In class object, how to auto update attributes?

Question

I have a class which has multiple attributes that are related, for example:

class SomeClass：
    def __init__(self, n=0):
        self.list = range(n)
               


        

Answer 1

Ignacio's @property solution is great but it recalculates the list every time you reference listsquare - that could get expensive. Mathew's solution is great, but now you have function calls. You can combine these with the 'property' function. Here I define a getter and a setter for my_list (I just couldn't call it 'list'!) that generates listsquare:

class SomeClass(object):

    def __init__(self, n=5):
        self.my_list = range(n)

    def get_my_list(self):
        return self._my_list

    def set_my_list(self, val):
        self._my_list = val
        # generate listsquare when my_list is updated
        self.my_listsquare = [x**2 for x in self._my_list]

    # now my_list can be used as a variable
    my_list = property(get_my_list, set_my_list, None, 'this list is squared')

x = SomeClass(3)
print x.my_list, x.my_listsquare
x.my_list = range(10)
print x.my_list, x.my_listsquare

This outputs:

[0, 1, 2] [0, 1, 4]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]