Section 16: Object-Oriented Programming

Question 1

Define a type:

Answer 1

• Helps regrouping different variables or different types to define our own type
• Part of Object-Oriented Programming

Question 2

Define

Object:

Answer 2

> An object is a collection of data and set of methods can be provided to work with it

• Python: object-oriented language.
  Means that it uses objects to represent data and provided methods related to them
• OOP way to use programmer-defined data types to organize code and data

OOP: object oriented programming

Question 3

How to define your own type in Python?

Answer 3

In Python: can define own data type

• Combine related pieces of information with each other into one variable

Programmer-defined type is also called a class

Question 4

Defining your own type:

What is a class?

Answer 4

> Programmer-defined type is also called a class

class keyword defined in Python

• When define a class, you can actually defining new type

Known classes: int, str, list, dict

```python
»> int
<class ‘int’>
~~~

Question 5

How are objects related to a newly defined class?

Answer 5

When you define a new class we create a new object type with the same name

• A class is a blueprint/template for a type of objectSpecifies attributes and what methods can operate on them
• An object is an instance of some class

Question 6

How is a new class usually defined?

Answer 6

Generally …

Define class using keyword class as follows:

```python
class MyNewType:
‘’’ a new data type ‘’’
~~~

• Class: MyNewType
• Full name of the type: \_\_main\_\_.MyNewType

Question 7

What is instantiation of a new object?

Answer 7

Creating a new object is called instantiation, we say the object is an instance of the class

Call the class as if it were a function

```python
my_object = MyNewType()
~~~

Question 8

How to add attributes to an object:

Answer 8

• Create variables that belong to specific object referenced by student_Bob
• Attributes only access through objects

Assign values to an object using the dot notation

Question 9

Constructors

What are methods?

Answer 9

Define functions associated with the class they will be defined within the class definition → methods

Question 10

What is a Constructor?

Answer 10

Constructor: (initalizer method) a method that creates a new object from the class

Constructor: special method when defining a class helps create a new object of that class → \_\_init\_\_

Question 11

\_\_init\_\_ Constructor (Initalizer Method)

What is the \_\_init\_\_ constructor?

Answer 11

• Constructor is a special method named \_\_init\_\_
  
  • Involved automatically when object is created
• Used to define attributes of a class t oset initial value

```python
def__init__(self):
~~~

• self refers to the object being initalized, not a keyword
• Can have a default value:

- Rename (constructor) `create_student` as `init` - add another input parameter `self` that refers to the object that is being initialized - Call the `init` constructor and provide parameters The first input argument `self` corresponds with the new object being created

Question 12

Class Attributes

Instance Attributes

Answer 12

Instance Attributes: Create variables store data specific to an object

• Access through variables through instance (instance = an object of a class)

Question 13

What are Class Attributes?

Answer 13

Class Attributes:

• Create variable store data related to entire class
• Instance shares the same class attributes

Define class attributes inside class → outside all methods

• Convention: place vars at top, below class header
• Access class attributes through instance and through class name (only modify through class name)

Question 14

Define OOP methods:

Answer 14

> Define functions associated with the class they will be defined within the class definition → methods

• Can define methods the say way as defined functions inside a module

Type:

1. Instance methods
2. Class methods
3. Static Methods

Question 15

Methods

What are the Instance Methods

and the self?

Answer 15

• Calling a method on an object is equivalent to passing that object as input to the method implicity
• TO indicate this, first argumenet of every instance method is always a reference to an instance of the class

Convention: name self (not a keyword)

Question 16

The \_\_str\_\_ method

What is the relation between the str() and print() functions?

Answer 16

• Use function str to convert object of built-in types into string
• Use function print with an object, interally function str is called

Question 17

What is the \_\_str\_\_ method?

Answer 17

The \_\_str\_\_ method:

• change representation of our class object by implementing method called \_\_str\_\_() in our class.

Header:

```python
def __str__ (self):
#must return a string
~~~

If you do that → when call print with instance of the class, this method is called automatically

Question 18

General structure for a docstring for a newly defined class:

Answer 18

Question 19

What is the alternative to creating a shallow copy like this one below?

• Aliasing (copying references) as above, only make program hard to read and more pronne to bugs

Answer 19

Copying Objects:

• Aliasing (copying references) as above, only make program hard to read and more pronne to bugs
• Import module copy and use to copy a function

copy() and deepcopy():

• Function copy.copy creates a ****shallow**** copy of the input object
• Need a deep copy use copy.deepcopy instead

Question 20

What is the difference between the module copy and deepcopy?

Answer 20

copy() and deepcopy():

• Function copy.copy creates a shallow copy of the input object
• Need a deep copy use copy.deepcopy instead

Question 21

What does the import module copy do?

Answer 21

When create a copy, new object contains the same data

**- Expect that the operator == would have compared data stored inside object
- By default, == simply compares identities of two objects

Default behavior of == is the same as the operator is**

Question 22

Define

Operator Overloading:

Answer 22

> Operator overloading refers to changing behavior of built-in operator, so works as we wish with programmer-defined data types

• Corresponding special method we can add to our classes to modify operator’s behavior

Modify behavior of ==: define the method __eq__ inside our class

Question 23

What is the \_\_eq\_\_ method?

Answer 23

Method __eq__ takes two inputs, self and other object, returns a boolean

Question 24

Operator Overriding

Examples of other default operators:

Answer 24

Override many kinds of default operators:

• \_\_It\_\_: less than operator
• \_\_gt\_\_: greater than operator
• \_\_le\_\_ / \_\_ge\_\_: less than or equal to / greater than or equals to
• \_\_eq\_\_ / \_\_ne\_\_: equal to ‘==’ not equal to !=