Question

Let’s say I have a class in Python:

class Foo(object):
    a = 1
    b = 2

I’d like to do some extra stuff when I access ‘a’ but NOT ‘b’. So, for example, let’s assume that the extra stuff I’d like to do is to increment the value of the attribute:

> f = Foo()
> f.a # Should output 2
> f.a # Should output 3
> f.a # Should output 4
> f.b # Should output 2, since I want the extra behavior just on 'a'

It feels like there is a way through __getattr__ or __getattribute__, but I couldn’t figure that out.

The extra thing can be anything, not necessarily related to the attribute (like print ‘Hello world’).

Thanks.

Answer 1

What you are looking for is a property, which can be used nicely as a decorator:

class Foo(object):
    _a = 2

    @property
    def a(self):
        Foo._a += 1
        return Foo._a - 1

    b = 2

The function is called whenever you try to access foo_instance.a, and the value returned is used as the value for the attribute. You can also define a setter too, which is called with the new value when the attribute is set.

This is presuming you want the odd set-up of class attributes you only ever access from instances. (_a and b here belong to the class – that is, there is only one variable shared by all instances – as in your question). A property, however, is always instance-owned. The most likely case is you actually want:

class Foo(object):
    def __init__(self):
        self._a = 2
        self.b = 2

    @property
    def a(self):
        self._a += 1
        return self._a - 1

Where they are instance attributes.