Eye of depth Python learning notes Chapter 7 class object oriented programming

Introduction

• Everything is the object
  A puppy, a chair, a credit card, a chocolate...

• All objects have their own internal attributes
  Dog breed, chair texture, credit card limit, chocolate taste...

• All actions are actions of the object
  Dog squatting, chair moving, credit card swiping, chocolate melting...

How: class is the carrier of object

Every cat of different age, color and quality is an object

They have one thing in common: they are all cats

We can abstract the common features of a class of objects and create a general class

# Create class class Cat(): """Simulated cat""" def __init__(self, name): """Initialize properties""" self.name = name def jump(self): """Simulated cat jumping""" print(self.name + " is jumping")

# Create instance with class my_cat = Cat("Loser") your_cat = Cat("Lucky")

# Call attribute print(my_cat.name) print(your_cat.name)

Loser Lucky

# Calling method my_cat.jump() your_cat.jump()

Loser is jumping Lucky is jumping

7.1 definition of category

Three elements: class name, attribute and method

7.1.1 class naming

• Have practical significance

• Hump Nomenclature - capitalized words
  Dog, CreditCard, ElectricCar

# class name: """It's better to leave two lines blank before class""" class Car(): """A brief introduction to this class""" pass """It's better to leave two lines blank after class"""

7.1.2 properties of class

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of the car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0

7.1.3 methods

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of the car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0 def get_main_information(self): # self can't save """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{}".format(self.brand, self.model, self.year)) def get_mileage(self): """Get total mileage""" return "Total mileage:{}kilometre".format(self.mileage)

My_Car = Car("Audi","sb",2019)

print(My_Car.get_mileage())

7.2 create an instance

7.2.1 instance creation

Assign an instance to an object, and pass in the corresponding parameters during instantiation
v = class name (required initialization parameters)

my_new_car = Car("Audi", "A6", 2018)

7.2.2 access properties

Instance name. Property name

print(my_new_car.brand) print(my_new_car.model) print(my_new_car.year)

Audi A6 2018

7.2.3 call method

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0 def get_main_information(self): # self can't save """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{}".format(self.brand, self.model, self.year)) def get_mileage(self): """Get total mileage""" return "Total mileage:{}kilometre".format(self.mileage)

Instance name. Method name (required parameters)

my_new_car = Car("Audi", "A6", 2018) my_new_car.get_main_information()

mileage = my_new_car.get_mileage() print(mileage)

7.2.4 modifying attributes

1. Direct modification

my_old_car = Car("BYD", "Song Dynasty", 2016)

Access first, then modify

print(my_old_car.mileage) my_old_car.mileage = 12000 print(my_old_car.mileage)

print(my_old_car.get_mileage())

2. Modify properties by method

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0 def get_main_information(self): # self can't save """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{}".format(self.brand, self.model, self.year)) def get_mileage(self): """Get total mileage""" return "Total mileage:{}kilometre".format(self.mileage) def set_mileage(self, distance): """Set total mileage""" self.mileage = distance

my_old_car = Car("BYD", "Song Dynasty", 2016) print(my_old_car.get_mileage()) my_old_car.set_mileage(8000) print(my_old_car.get_mileage())

3. Continue to expand

• Do not set negative mileage

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of the car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0 def get_main_information(self): # self can't save """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{}".format(self.brand, self.model, self.year)) def get_mileage(self): """Get total mileage""" print("Total mileage:{}kilometre".format(self.mileage)) def set_mileage(self, distance): """Set total mileage""" if distance >= 0: self.mileage = distance else: print("Mileage cannot be negative!") def increment_mileage(self, distance): """Total mileage accumulation""" if distance >= 0: self.mileage += distance else: print("New mileage cannot be negative!")

my_old_car = Car("BYD", "Song Dynasty", 2016) my_old_car.get_mileage() my_old_car.set_mileage(-8000) my_old_car.get_mileage()

• Add up mileage each time

my_old_car.get_mileage() my_old_car.set_mileage(8000) my_old_car.get_mileage() my_old_car.increment_mileage(500) my_old_car.get_mileage()

Summary

my_new_car = Car("Audi", "A6", 2018) my_cars = [my_new_car, my_old_car]

• It can contain a huge amount of information and create an infinite number of instances
• Highly anthropomorphic, in line with human's abstract and understanding of the objective world

7.3 class inheritance

Introduction

Take a look at the branch chain of human beings in the biological world

Biology -- zoology -- chordate -- mammalia -- primates -- human family -- human genus -- Homo sapiens

The process of increasing public features

[question]

Suppose the binary system: human =
Construct a class for everyone

Scenario 1:
They are independent and construct their own ethnic groups

Option two:
1. To extract the public characteristics of each race and establish the human genus;
2. Each race inherits the public characteristics of the upper class (human genus), and then adds its own special characteristics to construct its own race class.

Usually, we choose option 2, because it avoids too much duplication of labor

The so-called inheritance is the process of low-level abstraction inheriting high-level abstraction

7.3.1 simple inheritance

Parent class

class Car(): """Simulated vehicle""" def __init__(self, brand, model, year): """Initialize car properties""" # Equivalent to a variable within a class self.brand = brand # Brand of automobile self.model = model # Model of car self.year = year # Vehicle year of manufacture self.mileage = 0 # The total mileage of the new car is initialized to 0 def get_main_information(self): # self can't save """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{}".format(self.brand, self.model, self.year)) def get_mileage(self): """Get total mileage""" print("Total mileage:{}kilometre".format(self.mileage)) def set_mileage(self, distance): """Set total mileage""" if distance >= 0: self.mileage = distance else: print("Mileage cannot be negative!") def increment_mileage(self, distance): """Total mileage accumulation""" if distance >= 0: self.mileage += distance else: print("New mileage cannot be negative!")

