本篇内容介绍了“Python类的定义、类的约束、类的继承和调用怎么理解”的有关知识，在实际项目的操作过程或是学习过程中，不少人都会遇到这样的问题，接下来就让小编带大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！

一、类的约束

# _开头： 私有变量；
# __开问： 私有变量，不能被继承；
# __xxx__： 能被访问，不能被继承；
class A:
def __init__(self):
self._internal = 0 # 私有变量不能被访问
self.public = 1 # 可被访问
def public_method(self):
pass
def _private_method(self): # 私有方法不能被访问
pass
class B:
def __init__(self):
self.__private = 0 # 这个属性会在内存中被重新命名为_B__private

def __private_method(self): # 不能被访问，不能被继承
pass
def __private_method__(self): # 能被访问，不能被继承
pass

二、类的定义

2.1、创建创建

class Dog:
a = "0"; #相当于public static变量,全局的
"""__init__是一个默认的方法，且self为默认的，用self修饰的属性为public类型的类变量"""
def __init__(self, name, age):
self.name = name
self.age = age
self.sex = "1";#设置属性默认值

def sit(self):
print(self.name + "is now sitting" + "and sex is " + self.sex + Dog.a)

@classmethod
def user_name(cls, name): #注意这种注解的用法
return cls()

dog = Dog("kk", 12);
dog.sit()

2.1.1、类的导入

在python中分为文件、模块、类，其中文件和模块可划等价；所以导入有几种方式，比如dog.py文件中定义了两个Class，则在使用类中导入方法有以下几种：

• from car import Dog;#导入一个模块中的特定类，使用时则直接Car();
• import car;#导入一个模块中的所有类，使用时则需要car.Car();
• from car import *;#不推荐，容易引起命名冲突问题

from collections import OrderedDict; #使用标准类库
t = OrderedDict();

2.1.2、构造器

class Date:
# Primary constructor
def __init__(self, year, month, day):
self.year = year
self.month = month
self.day = day

# Alternate constructor
@classmethod
def today(cls):
t = time.localtime() #它接收一个class作为第一个参数，它被用来创建并返回最终的实例， 这个cls==__init__
return cls(t.tm_year, t.tm_mon, t.tm_mday)

a = Date(2020, 5, 10) # Primary
b = Date.today() # Alternate

减少构造函数的参数个数：

class Structure1:
# Class variable that specifies expected fields
_field_list = []

def __init__(self, *args):
if len(args) != len(self._field_list):
raise TypeError(f'Expected {len(self._field_list)} arguments')
# Set the arguments
for name, value in zip(self._field_list, args):
setattr(self, name, value)

# Example class definitions
class Course(Structure1):
# 这行只是为了一个准许入判断，没有太多实际意思，或是一个声明
_field_list = ['course_name', 'total_class', 'score']

c = Course('python', 30, 0.3);

关键字参数

class Structure2:
_field_list = []

def __init__(self, *args, **kwargs):
if len(args) > len(self._field_list):
raise TypeError(f'Expected {len(self._field_list)} arguments')
# Set all of the positional arguments
for name, value in zip(self._field_list, args):
setattr(self, name, value)

# Set the remaining keyword arguments
#是通过pop这种方式来检查的，在长度范围内如果pop出错则抛异常
for name in self._field_list[len(args):]:
setattr(self, name, kwargs.pop(name))

# Check for any remaining unknown arguments
if kwargs:
raise TypeError(f"Invalid argument(s): {','.join(kwargs)}")

# Example use
class Course(Structure2):
_field_list = ['course_name', 'total_class', 'score']

course_1 = Course('python', 30, 0.3)
course_2 = Course('python', 30, score=0.3)
course_3 = Course('python', total_class=30, score=0.3)

扩展关键字参数：

class Structure3:
# Class variable that specifies expected fields
_field_list = []

def __init__(self, *args, **kwargs):
if len(args) != len(self._field_list):
raise TypeError(f'Expected {len(self._field_list)} arguments')

# Set the arguments
for name, value in zip(self._field_list, args):
setattr(self, name, value)

# Set the additional arguments (if any)
extra_args = kwargs.keys() - self._field_list
for name in extra_args:
setattr(self, name, kwargs.pop(name))

if kwargs:
raise TypeError(f"Duplicate values for {','.join(kwargs)}")

