python类中的__call__、__init__、__getattr__、__setattr__、__setattr__详解

__new__(cls, *args, **kwargs)

创建对象时调用，返回当前对象的一个实例（注意：这里的第一个参数是cls即class本身）

__init__(self, *args, **kwargs)

创建完对象后调用，对当前对象的实例的一些初始化，无返回值

__call__(self, *args, **kwargs)

如果类实现了这个方法，相当于把这个类型的对象当作函数来使用，相当于 重载了括号运算符

class O(object):

    def __init__(self, *args, **kwargs):
        print "init"
        super(O, self).__init__(*args, **kwargs)

    def __new__(cls, *args, **kwargs):
        print "new", cls
        return super(O, cls).__new__(cls, *args, **kwargs)

    def __call__(self,  *args, **kwargs):
        print "call"

test = O('12')  # output new <class '__main__.O'> , init
test()  # output call

---

__str__

对象打印出来的字符串（返回用户看到的）

__repr

对象打印出来的字符串（返回开发者看到的）

class Student(object):
    def __init__(self, name):
        self.name = name

    def __str__(self):
        print '__str__'
        return 'name of this student is %s'%self.name

    def __repr__(self):
        return 'name of this student is %s'%self.name

print Student('Xiaoming')  # output: __str__, name of this student is Xiaoming
>>> print Student('Xiaoming')  # output: __repr__, name of this student is Xiaoming

---

__iter__

可以使类的实例是一个可迭代的对象，需实现next()方法

ex：

class Fib(object):

    def __init__(self):
        self.a, self.b = 0, 1 # 初始化两个计数器a，b

    def __iter__(self):
        return self # 实例本身就是迭代对象，故返回自己

    def next(self):
        self.a, self.b = self.b, self.a + self.b # 计算下一个值
        if self.a > 100000: # 退出循环的条件
            raise StopIteration();
        return self.a # 返回下一个值

>>> for n in Fib():
... print n
...
1
1
2
3
5
...
46368

75025

---

__getitem__

对象像list那样按照下标取出元素（注意：如若需要使用切片等方法需要自己实现）

class Fib(object):
    def __getitem__(self, n):
        a, b = 1, 1
        for x in range(n):
            a, b = b, a + b
        return a

f = Fib()
print f[0]  # output: 1
print f[1]  # output: 1
print f[10]  # output: 89

---

__getattr__(self, name)

当调用不存在的属性时访问这个方法

__get__(self, name)

无条件被调用，通过实例访问属性。如果class中定义了__getattr__()，则__getattr__()不会被调用（除非显示调用或引发AttributeError异常） 

__getattribute__((self, instance, owner)

如果class定义了它，则这个class就可以称为descriptor。owner是所有者的类，instance是访问descriptor的实 例，如果不是通过实例访问，而是通过类访问的话，instance则为None。（descriptor的实例自己访问自己是不会触发__get__，而 会触发__call__，只有descriptor作为其它类的属性才有意义。）（所以下文的d是作为C2的一个属性被调用） 

class C(object):
    a1 = 'abc'
    def __getattribute__(self, *args, **kwargs):
        print("__getattribute__() is called")
        return object.__getattribute__(self, *args, **kwargs)

    def __getattr__(self, name):
        print("__getattr__() is called ")
        return name + " from getattr"

    def __get__(self, instance, owner):
        print("__get__() is called", instance, owner)
        return self

    def foo(self, x):
        print(x)

class C2(object):
    d = C()

if __name__ == '__main__':
    c = C()
    c2 = C2()
    print(c.a1) 
    # __getattribute__() is called, 
    # abc
    print(c.a2)
    # __getattribute__() is called
    # __getattr__() is called 
    # a2 from getattr
    print(c2.d.a1)
    # ('__get__() is called', <__main__.C2 object at 0x00000000027444A8>, <class '__main__.C2'>)
    # __getattribute__() is called
    # abc
 
__setattr__(self, name)
设置属性的方法
class A(object):
    def __setattr__(self, *args, **kwargs):
        print 'call func set attr'
        return object.__setattr__(self, *args, **kwargs)
a = A()
a.a1 = 123
print a.a1