Access another child class from parent

Question

I have the code:

class Class1(object):
    class Class2:
        var1 = value1

    class Class3:
        var1 = Class2.var1 + value2

How can I access in Class3 the value I want from Class2?

@ReutSharabani Nope, that doesn't work because Class1 doesn't exist yet. Not sure if this is even possible as written — CrazyCasta, Jul 03 '15 at 10:23
Yes, Class1 is not constructed. Is there other way of accessing Class2 before Class1 is constructed? — berendeanicolae, Jul 03 '15 at 10:24
I am using reflection to build an object from the classes in a file and I would like to group some parameters this way. — berendeanicolae, Jul 03 '15 at 10:37
I'm not sure that's answered my question. Could you provide a less abstract example? I suspect that this is really an [XY problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — jonrsharpe, Jul 03 '15 at 11:00
Class1 contains some Programs and each class within Class1, contains the paths to various paths from those programs. Class3 is a program that is stored inside Class2's program. — berendeanicolae, Jul 03 '15 at 11:14
@user3668810 I might be wrong but from your last comment it looks like you are confusing classes and instances. — bruno desthuilliers, Jul 03 '15 at 12:08
@bruno desthuilliers I think I did not explain well. I use the following class to build an object so I can access paths like obj.Paths.Program1.location [Pastebin Snippet](http://pastebin.com/1KYp3Kpb) — berendeanicolae, Jul 03 '15 at 12:58

score 2 · Answer 1 · answered Jul 03 '15 at 10:30

2

You can use @classmethod's to access class variables without instantiating the class

In [17]: class Class1(object):
    class Class2:
        @classmethod
        def setvar(cls,value):
            cls.var1 = value
    class Class3:
        @classmethod
        def setvar(cls, value):
            cls.var1= value
    @classmethod
    def c2var(cls, val):
        cls.Class2.setvar(val)
    @classmethod
    def c3var(cls, val):
        cls.Class3.setvar(val + cls.Class2.var)


In [18]: Class1.c2var(5)

In [19]: Class1.c3var(9)

In [20]: Class1.Class2.var1
Out[20]: 5

In [21]: Class1.Class3.var1
Out[21]: 14

answered Jul 03 '15 at 10:30

shaktimaan

1,769
13
14

I would like to have values instead of methods because I am using reflection to build an object from the values from that class (and I would like to initialize the values inside the class). – berendeanicolae Jul 03 '15 at 10:34
That can not be done coz there is no way for python to confirm whether parent class has the attribute you want from it until class itself is defined. – shaktimaan Jul 03 '15 at 10:44
I that case, I think I can come up with some modifications to use your solution. Thanks! – berendeanicolae Jul 03 '15 at 10:49
@shaktimaan: you don't need anything to access *class* attributes without instanciating the class - a class is an object too (instance of it's metaclass), and as such works just like any other object. – bruno desthuilliers Jul 03 '15 at 12:05
1

@ bruno desthuilliers I didi not mean that the class needs to be instantiated before accessing its attributes. I meant that it needs to be defined atleast. In the case above you wont be able to access `Class1` inside `Class3` (as requested by user) as the `Class1` is itself in process of defining. – shaktimaan Jul 03 '15 at 12:16

score 1 · Answer 2 · edited May 23 '17 at 12:21

You can delay the computation by defining @classproperty:

class classproperty(object):
    def __init__(self, f):
        self.f = f
    def __get__(self, obj, owner):
        return self.f(owner)

Now all you need is this:

class Class1(object):
    class Class2:
        var1 = 1
    class Class3:
        @classproperty
        def var1(cls):
            return Class1.Class2.var1 + 1

Some output:

>>> Class1.Class3.var1
2
>>> Class1.Class2.var1 = 3
>>> Class1.Class3.var1
4

Access another child class from parent

2 Answers2