Subclass

Class subclass name (parent class name):

• Build a new class of electric vehicles

class ElectricCar(Car): """Simulated electric vehicle""" def __init__(self, brand, model, year): """Initialize EV properties""" super().__init__(brand, model,year) # Declare properties that inherit from the parent class

A = ElectricCar("au","23",90)

• Automatically inherit all methods of the parent class

my_electric_car = ElectricCar("NextWeek", "FF91", 2046) my_electric_car.get_main_information()

'Brand: NextWeek Model: FF91 Year of manufacture: 2046'

7.3.2 add attributes and methods to subclasses

class ElectricCar(Car): """Simulated electric vehicle""" def __init__(self, brand, model, year, bettery_size): """Initialize EV properties""" super().__init__(brand, model, year) # Declare properties that inherit from the parent class self.bettery_size = bettery_size # Battery capacity self.electric_quantity = bettery_size # Remaining battery capacity self.electric2distance_ratio = 5 # Power distance conversion factor 5km / kW.h self.remainder_range = self.electric_quantity*self.electric2distance_ratio # Remaining mileage def get_electric_quantit(self): """View current battery level""" print("Current battery capacity:{} kW.h".format(self.electric_quantity)) def set_electric_quantity(self, electric_quantity): """Set the remaining battery capacity and recalculate the battery capacity to support the mileage""" if electric_quantity >= 0 and electric_quantity <= self.bettery_size: self.electric_quantity = electric_quantity self.remainder_range = self.electric_quantity*self.electric2distance_ratio else: print("The power is not set in a reasonable range!") def get_remainder_range(self): """View remaining mileage""" print("The current power can still drive {} kilometre".format(self.remainder_range))

my_electric_car = ElectricCar("NextWeek", "FF91", 2046, 70) my_electric_car.get_electric_quantit() # Get current battery power my_electric_car.get_remainder_range() # Get the current remaining mileage

my_electric_car.set_electric_quantity(50) # Reset battery level my_electric_car.get_electric_quantit() # Get current battery power my_electric_car.get_remainder_range() # Get the current remaining mileage

7.3.3 method of overriding parent class polymorphism

class ElectricCar(Car): """Simulated electric vehicle""" def __init__(self, brand, model, year, bettery_size): """Initialize EV properties""" super().__init__(brand, model, year) # Declare properties that inherit from the parent class self.bettery_size = bettery_size # Battery capacity self.electric_quantity = bettery_size # Remaining battery capacity self.electric2distance_ratio = 5 # Power distance conversion factor 5km / kW.h self.remainder_range = self.electric_quantity*self.electric2distance_ratio # Remaining mileage def get_main_information(self): # Override parent method """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{} Mileage:{} kilometre" .format(self.brand, self.model, self.year, self.bettery_size*self.electric2distance_ratio)) def get_electric_quantit(self): """Check the current battery power and recalculate the power to support the mileage""" print("Current battery capacity:{} kW.h".format(self.electric_quantity)) def set_electric_quantity(self, electric_quantity): """Set the remaining battery capacity""" if electric_quantity >= 0 and electric_quantity <= self.bettery_size: self.electric_quantity = electric_quantity self.remainder_range = self.electric_quantity*self.electric2distance_ratio else: print("The power is not set in a reasonable range!") def get_remainder_range(self): """View remaining mileage""" print("The current power can still drive {} kilometre".format(self.remainder_range))

my_electric_car = ElectricCar("NextWeek", "FF91", 2046, 70) my_electric_car.get_main_information()

7.3.4 examples used in classes

Abstract the battery into an object
Clearer logic

class Bettery(): """Battery of analog electric vehicle""" def __init__(self, bettery_size = 70): self.bettery_size = bettery_size # Battery capacity self.electric_quantity = bettery_size # Remaining battery capacity self.electric2distance_ratio = 5 # Power distance conversion factor 5km / kW.h self.remainder_range = self.electric_quantity*self.electric2distance_ratio # Remaining mileage def get_electric_quantit(self): """View current battery level""" print("Current battery capacity:{} kW.h".format(self.electric_quantity)) def set_electric_quantity(self, electric_quantity): """Set the remaining battery capacity, and recalculate the battery capacity to support the mileage""" if electric_quantity >= 0 and electric_quantity <= self.bettery_size: self.electric_quantity = electric_quantity self.remainder_range = self.electric_quantity*self.electric2distance_ratio else: print("The power is not set in a reasonable range!") def get_remainder_range(self): """View remaining mileage""" print("The current power can still drive {} kilometre".format(self.remainder_range))

class ElectricCar(Car): """Simulated electric vehicle""" def __init__(self, brand, model, year, bettery_size): """Initialize EV properties""" super().__init__(brand, model, year) # Declare properties that inherit from the parent class self.bettery = Bettery(bettery_size) # Battery def get_main_information(self): # Override parent method """Get the main information of the car""" print("Brand:{} Model:{} Year of manufacture:{} Mileage:{} kilometre" .format(self.brand, self.model, self.year, self.bettery.bettery_size*self.bettery.electric2distance_ratio))

my_electric_car = ElectricCar("NextWeek", "FF91", 2046, 70) my_electric_car.get_main_information() # Access to main vehicle information

my_electric_car.bettery.get_electric_quantit() # Get current battery power

my_electric_car.bettery.set_electric_quantity(50) # Reset battery level

my_electric_car.bettery.get_electric_quantit() # Get current battery power

my_electric_car.bettery.get_remainder_range() # Get the current remaining mileage

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