实例化时调用__str__和__repr__

Question

我注意到，当我尝试创建一个类的实例时，__str__ 和 __repr__ 这两个方法会被调用。我在很多文档中看到，__str__ 是在打印操作时被调用的，而 __repr__ 的功能和 __str__ 类似。我发现当我尝试创建一个对象时，这些方法也会被调用。有人能帮我理解这是怎么回事吗？

class A:
    print "You are in Class A"
    #The init method is a special method used when instantiating a class
    #Python looks for this function if its present in the class the arguments passed during the   instantiation of a class
    #is based on the arguments defined in this method. The __init__ method is executed during instantiation

    def __init__(self):
        print self
        print "You aer inside init"

    #Once the instance of a class is created its called an object. If you want to call an object like a method
    #say x=A() here x is the object and A is the class
    #Now you would want to call x() like a method then __call__ method must be defined in that class
    def __call__(self):
        print "You are inside call"

    #The __str__ is called when we use the print function on the instance of the class
    #say x=A() where x is the instance of A. When we use print x then __str__ function will be called
    #if there is no __str__ method defined in the user defined class then by default it will print
    #the class it belongs to and also the memory address
    def __str__(self):
        print "You are in str"
        return "You are inside str"

    #The __repr__ is called when we try to see the contents of the object
    #say x=A() where x is the instance of the A. When we use x it prints a value in the interpreter
    #this value is the one returned by __repr__ method defined in the user-defined class
    def __repr__(self):
        print "You are in repr"
        return "This is obj of A"

class B:
    print "You are in Class B"

epradne@eussjlx8001:/home/epradne>python -i classes.py
You are in Class A
You are in Class B
>>> a=A() <-------- I create the object a here
You are in str <------- Why is __str__ method executed here??
You are inside str
You aer inside init
>>> A() <------------- I just call the class and see both __str__ and __repr__ are being executed
You are in str   
You are inside str
You aer inside init
You are in repr
This is obj of A
>>>                      

Answer 1

下面是为什么在你创建一个类的实例时会调用 __str__() 的原因：

def __init__(self):
    print self
    ^^^^^^^^^^ THIS

这里，Python 需要打印这个对象。为了做到这一点，它会把这个对象转换成字符串（通过调用 __str__()），然后打印出这个字符串。

另外，你会看到在第二个例子中调用了 __repr__()，因为交互式命令行想要打印出 A() 的结果。