Python对象类方法不继承

Question

In Python operator overloading is achieved by defining special method in the class. For example, to use + operator, define __add__ method. There are many other special methods for different operators. In addition to that, all these special operators comes from object class which is the base class for every other class. That means every class inherits special method like __add__ , __ge__ , __le__ and so on.

Now Consider the following code.

class test_class:
    def ring(self):
        print("ring")

obj = test_class() 
print(obj.__ge__) 

Output:

<method-wrapper '__ge__' of test_class object at 0x0000000002247EF0>

But when I try to access __add__ using obj object, it gives me following error:

AttributeError: 'test_class' object has no attribute '__add__'

Why is that ? Does that mean __add__ method doesn't inherit from the object class ? What's exactly going on ?

Answer 1

The __add__ function is not an attribute of object . It is a function found in the operator module. You can provide your own implementation of this by implementing your own version of these methods in your class.

This is also explained in the documentation here .

Observe:

>>> class Foo:
...     def __add__(self, x):
...         print("I took over the add with {}".format(x))
...
>>>
>>>
>>> f = Foo()
>>> f + 100
I took over the add with 100
>>>

Furthermore, even if you look at what is provided to you from object , you will see:

>>> dir(object)
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']
>>>

There is no implementation of the __add__ you are looking to override. These operators are overridable by declaring them yourself in your class.

To provide some context around why operators like __eq__ (and other comparison operators) operator might be implemented as part of object and not __add__ as an example, is because, per the documentation as well ( here ), these equality operators are rich-comparison methods and they provide their own implementation in the base object. If you take a look at the implementation of the object class, you will see a docstring mention of what will be returned when you actually use these operators with instance objects:

def __ge__(self, *args, **kwargs): # real signature unknown
    """ Return self>=value. """
    pass

As an example, you can see it mentions that when you try to perform a ">" with instances that inherit from object, they will use that implementation.