CODEWITHSUNDEEP
pythonclassoop
ViFI
ViFI

Reputation: 1049

Python class shared variables gives unexpected output

Based on the explanation on Shared Variables in Python Class here , I expected the following code to give output as :

123 123
200 200
300 300

But it is 

123 123
200 123
200 300

Code:

class A:
    abc = 123
    def __init__(self, a,b,c):
        self._a = a
        self._b = b
        self._c = c


if __name__ == '__main__':
    a = A(2, 4, 6)
    b = A(3, 9, 27)

    print a.abc , b.abc
    a.abc = 200
    print a.abc , b.abc
    A.abc = 300
    print a.abc , b.abc

Can somebody please help understand this ? My impression is that shared variables are same as static variables in C++ classes. Any insights to bust that myth, if it is so, would be helpful too.

Upvotes: 1

Views: 105

Answers (4)

ViFI
ViFI

Reputation: 1049

@Scott Hunter's answer explains the behavior of code in question best but I would like to add C++ perspective to it here. Unfortunately it couldn't be added as comment in that answer as it is too long.

As in C++, to access static members name needs to be qualified by class name (e.g. A::abc or A::func_static() ) , in Python also you have to access the shared variable outside the class using Class name as qualifier.

Within the class, C++ allows to use the static variable name as like other member variables (i.e. without any qualifier) and same is true with Python as well, as shared variables can be accessed using self such as self.abc here.

Only thing which is different here is Python allows you to create an instance variable with the same name as shared variable for that instance only.

Upvotes: 0

ospahiu
ospahiu

Reputation: 3525

You are creating a new instance variable -> a.abc and setting it to 200. Access the shared static variable instead.

a = A(2, 4, 6)
b = A(3, 9, 27)
print a.abc , b.abc
A.abc = 200  # Set static class variable, instead of creating new instance member
print a.abc , b.abc
A.abc = 300
print a.abc , b.abc

I recommend reading the very informative Python official docs on [9] Classes.

Upvotes: 4

timakro
timakro

Reputation: 1841

Your expected output would mean that you can't change the attribute of an object without changing the attribute of every instance of the objects class. That would obviously break the core of the object orientation idea. I think you can overwrite "shared" variables because this gives just more possibilities.

Upvotes: -1

Scott Hunter
Scott Hunter

Reputation: 49813

Initially, the class A has an abc defined to be 123, which each of a and b use since neither has an abc of their own.

Then you execute a.abc = 200, which creates an abc for a; b still uses the one from A.

Then you execute A.abc = 300, which updates the abc for A, which b still looks to, so it see the new value. But a has its own abc, and so doesn't care.

Upvotes: 4

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