python use lazy assignment when passing lists to methods

Question

I have a python class A and its child B. I make an object of class B passing some list-type arguments, which are appended in B and the result stored in class A. I would like to be able to change one of those initial arguments and see the change reflected without further ado. Let me explain with an example:

class A():
    def __init__(self,data):
        self.a=data

class B(A):
    def __init__(self,x,y):
        self.x=x
        self.y=y
        A.__init__(self,self.x+self.y)

    def change_x(self,x):
        self.x = x

Now, wen i run

test = B([1, 2], [3,4])
print(test.a)

I obviously get [1,2,3,4]. If I change part of the list as follows:

test.change_x([3,4])
print(test.a)

I would like to get [3,4,3,4], instead of course, I receive [1,2,3,4]. Of course test.a is only evaluated during instantiation.

I understand why this is not the case and I read about generators but don't manage to figure out how to implement this. I don't want to end up with an iterable that i can only iterate once.

Could anyone help me? A clean way to solve this?

I could of course try something like: def change_x(self,x): self.__init__(x,self.y) But I'm not sure whether this is a good approach? — chrisvp
– chrisvp, Commented Aug 21, 2016 at 21:25
There isn't really a way to do that with your current design. In your example, why is A a class at all (and why is it a superclass of B)? What you could do is give B a method that does the equivalent of self.a.data, but computes the result dynamically. Basically, when you create a, you are creating it with some data, and a knows nothing about where that data came from. If you want to get updated data, it may be easier to do it in B and not involve a at all. — BrenBarn
– BrenBarn, Commented Aug 21, 2016 at 21:27
Hi, I tried to make kind of a minimal working design, which lost all meaning. I'm implementing kind of a network protocol, class A is just the container for a packet. class B would have knowledge of header, cargo, trailer split. class, e.g., wifi, would be derrived from B and describe the actual header, and trailer. class A would have a transmit function, class B would just concatenate data and perhaps calculate a crc. — chrisvp
– chrisvp, Commented Aug 22, 2016 at 6:00

Karin · Accepted Answer · 2016-08-21 21:36:25Z

6

Is there a reason to have the class A at all? You could create a property or method a that returns what you want within class B.

class B():
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def change_x(self, x):
        self.x = x

    @property
    def a(self):
        return self.x + self.y


test = B([1, 2], [3, 4])
print(test.a)  # [1, 2, 3, 4]

test.change_x([3, 4])
print(test.a)  # [3, 4, 3, 4]

Small note: With this implementation, the change_x method shouldn't be necessary. It's generally more Pythonic to just access attributes directly (e.g. test.x = x) than to use getters and setters.

edited Aug 21, 2016 at 21:36

answered Aug 21, 2016 at 21:30

Karin

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1 Comment

chrisvp Over a year ago

Hi Karin, thank you for your reply. As I just commented above: Hi, I tried to make kind of a minimal working design, which lost all meaning. I'm implementing kind of a network protocol, class A is just the container for a packet. class B would have knowledge of header, cargo, trailer split. class, e.g., wifi, would be derrived from B and describe the actual header, and trailer. class A would have a transmit function, class B would just concatenate data and perhaps calculate a crc change_x could e.g. rather be def mess_up_crc(self) (for testing purposes)

Israel Unterman · Accepted Answer · 2016-08-21 21:28:49Z

1

You need to implement it "manually". For example, like so:

class B(A):
    def __init__(self,x,y):
        self.x=x
        self.y=y
        self.spread_changes()

    def change_x(self,x):
        self.x = x
        self.spread_changes()

    def spread_changes(self):
        A.__init__(self,self.x+self.y)

answered Aug 21, 2016 at 21:28

Israel Unterman

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Comments

chrisvp · Accepted Answer · 2016-08-23 05:35:53Z

0

I ended up storing all parts of self.a (in A) directly by slicing/indexing from it's inheriting classes. I added a method "B.update_crc(self)" as suggested by Israel Unterman to reevaluate certain parts of self.a in case of a modification.

Thanks, my first approach just wan't the right one :)

answered Aug 23, 2016 at 5:35

chrisvp

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