# Example use
if __name__ == '__main__':
class Course(Structure3):
_field_list = ['course_name', 'total_class', 'score']

course_1 = Course('python', 30, 0.3)
course_2 = Course('python', 30, 0.3, date='8/5/2020')

2.1.3、类属性

要创建一个新的实例属性，可以通过描述器的形式来定义它的功能，一个描述器就是一个实现了3个核心属性访问操作的类，分别对应get\set\delete这三个特殊的方法。

# Descriptor attribute for an integer type-checked attribute
class Integer:
def __init__(self, name):
self.name = name
"""下面三个方法只是一个更严格的定义，可以不需要，要使用上面的描述器，需要把描述器放入到一个class中，这样所有对描述器的访问都会被get/set/delete所捕获"""
def __get__(self, instance, cls):
if not instance:
return self
else:
return instance.__dict__[self.name]
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Expected an int object')
instance.__dict__[self.name] = value
def __delete__(self, instance):
del instance.__dict__[self.name]

示例1：

class Point:
"""实例变量，和下面的x,y不是一回事"""
x = Integer('x')
y = Integer('y')

def __init__(self, x, y):
self.x = x
self.y = y
print(Point.x.name) # x
point = Point(3, 5)
print(f'point x = {point.x}') #3
print(f'point y = {point.y}') #5
point.y = 6
print(f'after change,point y = {point.y}') #6

三、类的继承

ptyhon在实现继承时会用一个叫MRO列表的算法实现，它有三条规则：1、子类会先于父类；2、多个父类会根据它们在列表中的顺序被检查；3、如果对下一个类有两个合法的选择，则返回第一个合法的父类；

3.1、单继承

class A:
def __init__(self):
self.x = 0
class B(A):
def __init__(self):
super().__init__() #这行需要注意，也可以不写，但不写时就不会调用父类的init方法
self.y = 1

3.2、多继承

class Base:
def __init__(self):
print('call Base.__init__')
class A(Base):
def __init__(self):
Base.__init__(self)
print('call A.__init__')

class B(Base):
def __init__(self):
Base.__init__(self)
print('call B.__init__')
"""多继承的实现"""
class C(A,B):
def __init__(self):
A.__init__(self)
B.__init__(self)
print('call C.__init__')
c = C()
# call Base.__init__
# call A.__init__
# call Base.__init__
# call B.__init__
# call C.__init__

3.3、调用父类方法

class Proxy:
def __init__(self, obj):
self._obj = obj

def __getattr__(self, name):
return getattr(self._obj, name)

def __setattr__(self, name, value):
if name.startswith('_'):
"""调用父类方法"""
super().__setattr__(name, value)
else:
setattr(self._obj, name, value)

proxy = Proxy({})
proxy.__setattr__("_name", "hm")

3.4、属性扩展

3.4.1、完全扩展

# 父类
class Person:
def __init__(self, name):
self.name = name

# defined Getter function， auto to call the sign name.setter when it be build
@property
def name(self):
return self._name

# defined Setter function
@name.setter
def name(self, value):
if not isinstance(value, str):
raise TypeError('Expected a string')
self._name = value

# defined Deleter function
@name.deleter
def name(self):
raise AttributeError("Can't delete attribute")

"""子类"""
class SubPerson(Person):
@property
def name(self):
print('Getting name')
return super().name

@name.setter
def name(self, value):
print(f'Setting name to {value}')
super(SubPerson, SubPerson).name.__set__(self, value)

@name.deleter
def name(self):
print('Deleting name')
super(SubPerson, SubPerson).name.__delete__(self)

"""测试"""
sub_person = SubPerson('Guido')
print(f'name is: {sub_person.name}')

3.4.2、单独扩展

class SubPerson(Person):
@Person.name.getter
def name(self):
print('Getting name')
return super().name # or super(SubPerson, SubPerson).name.__set__(self, value)
sub_p = SubPerson('Bill')

#不能用property的原因是，property其实是get、set、del函数的集合，各有各的用处。下面才是正确的扩展方式，所以下面的代码是不工作的
class SubPerson(Person):
@property # Doesn't work
def name(self):
print('Getting name')
return super().name
#如果要用property属性则要用下面的编码实现
class SubPerson(Person):
@property
def name(self):
print('Getting name')
return super().name
@name.setter
def name(self, value):
print(f'Setting name to {value}')
super(SubPerson, SubPerson).name.__set__(self, value)
@name.deleter
def name(self):
print('Deleting name')
super(SubPerson, SubPerson).name.__delete__(self)

