Question

Consider the following simple example class, which has a property that exposes a modified version of some internal data when called:

class Foo(object):
    def __init__(self, value, offset=0):
        self._value = value
        self.offset = offset

    @property
    def value(self):
        return self._value + self.offset

    @value.setter
    def value(self, value):
        self._value = value

The value.setter works fine for regular assignment, but of course breaks down for compound assignment:

>>> x = Foo(3, 2)
>>> x.value
5
>>> x.value = 2
>>> x.value
4
>>> x.value += 5
>>> x.value
11

The desired behavior is that x.value += 5 should be equivalent to x.value = x._value + 5, as opposed to x.value = x.value + 5. Is there any way to achieve this (with the property interface) purely within the Foo class?

Answer 1

@ezod, there is no direct way to do what you’re asking for, even with the descriptors protocol.
That kind behaviour of value property totally breaks the semantics of += operator.