深入讲解Python中面向对象编程的相关知识

 Python从第一天开始就是面向对象的语言。正因为如此，创建和使用类和对象是非常地容易。本章将帮助您在使用Python面向对象编程的技术方面所有提高。

如果没有任何以往面向对象(OO)的编程的经验，那么可能要了解一些基本的入门课程就可以了，或者至少某种形式的教程，让你有了解基本概念。

但是，这里会比较少地介绍面向对象编程(OOP)：
OOP术语概述

•     类: 用户定义的原型对象，它定义了一套描述类的任何对象的属性。属性是数据成员(类变量和实例变量)和方法，通过点符号访问。
•     类变量：这是一个类的所有实例共享的变量。类变量在类，但外面的任何类的方法定义。类变量不被用作经常作为实例变量。
•     数据成员：保存与类和对象关联的数据的类变量或实例变量。
•     函数重载：一个以上的行为特定功能的分配。执行的操作所涉及的对象(自变量)的类型不同而不同。
•     实例变量：所定义的方法内，只属于一个类的当前实例的变量。
•     继承：类的特点，即都是由它派生其他类的转移。
•     实例：某一类的一个单独对象。属于类Circle一个obj对象，例如，是类Circle的一个实例。
•     实例化：创建一个类的实例。
•     Method : 一种特殊的函数，函数在类定义中定义。
•     对象：这是由它的类中定义的数据结构的唯一实例。一个对象包括两个数据成员(类变量和实例变量)和方法。
•     运算符重载：一个以上的函数功能，特定的操作符分配。

创建类：

类语句将创建一个新的类定义。类的名称紧跟在关键字class后跟一个冒号，如下所示：

class ClassName:
  'Optional class documentation string'
  class_suite

类有一个文档字符串，它可以通过类名.__ doc__访问。

class_suite由所有定义的类成员，数据属性与函数组件的语句。

例子

下面是一个简单的Python类的例子：

class Employee:
  'Common base class for all employees'
  empCount = 0

  def __init__(self, name, salary):
   self.name = name
   self.salary = salary
   Employee.empCount += 1

  def displayCount(self):
   print "Total Employee %d" % Employee.empCount

  def displayEmployee(self):
   print "Name : ", self.name, ", Salary: ", self.salary

empCount是一个类变量，其值将是这个类的所有实例共享。这可以从类中或外部进行访问，访问形式为 Employee.empCount。

第一个方法__init__()是一种特殊的方法，这就是所谓的类构造函数或当创建该类的一个新实例Python调用的初始化方法。

声明就像正常函数中一样，不同的是第一个参数到每个方法是类的方法。 Python增加了self参数列表;不需要把调用的方法都它列入。

创建实例对象：

要创建一个类的实例，调用类名并传递任何参数给__init__方法接收。

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)

访问属性：

可以访问使用点运算符来访问对象的属性。而类变量使用类名来访问，如下所示：

emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

现在，把所有的概念放在一起：

#!/usr/bin/python

class Employee:
  'Common base class for all employees'
  empCount = 0

  def __init__(self, name, salary):
   self.name = name
   self.salary = salary
   Employee.empCount += 1

  def displayCount(self):
   print "Total Employee %d" % Employee.empCount

  def displayEmployee(self):
   print "Name : ", self.name, ", Salary: ", self.salary

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)
emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

当执行上面的代码，产生以下结果：

Name : Zara ,Salary: 2000
Name : Manni ,Salary: 5000
Total Employee 2

在任何时候可以添加，删除或修改类和对象的属性：

emp1.age = 7 # Add an 'age' attribute.
emp1.age = 8 # Modify 'age' attribute.
del emp1.age # Delete 'age' attribute.

除了使用正常的语句来访问属性，可以使用以下函数：

• getattr(obj, name[, default]) : 访问对象的属性。
• hasattr(obj,name) : 检查一个属性是否存在。
• setattr(obj,name,value) : 设置一个属性。如果属性不存在，那么它将被创建。
• delattr(obj, name) : 要删除一个属性。

hasattr(emp1, 'age')  # Returns true if 'age' attribute exists
getattr(emp1, 'age')  # Returns value of 'age' attribute
setattr(emp1, 'age', 8) # Set attribute 'age' at 8
delattr(empl, 'age')  # Delete attribute 'age'

内置的类属性：

每个Python类会继续并带有内置属性，他们可以使用点运算符像任何其他属性一样来访问：

• __dict__ : 字典包含类的命名空间。
• __doc__ : 类的文档字符串，或None如果没有定义。
• __name__: 类名称。
• __module__: 在类中定义的模块名称。此属性是在交互模式其值为“__main__”。
• __bases__ : 一个可能是空的元组包含了基类，其在基类列表出现的顺序。

对于上面的类，尝试访问这些属性：

#!/usr/bin/python

class Employee:
  'Common base class for all employees'
  empCount = 0

  def __init__(self, name, salary):
   self.name = name
   self.salary = salary
   Employee.empCount += 1

  def displayCount(self):
   print "Total Employee %d" % Employee.empCount

  def displayEmployee(self):
   print "Name : ", self.name, ", Salary: ", self.salary

print "Employee.__doc__:", Employee.__doc__
print "Employee.__name__:", Employee.__name__
print "Employee.__module__:", Employee.__module__
print "Employee.__bases__:", Employee.__bases__
print "Employee.__dict__:", Employee.__dict__

当执行上面的代码，产生以下结果：

Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
Employee.__bases__: ()
Employee.__dict__: {'__module__': '__main__', 'displayCount':
<function displayCount at 0xb7c84994>, 'empCount': 2,
'displayEmployee': <function displayEmployee at 0xb7c8441c>,
'__doc__': 'Common base class for all employees',
'__init__': <function __init__ at 0xb7c846bc>}

