使用Super设置父类属性

Question

Can you set parent class attributes using super() in python 2.7? I would expect the following code to work, but it does not:

class A(object):
    val_a = 1

class B(A):
    val_b = 2

    def set_a(self, val):
        super(B,self).__class__.val_a = val

b = B()

b.set_a(3)

It gives the following error:

TypeError: can't set attributes of built-in/extension type 'super'

Is there a correct way to do this?

Answer 1

Can you set static class attributes using super() in python 2.7?

No.

I would expect the following code to work, but it does not

First, __class__ actually gets special-cased by super , to return the super instance's class, rather than performing the MRO search super usually does.

Second, there's no B.__dict__['__class__'] entry for super to bypass, so even if __class__ wasn't special-cased, it'd still be B instead of A .

Third, if you want to refer specifically to class A , rather than whatever happens to come next in self 's MRO, you should just refer to A directly.

---

Long story short: use A , not super .

Answer 2

Reference A.val_a directly. val_a in this case is a "static" variable, not an "attribute" of an object instance.

The super call tries to get the object of self instance, and thus access to attribute super(B, self).val_a will fail.

class A(object):
    val_a = 1

class B(A):
    val_b = 2

    def set_a(self, val):
        A.val_a = val 

b = B()
b.set_a(3)

print b.val_a

Answer 3

You can't do it with super() , but you can search through base classes of the subclass and do it that way.

Here's what I mean:

class Other(object):
    val_a = 42

class A(Other):
    val_x = 1

    def __str__(self):
        return '{}(val_a={})'.format(self.__class__.__name__, self.val_a)

class B(A):
    val_b = 2

    def set_a(self, val):
        for cls in self.__class__.__mro__[1:]:
            try:
                object.__getattribute__(cls, 'val_a')
            except AttributeError:
                continue
            print('Updating class: {}'.format(cls.__name__))
            cls.val_a = val
            break
        else:
            raise RuntimeError("Can't find a base class with attribute 'val_a'")

b = B()
print(b)  # -> B(val_a=1)                     # B(val_a=42)
print('Other.val_a: {}'.format(Other.val_a))  # Other.val_a: 42
print('A.val_a: {}'.format(A.val_a))          # A.val_a: 42
print('B.val_a: {}'.format(B.val_a))          # B.val_a: 42
print('')
b.set_a(3)                                    # Updating class: Other
print('')
print(b)                                      # B(val_a=3)
print('Other.val_a: {}'.format(Other.val_a))  # Other.val_a: 3
print('A.val_a: {}'.format(A.val_a))          # A.val_a: 3
print('B.val_a: {}'.format(B.val_a))          # B.val_a: 3

Answer 4

According to your code you are trying to make the change for the instance. Then, the function set_a needs to be as below.

class B(A):
    val_b = 2

    def set_a(self, val):
        self.val_a = val

But if you need the change to be applicable for the class level, you can use a class method and you can't use super()

class B(A):
    val_b = 2

    @classmethod
    def set_a_class(cls, val):
        cls.val_a = val

B.set_a_class(5)
print B.val_a

b = B()
print b.val_a

will return

5

5

Answer 5

See Static variable inheritance in Python

class A(object):
    val_a = 1


class B(A):
    val_b = 2
    def set_a(self, val):
        type(self).val_a = val


b1 = B()
b1.set_a(3)
b2 = B()
b2.set_a(4)
print type(b1).val_a
print type(b2).val_a
print A.val_a
print B.val_a