python中的类函数，实例函数，类变量，实例变量

1. 类函数/实例函数（class method & instance method），类变量/实例变量（class variable & instance variable）：
   根据不同的环境中，以上叫法可能有所不同
    在上面程序中只涉及到了实例函数和实例变量。
    1. 实例函数。（实例方法，instance function，instance method）
        在class定义中定义的函数一般是实例函数，即实例的函数。如上面BinaryTree中定义的函数均为实例函数。实例函数的第一个形参必然是self，代表class创建的instance。实例函数不管是在class里面还是在class外面都需要使用object.instance_method()来调用（在class中，self即表示该class的实例，因此使用self.instance_method()调用）。一般如果实例函数不允许在class外面访问（即调用，使用）的话，在变量名或函数名前面加双下划线("__")。
    2. 实例变量。（实例属性，instance variable，instance attribute）
       在class定义中，使用self.variable定义的变量即为实例变量。如上面BinaryTree中self.value，self.left，self.right，self.parent均为实例变量。一般如果实例变量不允许在class外面访问（即调用，使用）的话，在变量名或函数名前面加双下划线("__")，使其变为私有变量。一般可以单独写非私有实例函数返回实例私有变量的值，从而在class外面访问实例私有变量。
    3. 类变量。（类属性，class variable， class attribute）
       在class里面的最外层定义的变量即为类变量。类变量不用self.variable来定义，区分于实例变量。同时，类变量处于与实例函数/类函数同等的位置（class里最外层定义），区分于局部变量。
    4. 类函数。（类方法，class variable，class attribute）
       即在类中定义的函数。关于类函数的解释（未完待续）。

下面例子说明：
   1. 类变量的作用；
   2. 类变量和实例变量之间的区别；
   3. 类变量如何转化用作实例变量以及此时二者之间的差别；

class Spam:
    numInstances = 0 # Declare a class attribute
    def __init__(self):
        Spam.numInstances = Spam.numInstances + 1 # count how many instances created
        print("New Spam instance created")
        if Spam.numInstances == 2:
            print("You have created two instance of Spam")
    def printNumInstances(): # class method
        print("Number of instances created: %s" % Spam.numInstances)

obj_1 = Spam()
obj_2 = Spam()
obj_3 = Spam()
# 1. Call of class method
Spam.printNumInstances()
# 2. Access of class variable from outside
Spam.numInstances = 5
# 3. Check if outside access has changed class variable's value
Spam.printNumInstances()
print("Number of instances created: %s" % Spam.numInstances)
# 4. Access of class variable through instance
print("numInstances's value in obj_1: %s" % obj_1.numInstances)
# Access with instance and a copy(reference) of class varialbe are generated as the instance's instance variable
obj_1.numInstances = 10
# Now obj_1's instance has become its independent instance, and has nothing to do with Spam.numInstances, this is like the following process:
# obj_1.numInstances = Spam.numInstances // When you access obj_1.numInstances, you are actually access the same object that Spam.numInstances pointed to (obj_1.numInstances also point to this object)
# obj_1.numInstances = 10 // And now obj_1.numInstances point to another object "10", and has nothing to do with "Spam.numInstances" from now on
print("numInstances's value in obj_1: %s" % obj_1.numInstances)
print("numInstances's value in obj_2: %s" % obj_2.numInstances)
# At this time, obj_2's numInstance is still a reference of Spam.numInstances
obj_2.numInstances = 20
# Now obj_2's instance has become its independent instance, and has nothing to do with Spam.numInstances
print("numInstances's value in obj_2: %s" % obj_2.numInstances)
print("numInstances's value in Spam: %s" % Spam.numInstances)

>>>
New Spam instance created
New Spam instance created
You have created two instance of Spam
New Spam instance created
Number of instances created: 3
Number of instances created: 5
Number of instances created: 5
numInstances's value in obj_1: 5
numInstances's value in obj_1: 10
numInstances's value in obj_2: 5
numInstances's value in obj_2: 20
numInstances

下面例子说明私有变量在类内外的可访问性：
    1. 不管是类变量还是实例变量，当我们设定其为私有变量之后，外部无法直接访问到它。只能通过类函数或实例函数来访问；
   2. 但外部却可以写出如下line 17的句子，此时这个"__numInstances"变量指的是什么呢？相对于类来说它是私有的吗？

class Spam:
    __numInstances = 0 # Declare a class attribute
    def __init__(self):
        Spam.__numInstances = Spam.__numInstances + 1 # count how many instances created
        print("New Spam instance created")
        if Spam.__numInstances == 2:
            print("You have created two instance of Spam")
    def printNumInstances(): # class method
        print("Number of instances created: %s" % Spam.__numInstances)

obj_1 = Spam()
obj_2 = Spam()
obj_3 = Spam()
# 1. Call of class method
Spam.printNumInstances()
# 2. Access of class variable from outside
Spam.__numInstances = 5
# 3. Check if outside access has changed class variable's value
Spam.printNumInstances()
print("After outside change, number of instances created: %s" % Spam.__numInstances)
# 4. Access of class variable through instance
print("numInstances's value in obj_1: %s" % obj_1.__numInstances)
obj_1.__numInstances = 10
print("numInstances's value in obj_1: %s" % obj_1.__numInstances)
print("numInstances's value in obj_2: %s" % obj_2.__numInstances)
print("numInstances

>>>
New Spam instance created
New Spam instance created
You have created two instance of Spam
New Spam instance created
Number of instances created: 3
Number of instances created: 3
After outside change, number of instances created: 5
numInstances's value in obj_1: 5
numInstances's value in obj_1: 10
numInstances's value in obj_2: 5
numInstances's value in Spam: 5
>>>