销毁对象（垃圾回收）：

Python的删除不需要的对象（内建类型或类的实例），自动释放内存空间。由Python定期回收的内存块不再使用的过程被称为垃圾收集。

Python的垃圾回收器在程序执行过程中运行，当一个对象的引用计数为零时触发。一个对象的引用计数改变为指向它改变别名的数量。

当它分配一个新的名字或放置在容器（列表，元组或字典）的对象的引用计数增加。当对象的引用计数减少使用 del 删除，其基准被重新分配，或者它的引用超出范围。当一个对象的引用计数变为零，Python会自动地收集它。

a = 40   # Create object <40>
b = a    # Increase ref. count of <40>
c = [b]   # Increase ref. count of <40>

del a    # Decrease ref. count of <40>
b = 100   # Decrease ref. count of <40>
c[0] = -1  # Decrease ref. count of <40>

当垃圾回收器销毁孤立的实例，并回收其空间一般不会注意到。但是，一个类可以实现特殊方法__del__()，称为析构函数被调用时，该实例将被摧毁。这个方法可以用于清理所用的一个实例的任何非内存资源。
例子：

__del__()析构函数打印实例，它即将被销毁的类名：

#!/usr/bin/python

class Point:
  def __init( self, x=0, y=0):
   self.x = x
   self.y = y
  def __del__(self):
   class_name = self.__class__.__name__
   print class_name, "destroyed"

pt1 = Point()
pt2 = pt1
pt3 = pt1
print id(pt1), id(pt2), id(pt3) # prints the ids of the obejcts
del pt1
del pt2
del pt3

当执行上面的代码，它产生以下结果：

3083401324 3083401324 3083401324
Point destroyed

注意：理想情况下，应该定义类的单独的文件，那么应该使用import语句将其导入主程序文件。详细请查看Python- 模块章节，导入模块和类的更多细节。
类继承：

不用从头开始，可以通过上面列出的括号父类的新类名后，从一个已经存在的类派生它创建一个类。

子类继承父类的属性，可以使用父类的这些属性，就好像它们是在子类中定义的一样。子类也可以覆盖父类的数据成员和方法。
语法

派生类的声明很像它们的父类; 从基类的列表后给出类名继承：

class SubClassName (ParentClass1[, ParentClass2, ...]):
  'Optional class documentation string'
  class_suite

例子

#!/usr/bin/python

class Parent:    # define parent class
  parentAttr = 100
  def __init__(self):
   print "Calling parent constructor"

  def parentMethod(self):
   print 'Calling parent method'

  def setAttr(self, attr):
   Parent.parentAttr = attr

  def getAttr(self):
   print "Parent attribute :", Parent.parentAttr

class Child(Parent): # define child class
  def __init__(self):
   print "Calling child constructor"

  def childMethod(self):
   print 'Calling child method'

c = Child()     # instance of child
c.childMethod()   # child calls its method
c.parentMethod()   # calls parent's method
c.setAttr(200)    # again call parent's method
c.getAttr()     # again call parent's method

当执行上面的代码，产生以下结果：

Calling child constructor
Calling child method
Calling parent method
Parent attribute : 200

类似的方式，可以按如下继承多个父类的类：

复制代码 代码如下:

class A: # define your class A
.....

class B: # define your calss B
.....

class C(A, B): # subclass of A and B
.....

可以使用issubclass()或isinstance()函数来检查两个类和实例的关系。

• issubclass(sub, sup) 如果给定的子类子确实是超sup的子类布尔函数返回true。
• isinstance(obj, Class) 如果obj是Class类的实例，或者是类的一个子类的实例布尔函数返回true

重写方法：

可以覆盖父类的方法。原因之一重写父的方法，因为可能想在子类特殊或实现不同的功能。
例子

#!/usr/bin/python

class Parent:    # define parent class
  def myMethod(self):
   print 'Calling parent method'

class Child(Parent): # define child class
  def myMethod(self):
   print 'Calling child method'

c = Child()     # instance of child
c.myMethod()     # child calls overridden method

当执行上面的代码，产生以下结果：

Calling child method

基础重载方法：

下表列出了一些通用的功能，可以在类重写中：

重载运算符：

假设要创建了一个Vector类来表示二维向量，当使用加运算符来增加他们发生了什么？最有可能是Python会屌你。

可以，但是定义__add__方法在类中进行矢量相加，再加上操作符的行为会按预期：
例子：

#!/usr/bin/python

class Vector:
  def __init__(self, a, b):
   self.a = a
   self.b = b

  def __str__(self):
   return 'Vector (%d, %d)' % (self.a, self.b)

  def __add__(self,other):
   return Vector(self.a + other.a, self.b + other.b)

v1 = Vector(2,10)
v2 = Vector(5,-2)
print v1 + v2

当执行上面的代码，产生以下结果：

Vector(7,8)

数据隐藏：

对象的属性可以是或可以不在类定义外部可见。对于这些情况，可以命名以双下划线前缀属性，这些属性将无法直接让外部可视。
例子：

#!/usr/bin/python

class JustCounter:
  __secretCount = 0

  def count(self):
   self.__secretCount += 1
   print self.__secretCount

counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

当执行上面的代码，产生以下结果：

1
2
Traceback (most recent call last):
 File "test.py", line 12, in <module>
  print counter.__secretCount
AttributeError: JustCounter instance has no attribute '__secretCount'

Python的保护成员通过内部更改名称以包含类名。可以访问这些属性通过object._className__attrName。如果想更换最后一行，那么它会工作如下：

.........................
print counter._JustCounter__secretCount

当执行上面的代码，产生以下结果：

1
2
2

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