Can I link two variables such that one is double of another in python?

Question

So basically I have two lists a and b and have defined them like a=b=[10] so changing one will change the other-

a=b=[10]
a[0]+=1
a
>>>[11]
b
>>>[11]

Is there a way to do this but instead make it give double of the variable? Desired output -

a=b=[10]
#some code
a
>>>[10]
b
>>>[20]

You could do this with instance attributes (make `instance.b` be a property, that calculates a value from `instance.a`), but not with plain variables. — jasonharper, Dec 17 '21 at 04:32
The problem is that once you do it you cannot change the values like - ```class foo: def __init__(self,a): self.a=a self.b=a*2``` then if you call it ```c=foo(2)``` then do ```c.a=3``` then ```c.b``` does not change to 6 — sr0812, Dec 17 '21 at 04:36

score 1 · Answer 1 · answered Dec 17 '21 at 05:11

You can create a new class that inherits from list and create a property that returns the modified values:

class new_list(list):
    @property
    def double(self):
        return [i * 2 for i in self]

        
a = new_list([10])

a.append(20)
a.append(30)
print(a, a.double)

[10, 20, 30] [20, 40, 60]

The advantage with this approach is that you still be able to use all methods from list directly.

score 0 · Accepted Answer · answered Dec 17 '21 at 04:41

0

I'm not sure if I get what you're trying to do correctly, but something like this should work.

prop = property(lambda self: self.a * 2)
# Dynamically creating a class with a default value for `a` and `b`
Class = type('ClassName', (object,), {"a": 0, "b": prop})

c = Class()
c.a = [10]
print(c.a, c.b)  # ([10], [10, 10])

answered Dec 17 '21 at 04:41

Wiggy A.

496
3
16

Thank you!, this is not exactly what I wanted, but it solved my problem nonetheless. – sr0812 Dec 17 '21 at 04:46

DEBASHISROY ROY · Answer 3 · 2021-12-17T04:42:47.713

0

with a=b=[0] you don't actually have two lists. The assignment just copies the reference to the list, not the actual list, so both liat 'a' and list 'b' refer to the same list after the assignment.

a=b=[10]
print(a) // [10]
b = list(a)[0] * 2 
a[0] += 1

print(a) // [11]
print(b) // [20]

To actually copy a into b you have various option simplest one would be using using the built in list() function to make a actual copy . Thank You.

edited Dec 17 '21 at 04:42

answered Dec 17 '21 at 04:41

DEBASHISROY ROY

1
2

I don't want that, I want to make it so that when I modify one of the variables, the other one is also modified – sr0812 Dec 17 '21 at 04:42
okay, I think you need a event system here. Idea is to listen for a value change event in list 'a' and then trigger a method which would modify list 'b' – DEBASHISROY ROY Dec 17 '21 at 05:20
this might be helpful https://stackoverflow.com/questions/2612802/list-changes-unexpectedly-after-assignment-why-is-this-and-how-can-i-prevent-it – DEBASHISROY ROY Dec 17 '21 at 05:21

score 0 · Answer 4 · answered Dec 17 '21 at 04:46

0

Because a=b=[10] is equal to b=[10];a=b so they are the same thing with only one instance. You can just

b=[a[0]*2]

or

b=[x*2 for x in a]

answered Dec 17 '21 at 04:46

Ben

141
8

Can I link two variables such that one is double of another in python?

4 Answers4