How to call a property of the base class if this property is being overwritten in the derived class?

Question

I'm changing some classes of mine from an extensive use of getters and setters to a more pythonic use of properties.

But now I'm stuck because some of my previous getters or setters would call the corresponding method of the base class, and then perform something else. But how can this be accomplished with properties? How to call the property getter or setter in the parent class?

Of course just calling the attribute itself gives infinite recursion.

class Foo(object):

    @property
    def bar(self):
        return 5

    @bar.setter
    def bar(self, a):
        print a

class FooBar(Foo):

    @property
    def bar(self):
        # return the same value
        # as in the base class
        return self.bar # --> recursion!

    @bar.setter
    def bar(self, c):
        # perform the same action
        # as in the base class
        self.bar = c    # --> recursion!
        # then do something else
        print 'something else'

fb = FooBar()
fb.bar = 7

Solution 1

You might think you could call the base class function which is called by property:

class FooBar(Foo):

    @property
    def bar(self):
        # return the same value
        # as in the base class
        return Foo.bar(self)

Though this is the most obvious thing to try I think - it does not work because bar is a property, not a callable.

But a property is just an object, with a getter method to find the corresponding attribute:

class FooBar(Foo):

    @property
    def bar(self):
        # return the same value
        # as in the base class
        return Foo.bar.fget(self)

Solution 2

super() should do the trick:

return super().bar

In Python 2.x you need to use the more verbose syntax:

return super(FooBar, self).bar

Solution 3

There is an alternative using super that does not require to explicitly reference the base class name.

Base class A:

class A(object):
    def __init__(self):
        self._prop = None

    @property
    def prop(self):
        return self._prop

    @prop.setter
    def prop(self, value):
        self._prop = value

class B(A):
    # we want to extend prop here
    pass

In B, accessing the property getter of the parent class A:

As others have already answered, it's:

super(B, self).prop

Or in Python 3:

super().prop

This returns the value returned by the getter of the property, not the getter itself but it's sufficient to extend the getter.

In B, accessing the property setter of the parent class A:

The best recommendation I've seen so far is the following:

A.prop.fset(self, value)

I believe this one is better:

super(B, self.__class__).prop.fset(self, value)

In this example both options are equivalent but using super has the advantage of being independent from the base classes of B. If B were to inherit from a C class also extending the property, you would not have to update B's code.

Full code of B extending A's property:

class B(A):
    @property
    def prop(self):
        value = super(B, self).prop
        # do something with / modify value here
        return value

    @prop.setter
    def prop(self, value):
        # do something with / modify value here
        super(B, self.__class__).prop.fset(self, value)

One caveat:

Unless your property doesn't have a setter, you have to define both the setter and the getter in B even if you only change the behaviour of one of them.

Solution 4

try

@property
def bar:
    return super(FooBar, self).bar

Although I'm not sure if python supports calling the base class property. A property is actually a callable object which is set up with the function specified and then replaces that name in the class. This could easily mean that there is no super function available.

You could always switch your syntax to use the property() function though:

class Foo(object):

    def _getbar(self):
        return 5

    def _setbar(self, a):
        print a

    bar = property(_getbar, _setbar)

class FooBar(Foo):

    def _getbar(self):
        # return the same value
        # as in the base class
        return super(FooBar, self)._getbar()

    def bar(self, c):
        super(FooBar, self)._setbar(c)
        print "Something else"

    bar = property(_getbar, _setbar)

fb = FooBar()
fb.bar = 7

Solution 5

Some small improvements to Maxime's answer:

• Using __class__ to avoid writing B. Note that self.__class__ is the runtime type of self, but __class__ without self is the name of the enclosing class definition. super() is a shorthand for super(__class__, self).
• Using __set__ instead of fset. The latter is specific to propertys, but the former applies to all property-like objects (descriptors).

class B(A):
    @property
    def prop(self):
        value = super().prop
        # do something with / modify value here
        return value

    @prop.setter
    def prop(self, value):
        # do something with / modify value here
        super(__class__, self.__class__).prop.__set__(self, value)

View more solutions

Comments

[Aaron Maenpaa]

TypeError: super() takes at least 1 argument (0 given)

[shylent]

I guess (well, judging by the link :P), this answer is python3-related. In python3 super() can take zero arguments, yes.

[Pankrat]

super() works without arguments in Python 3 and is equivalent to super(MyClass, self) but more readable in my book.

[akaihola]

you are: he's talking about properties, not plain methods

[akaihola]

This works fine if you write the base class. But what if you extend a third-party base class which uses the same name for the property and the getter?

[nerdoc]

Why should I call Foo.bar.fset(self, c) in an inherited setter? Why not Foo.bar.fset(c) - without the "self" -I thought this is implicitely passed?

[hithwen]

I get a TypeError: 'property' object is not callable

[hithwen]

In python 2.7 I get AttributeError: 'super' object has no attribute 'bar'

[Rob Smallshire]

super().bar seems to work fine for the getter, but doesn't work for assignment through the base property in an overridden setter. If I do super().bar = 3 I get AttributeError: 'super' object has no attribute 'bar'

[Pankrat]

Good point Rob, didn't know that. Here's more information: stackoverflow.com/questions/10810369/…

[Ethan Furman]

While this works for getting, it fails for setting with an AttributeError.

[Tadhg McDonald-Jensen]

@nerdoc where does the self get implies from the chain Foo.bar.fset?

[nerdoc]

Just a thought - AFAIK self is always implicitly passed. If you do a foo = Foo() \ foo.bar(c) there is no self passed, but bar() receives it from Python. I'm no Python expert, more or less a beginner too. It's just a thought.

[Sang]

Hello, this is very helpful. I have a follow-up question to this. This setup works well; however, it stops working when I set init for B to define additional properties. Is there a way to have a separate init for B? Thank you

[Art]

Could you please explain how super(B, self.__class__) works exactly with super(class, class)? Where is it documented?

[TallChuck]

self is only implicitly passed when the method is called on an instance of a class. For example, if I have a class A with a method b(self, arg), and I create an instance c = A(), then calling c.b(arg) is equivalent to A.b(c, arg)

[michael]

The above works for the getter. For the setter: super(self.__class__, self.__class__).bar.__set__(self, value) # See bugs.python.org/issue14965

[Eric]

I've suggested some small tweaks to this answer in my answer

[Matteo Ferla]

I thought I'd add in case someone has a reason not to want to call the first base class by name even if it obfuscates the code... Foo is self.__class__.__bases__[0] (handly trick also for super(self.__class__.__bases__[1], self).

[CharlesB]

What''s unclear to me is the difference between super(__class__, self) and super(__class__, self.__class__), and why we need the latter for setting property of the ancestor class?

[Eric]

From the help() for super, there are two overloads - super(type, obj), which is the first case, and super(type, type2), which is the second case. We're accessing .prop as a class attribute not an instance attribute, which is why the latter is needed.

[Will Razen]

super(type(self), type(self)).setter.fset(self, value) doesn't work adequately with multiple inheritance. Try my solution duper(super()).setter = value: gist.github.com/willrazen/bef3fcb26a83dffb6692e5e10d3e67ac

[Eric]

In what case does it not work adequately?

[Eric]

super(Child, Child) is wrong, it should be Super(Child, self.__class__)