四、类的调用

import time
class Date:
# Primary constructor
def __init__(self, year, month, day):
self.year = year
self.month = month
self.day = day
# Alternate constructor
@classmethod
def today(cls):
t = time.localtime() #它接收一个class作为第一个参数，它被用来创建并返回最终的实例， 这个cls==__init__
return cls(t.tm_year, t.tm_mon, t.tm_mday)

"""普通调用"""
c = Date(2010, 12, 12)

"""类方法在继承中使用"""
class NewDate(Date):
pass
c = Date.today() # Creates an instance of Date (cls=Date)
d = NewDate.today() # Creates an instance of NewDate (cls=NewDate)

五、抽象类

from abc import ABCMeta, abstractmethod
class IStream(metaclass=ABCMeta):
@abstractmethod
def read(self, max_bytes=-1):
pass
@abstractmethod
def write(self, data):
pass
"""不能被实例化"""
#a = IStream()

class SocketStream(IStream):
def read(self, max_bytes=-1):
pass
def write(self, data):
pass
"""检查"""
def serialize(obj, stream):
if not isinstance(stream, IStream):
raise TypeError('Expected an IStream')
pass

5.1、强制类型检查

from abc import ABCMeta, abstractmethod
class IStream(metaclass=ABCMeta):
@abstractmethod
def read(self, max_bytes=-1):
pass
@abstractmethod
def write(self, data):
pass
import io
# Register the built-in I/O classes as supporting our interface
IStream.register(io.IOBase)

# Open a normal file and type check
f = None #open('test.txt')
print(f'f object is IStream type: {isinstance(f, IStream)}')
#f object is IStream type: False

六、类的比较

from functools import total_ordering
class Room:
def __init__(self, name, length, width):
self.name = name
self.length = length
self.width = width
self.square_feet = self.length * self.width
@total_ordering
class House:
def __init__(self, name, style):
self.name = name
self.style = style
self.rooms = list()
@property
def living_space_footage(self):
return sum(r.square_feet for r in self.rooms)
def add_room(self, room):
self.rooms.append(room)
def __str__(self):
return f'{self.name}: {self.living_space_footage} square foot {self.style}'
def __eq__(self, other):
return self.living_space_footage == other.living_space_footage

def __lt__(self, other):
return self.living_space_footage < other.living_space_footage
# Build a few houses, and add rooms to them
h1 = House('h1', 'Cape')
h1.add_room(Room('Master Bedroom', 14, 21))
h1.add_room(Room('Living Room', 18, 20))
h1.add_room(Room('Kitchen', 12, 16))
h1.add_room(Room('Office', 12, 12))

h2 = House('h2', 'Ranch')
h2.add_room(Room('Master Bedroom', 14, 21))
h2.add_room(Room('Living Room', 18, 20))
h2.add_room(Room('Kitchen', 12, 16))

h3 = House('h3', 'Split')
h3.add_room(Room('Master Bedroom', 14, 21))
h3.add_room(Room('Living Room', 18, 20))
h3.add_room(Room('Office', 12, 16))
h3.add_room(Room('Kitchen', 15, 17))
houses = [h1, h2, h3]

print(f'Is {h1} bigger than {h2}: {h1 > h2}')
print(f'Is {h2} smaller than {h3}: {h2 < h3}')
print(f'Is {h2} greater than or equal to {h1}: {h2 >= h1}')
print(f'Which one is biggest in houses: {max(houses)}')
print(f'Which is smallest in houses: {min(houses)}')

""""""
# Is h1: 990 square foot Cape bigger than h2: 846 square foot Ranch: True
# Is h2: 846 square foot Ranch smaller than h3: 1101 square foot Split: True
# Is h2: 846 square foot Ranch greater than or equal to h1: 990 square foot Cape: False
# Which one is biggest in houses: h3: 1101 square foot Split
# Which is smallest in houses: h2: 846 square foot Ranch
# """"""
class House:
def __eq__(self, other):
pass
def __lt__(self, other):
pass
# Methods created by @total_ordering
__le__ = lambda self, other: self < other or self == other
__gt__ = lambda self, other: not (self < other or self == other)
__ge__ = lambda self, other: not (self < other)
__ne__ = lambda self, other: not self == other

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