9.1. Classes and Instances

9.1.1. Rationale

• Object Oriented Paradigm

• Model world as objects that interacts with each other

class

Templates for objects.

instance

object

Object created from class.

class <name>:
    pass

<instance> = <name>()

class MyClass:
    pass

my = MyClass()

9.1.2. Classes

• Capitalized CamelCase name convention

• Classes are templates for objects

Listing 9.1. Defining class. Classes should have capitalized name

class Astronaut:
    pass

Listing 9.2. Multi-word class names should use CamelCase

class AstronautPilot:
    pass

9.1.3. Instances

• Instances are also known as Objects

• Two newlines between class and code

• snake_case names

Listing 9.3. One class and one instance

class Astronaut:
    pass


watney = Astronaut()

Listing 9.4. One class and three instances

class Astronaut:
    pass


watney =  = Astronaut()
twardowski = Astronaut()
jimenez = Astronaut()

Listing 9.5. Two classes and two instances

class Astronaut:
    pass

class Cosmonaut:
    pass


mark = Astronaut()
ivan = Cosmonaut()

Listing 9.6. Two classes and four instances (two instances of an Astronaut class, and two of a Cosmonaut class)

class Astronaut:
    pass

class Cosmonaut:
    pass


mark = Astronaut()
melissa = Astronaut()
ivan = Cosmonaut()
jan = Cosmonaut()

9.1.4. Class vs Instance

Figure 9.1. Intuition definition: Class is a blueprint, instances are homes made from this plan. Image source: [ImgFre19]

9.1.5. Convention

• Never print in a class

• One class per file - when classes are long

• All classes in one file - when classes are short

• You can mix classes and functions in one file

Listing 9.7. Classes and Objects

class AstronautEngineer:
    pass

class AstronautScientist:
    pass

class AstronautPilot:
    pass


mark_watney = AstronautScientist()
melissa_lewis = AstronautScientist()
jose_jimenez = AstronautEngineer()
jan_twardowski = AstronautPilot()

def hello():
    pass

class Astronaut:
    pass


mark = Astronaut()
jose = Astronaut()

9.1.6. Assignments

9.1.6.1. Example

• Complexity level: easy

• Lines of code to write: 5 lines

• Estimated time of completion: 5 min

• Solution: solution/oop_class_example.py

English

1. Define class Iris

2. Create instance setosa of a class Iris

3. Create instance virginica of a class Iris

4. Create instance versicolor of a class Iris

Polish

1. Zdefiniuj klasę Iris

2. Stwórz instancję setosa klasy Iris

3. Stwórz instancję virginica klasy Iris

4. Stwórz instancję versicolor klasy Iris

Solution

class Iris:
    pass


setosa = Iris()
versicolor = Iris()
virginica = Iris()

The whys and wherefores

• Classes and Instances

9.1.6.2. Class instantiation

• Complexity level: easy

• Lines of code to write: 9 lines

• Estimated time of completion: 5 min

• Solution: solution/oop_class_instance.py

English

1. Define class Astronaut

2. Define class SpaceAgency

3. Create instance twardowski of a class Astronaut

4. Create instance watney of a class Astronaut

5. Create instance nasa of a class SpaceAgency

6. Create instance esa of a class SpaceAgency

7. Create instance polsa of a class SpaceAgency

Polish

1. Zdefiniuj klasę Astronaut

2. Zdefiniuj klasę SpaceAgency

3. Stwórz instancję twardowski klasy Astronaut

4. Stwórz instancję watney klasy Astronaut

5. Stwórz instancję nasa klasy SpaceAgency

6. Stwórz instancję esa klasy SpaceAgency

7. Stwórz instancję polsa klasy SpaceAgency

The whys and wherefores

• Classes and Instances