fundamentals of programming (python) object-oriented...

Fundamentals of Programming(Python)

Object-Oriented Programming

Ali TaheriSharif University of Technology

Spring 2019

Outline1. Python Data Types

2. Classes and Objects

3. Defining Classes

4. Working with Objects

5. Customizing Initializer

6. Adding Methods

7. Special Methods

8. Composition

9. Mutability

10.Sameness

11.Copying

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Python Data TypesSo far we have seen several data types in Python

Some of these data types are primitive data types◦ Integer, Float, String, Boolean

Some of them are more complicated◦ List, Dictionary, Set, File

In Python, we can build our own data types◦ Using the other available types

◦ This is the subject of Object-Oriented programming

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Classes and ObjectsAn object is simply a variable

A Class is simply a data type of an object

We have been using objects!◦ Just didn’t call them objects

For example: str is a data type (or class)◦ We created objects of type (class) string

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animal = "cow"

fruit = "apple"

Classes and ObjectsObject vs Variable◦ Objects may contain variables, called attributes

◦ Objects may contain methods

Class vs Type◦ Programmer can define his own classes

◦ Attributes and methods are defined within class

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Defining ClassesClass header ◦ Keyword class begins definition

◦ Followed by name of class and colon ( : )

Initializer method __init__◦ Executes each time an object is created

◦ Initializes attributes of class

Object reference◦ All methods must at least specify this one parameter

◦ Represents object of class from which a method is called

◦ Called self by convention

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Defining Classes

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Working with ObjectsInstantiating objects:

Accessing objects’ attributes:

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p = Point()

q = Point()

p.x = 3

q.x = p.y + 5

q.y = p.x * q.x

print(p.x, p.y, q.x, q.y)

Customizing Initializer

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Adding Methods

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Adding Methods

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class Point:

def __init__(self, x=0, y=0):

self.x = x

self.y = y

def distance_from_origin(self):

return ((self.x**2) + (self.y**2)) ** 0.5

def distance(self, other):

dx = self.x - other.x

dy = self.y - other.y

return (dx**2 + dx**2) ** 0.5

p = Point(2, 6)

q = Point(-1, 4)

d = p.distance(q)

print(d)

Special Methods

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Operator Method Description

+ __add__(self, other) Addition

- __sub__(self, other) Subtraction

* __mul__(self, other) Multiplication

/ __truediv__(self, other) Division

< __lt__(self, other) Less than

<= __le__(self, other) Less that or equal

> __gt__(self, other) Greater than

>= __ge__(self, other) Greater than or equal

== __eq__(self, other) Equal to

!= __ne__(self, other) Not equal to

Special Methods

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Operator Method Description

[index] __getitem__(self, index) Index operator

in __contains__(self, value) Check membership

len __len__(self) The number of elements

str __str__(self) The string representation

Special Methods

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class Point:

def __init__(self, x=0, y=0):

self.x = x

self.y = y

def __str__(self):

return '(%d,%d)' % (self.x, self.y)

def __getitem__(self, index):

if index == 0:

return self.x

elif index == 1:

return self.y

else:

raise IndexError('Point index out of range.')

>>> p = Point(2,3)

>>> print(p)

(2,3)

>>> print(p[1])

3

Composition

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Mutability

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SamenessWhen dealing with objects, the concept of sameness can be ambiguous◦ Do the two objects contain the same data?

◦ Do they represent the same object?

Shallow Equality◦ Only compares the references, not the contents of the objects

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SamenessThe equality operator (==) performs shallow equality check by default on user-defined classes◦ Unless the __eq__() method is implemented

Deep Equality◦ Compares the contents of the objects

◦ Must be implemented manually through __eq__() or another custom method

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>>> p = Point(2,3)

>>> q = Point(2,3)

>>> p == q

False

SamenessDeep Equality

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>>> p = Point(2,3)

>>> q = Point(2,3)

>>> p == q

True

class Point:

def __init__(self, x=0, y=0):

self.x = x

self.y = y

def __eq__(self, other):

return (self.x == other.x and

self.y == other.y)

CopyingThe assignment operator make aliases◦ Changing one results in changing the other

◦ What if we want separate duplicates?

We can copy objects using the copy module◦ Shallow copy

◦ Deep copy

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>>> p = Point(2,3)

>>> q = p

>>> p is q

True

CopyingThe assignment operator make aliases◦ Changing one results in changing the other

◦ What if we want separate duplicates?

We can copy objects using the copy module◦ Shallow copy

◦ Deep copy

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>>> p = Point(2,3)

>>> q = p

>>> q.x = 5

>>> print(p)

(5,3)

CopyingShallow Copy◦ Copies the object, but doesn’t copy the embedded objects unless

they are immutable

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CopyingDeep Copy◦ Copies the object, and recursively copies the embedded objects

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class Rectangle:

""" A class to manufacture rectangle objects """

def __init__(self, corner, w, h):

self.corner = corner

self.width = w

self.height = h

def __eq__(self, other):

return (self.corner == other.corner and

self.width == other.width and

self.height == other.height)

CopyingDeep Copy◦ Copies the object, and recursively copies the embedded objects

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>>> import copy

>>> r = Rectangle(Point(1,4), 7, 2)

>>> s = copy.copy(r)

>>> r == s

True

>>> s.corner.x = 5

>>> r == s

True

>>> t = copy.deepcopy(r)

>>> r == t

True

>>> t.corner.x = 6

>>> r == t

False