed to perform required task
• Example: deposit method amount to be deposited

The __str__ method
• Object’s state: the values of the object’s
attribute at a given moment
• __str__ method: return a string version
of the object, typically the state of its
internal data
• Automatically called when the object is
passed as an argument to the
print function
• Automatically called when the object is
passed as an argument to the str function

Working With Instances
• Instance attribute: belongs to a specific
instance of a class
• Created when a method uses the self
parameter to create an attribute
• Can be local to a method, but continues to
exist after that method completes
• If many instances of a class are
created, each would has its own set of
attributes

Accessor and Mutator Methods
• Typically, all of a class’s data attributes
are private and provide methods to
access and change them
• Accessor methods: return a value from
a class’s attribute without changing it
• Safe way for code outside the class to retrieve
the value of attributes
• Mutator methods: store or change the
value of a data attribute
• You DO NOT have to have mutator methods
for all (or any) internal attributes

Passing Objects as
Arguments
• Methods and functions often need to
accept objects as arguments
• When you pass an object as an
argument, you are actually passing a
reference to the object
• The receiving method or function has access
to the actual object
• Methods of the object can be called within the
receiving function or method, and data attributes
may be changed using mutator methods

Other methods
• generally methods with the _ _name_ _
format are not meant to be called directly
• Instead we define them and then the are
called with other operators
_ _init_ _ ClassName()
_ _len_ _ len()
_ _str_ _ str
_ _add_ _ + _ _eq_ _ ==
_ _lt_ _ < _ _le_ _ <=
_ _gt_ _ > _ _ge_ _ >=

Displaying New Classes in
Data Structures
Output of
print. Great!
Output of
print of list. Yuck!

_ _str_ _ and _ _ repr_ _
• print calls the _ _str_ _ method on
objects sent to it
• a data structure calls the _ _repr_ _
method on the objects inside it to
• repr for representation
• Like _ _str_ _ but should display the
object in a way that we could use to
rebuild the object

_ _repr_ _ method for Circle

Techniques for Designing
Classes
• UML diagram: standard diagrams for
graphically depicting object-oriented
systems
• Stands for Unified Modeling Language
• General layout: box divided into three
sections:
• Top section: name of the class
• Middle section: list of data attributes
• Bottom section: list of class methods

Finding the Classes in a
Problem
• When developing object oriented
program, first goal is to identify classes
• Typically involves identifying the real-world
objects that are in the problem
• Technique for identifying classes:
1. Get written description of the problem domain
2. Identify all nouns in the description, each of
which is a potential class
3. Refine the list to include only classes that are
relevant to the problem

Problem (cont’d.)
1. Get written description of the problem
domain
• May be written by you or by an expert
• Should include any or all of the following:
• Physical objects simulated by the program
• The role played by a person
• The result of a business event
• Recordkeeping items

Problem (cont’d.)
2. Identify all nouns in the description,
each of which is a potential class
• Should include noun phrases and pronouns
• Some nouns may appear twice

Problem (cont’d.)
3. Refine the list to include only classes
that are relevant to the problem
• Remove nouns that mean the same thing
• Remove nouns that represent items that the
program does not need to be concerned with
• Remove nouns that represent objects, not
classes
• Remove nouns that represent simple values
that can be assigned to a variable

Identifying a Class’s
Responsibilities
• A classes responsibilities are:
• The things the class is responsible for
knowing
• Identifying these helps identify the class’s data
attributes
• The actions the class is responsible for doing
• Identifying these helps identify the class’s methods
• To find out a class’s responsibilities
look at the problem domain
• Deduce required information and actions

Summary
• This chapter covered:
• Procedural vs. object-oriented programming
• Classes and instances
• Class definitions, including:
• The self parameter
• Data attributes and methods
• __init__ and __str__ functions
• Hiding attributes from code outside a class
• Storing classes in modules
• Designing classes

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