类的实例

>>> class newClass():
    pass
# 像调用函数一样的表达式直接进行类的实例化：
>>> newObj = newClass()

>>> class newClass():
    pass

>>> newObj = newClass()
>>> class newClass():
    def __init__(self,name):
        self.name = name
# 如果类的__init__函数中有参数且不是默认参数，那么进行实例化时必须将参数传进去，否则将会报错：        
>>> newObj = newClass()
Traceback (most recent call last):
  File "<pyshell#21>", line 1, in <module>
    newObj = newClass()
TypeError: __init__() missing 1 required positional argument: ‘name‘
>>> newObj = newClass(‘new class‘)
>>> newObj.name
‘new class‘
>>> 

# 这里可以看到__init__函数的参数使用和普通的函数中的参数是一样的：
>>> class newClass():
    def __init__(self,name=‘new class‘):
        self.name = name
>>> newObj = newClass()
>>> newObj.name
‘new class‘
>>> 

>>> class InstTrack():
    count = 0
    def __init__(self):
        InstTrack.count += 1
    def __del__(self):
        InstTrack.count -= 1
    def howMany(self):
        return InstTrack.count

    
>>> a = InstTrack()
>>> b = InstTrack()
>>> id(a)
1786701000432
>>> id(b)
1786701000544
>>> a.howMany()
2
>>> b.howMany()
2
>>> del a
>>> b.howMany()
1
>>> del b
>>> InstTrack.count
0

# 如果返回非None值在实例化时会报错：
>>> class MyClass():
    def __init__(self):
        print("Initialized...")
        return 1

    
>>> mc = MyClass()
Initialized...
Traceback (most recent call last):
  File "<pyshell#48>", line 1, in <module>
    mc = MyClass()
TypeError: __init__() should return None, not ‘int‘
# 返回Nnone时就不会报错了：
>>> class MyClass():
    def __init__(self):
        print("Initialized...")
        return None

    
>>> mc = MyClass()
Initialized...
>>> 

>>> class Example():
    pass
>>> ex.user = ‘root‘
>>> ex = Example()
>>> ex.pri = 775
>>> dir(Example)
[‘__class__‘, ‘__delattr__‘, ‘__dict__‘, ‘__dir__‘, ‘__doc__‘, ‘__eq__‘, ‘__format__‘, ‘__ge__‘, ‘__getattribute__‘, ‘__gt__‘, ‘__hash__‘, ‘__init__‘, ‘__le__‘, ‘__lt__‘, ‘__module__‘, ‘__ne__‘, ‘__new__‘, ‘__reduce__‘, ‘__reduce_ex__‘, ‘__repr__‘, ‘__setattr__‘, ‘__sizeof__‘, ‘__str__‘, ‘__subclasshook__‘, ‘__weakref__‘]
>>> 
>>> dir(ex)
[‘__class__‘, ‘__delattr__‘, ‘__dict__‘, ‘__dir__‘, ‘__doc__‘, ‘__eq__‘, ‘__format__‘, ‘__ge__‘, ‘__getattribute__‘, ‘__gt__‘, ‘__hash__‘, ‘__init__‘, ‘__le__‘, ‘__lt__‘, ‘__module__‘, ‘__ne__‘, ‘__new__‘, ‘__reduce__‘, ‘__reduce_ex__‘, ‘__repr__‘, ‘__setattr__‘, ‘__sizeof__‘, ‘__str__‘, ‘__subclasshook__‘, ‘__weakref__‘, ‘pri‘, ‘user‘]
>>> 

>>> class Example():
    pass
>>> ex = Example()
>>> ex.user = ‘root‘
>>> ex.pri = 775
>>> ex.__dict__
{‘pri‘: 775, ‘user‘: ‘root‘}
>>> 

>>> ex.__class__
<class ‘__main__.Example‘>
>>> 

>>> class Example():
    version = 1.2    #定义了一个类的静态成员属性
>>> ex = Example()
# 在实例中没有创建属性时会通过类以及基类寻找属性，找到后可返回：
>>> ex.version
1.2
# 类可以调用自己的静态属性
>>> Example.version
1.2
# 类修改类属性：
>>> Example.version = 1.3
>>> Example.version
1.3
# 此时实例属性查看时也发生了变化：
>>> ex.version
1.3
>>> 
# 但是此处给实例新增加了一个同名的属性，这个属性是这个实例的属性，与类的属性是分开的：
>>> ex.version = 1.0
>>> ex.version
1.0
# 可以看到类属性是没有变化的：
>>> Example.version
1.3
# 此时再实例化一个新实例，它的属性值也是默认的类属性：
>>> ex2 = Example()
>>> ex2.version
1.3
# 此时再次删除ex实例属性后，再次访问到的是类属性的值： 
>>> del ex.version
>>> ex.version
1.3

>>> class SaySomething():
    def __init__(self,sayWord = ‘hello,world!‘):
        self.sayWord = sayWord
    def say(self):
        print(self.sayWord)
>>> say1 = SaySomething()
# 通过实例调用绑定的方法：
>>> say1.say()
hello,world!
# 通过类调用绑定的方法时会报错：
>>> SaySomething.say()
Traceback (most recent call last):
  File "<pyshell#150>", line 1, in <module>
    SaySomething.say()
TypeError: say() missing 1 required positional argument: ‘self‘
>>> 

>>> class SayOneWord(SaySomething):
    def __init__(self):
        SaySomething.__init__(self)

        
>>> say2 = SayOneWord()
>>> say2.say()
hello,world!
>>>