Dynamically update attributes of an object that depend on the state of other attributes of same object

Question

Say I have an class that looks like this:

class Test(object):
   def __init__(self, a, b):
      self.a = a
      self.b = b
      self.c = self.a + self.b 
         


        

Answer 1

The simplest solution is to make c a read-only property:

class Test(object):

    def __init__(self, a, b):
        self.a = a
        self.b = b

    @property
    def c(self):
        return self.a + self.b

Now every time you access test_instance.c, it calls the property getter and calculates the appropriate value from the other attributes. In use:

>>> t = Test(2, 4)
>>> t.c
6
>>> t.a = 3
>>> t.c
7

Note that this means that you cannot set c directly:

>>> t.c = 6

Traceback (most recent call last):
  File "", line 1, in 
    t.c = 6
AttributeError: can't set attribute