Difference between _ get __,__ getattr __,__ getattribute _ in python3, python3 _ get __

Difference between _ get __,__ getattr __,__ getattribute _ in python3, python3 _ get __

_ Get __,__ getattr _ and _ getattribute are both methods for accessing attributes, but they are not the same.
Object. _ getattr _ (self, name)
If the attribute cannot be found in a common location, getattr is called to return a value or AttributeError.

Object. _ getattribute _ (self, name)
It is called unconditionally and accessed through an instance. If _ getattr _ () is defined in the class, _ getattr _ () is not called unless it is displayed as a call or an AttributeError exception is thrown)

Object. _ get _ (self, instance, owner)
If a class is defined, this class can be called a descriptor. The owner is the owner's class, And the instance is the instance that accesses the descriptor. If it is accessed not through the instance but through the class, the instance is None. (When a descriptor instance accesses itself, it will not trigger _ get __, but will trigger _ call __. only the descriptor as an attribute of other classes makes sense .) (So the following d is called as an attribute of C2)

class C(object):    a = 'abc'    def __getattribute__(self, *args, **kwargs):        print("__getattribute__() is called")        return object.__getattribute__(self, *args, **kwargs)#        return "haha"    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.a)    print(c.zzzzzzzz)    c2.d    print(c2.d.a)

 

The output result is:

__getattribute__() is calledabc__getattribute__() is called__getattr__() is called zzzzzzzz from getattr__get__() is called <__main__.C2 object at 0x16d2310> <class '__main__.C2'>__get__() is called <__main__.C2 object at 0x16d2310> <class '__main__.C2'>__getattribute__() is calledabc

 

Conclusion: we can see thatInstanceAll access properties pass through the _ getattribute _ function. If the attribute does not exist, you still need to access _ getattribute __, but then access _ getattr __. This is like an exception handling function.
Every time you access descriptor (that is, the class that implements _ get _), it passes through the _ get _ function first.

Note that when you use a class to access a non-existent variable, it does not pass through the _ getattr _ function. The descriptor does not have this problem, but identifies the instance as none.

From: http://luozhaoyu.iteye.com/blog/1506426