LWZ LWZ - 2 months ago 20
Python Question

In class object, how to auto update attributes?

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

class SomeClass:
def __init__(self, n=0):
self.list = range(n)
self.listsquare = [ x**2 for x in self.a ]


If I make an object normally that would no problem, with

a = SomeClass(10)


I will get 2 lists,
a.list
and
a.listsquare
.

Now if I want to make a empty object first, and assign one attribute to it, I want the other attributes to be automatically updated, for example if I do

b = SomeClass()
b.list = range(5,10)


I want
b.listsquare
to be automatically updated, and also the other way around (assign
b.listsquare
and auto update
b.list
). Is this possible? Is Class the right choice for this?




Thanks to you all, but I'm completely overwhelmed by all the different answers. Can anyone give a complete solution so I can learn write my own?

I would like to achieve a class
Foo
with 3 attributes
length
,
list
and
listsquare
such that:


  1. If I do
    a = Foo(3)
    , I get
    a.length = 3
    ,
    a.list = [0, 1, 2]
    ,
    a.listsquare = [0, 1, 4]
    .

  2. If I do
    b = Foo().list = [5, 6]
    , I get
    b.length = 2
    ,
    b.listsquare = [25, 36]
    .

  3. If I do
    c = Foo().listsquare = [4, 9]
    , I get
    c.length = 2
    ,
    c.list = [2, 3]
    .


Answer

if updating one property due to an update on another property is what you're looking for (instead of recomputing the value of the downstream property on access) use property setters:

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

    @property
    def list(self):
        return self._list
    @list.setter
    def list(self, val):
        self._list = val
        self._listsquare = [x**2 for x in self._list ]

    @property
    def listsquare(self):
        return self._listsquare
    @listsquare.setter
    def listsquare(self, val):
        self.list = [int(pow(x, 0.5)) for x in val]

>>> c = SomeClass(5)
>>> c.listsquare
[0, 1, 4, 9, 16]
>>> c.list
[0, 1, 2, 3, 4]
>>> c.list = range(0,6)
>>> c.list
[0, 1, 2, 3, 4, 5]
>>> c.listsquare
[0, 1, 4, 9, 16, 25]
>>> c.listsquare = [x**2 for x in range(0,10)]
>>> c.list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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