Why is 'is' returning true for a different instance in Python

Question

This is a python beginner question:

class MyClass:
    variable = "test"

    def function(self):
        print("test")

X = MyClass()
Y = MyClass()

print (X.variable == Y.variable)    // expected true, works
print (X.variable is Y.variable)    // expected false, but return true; why?

Since there are two instances of the class, why is the second test with 'is' returning true since the variables should be difference instances?

Related: [How to avoid having class data shared among instances?](//stackoverflow.com/q/1680528) — Aran-Fey, Sep 09 '18 at 18:33

Kirollos Morkos · Answer 1 · 2018-09-09T18:48:37.000

1

In the example you've provided, variable is a class attribute on MyClass. All instances of MyClass will share the same variable instance. If you want to provide an instance variable to an instance of the class, you would do so in the __init__ method like so:

class MyClass:
    shared_var = "I'm a property on the class"

    def __init__(self):
        self.my_var = "I'm a property on an instance"

Note, however, that even if you were to do the following:

x = MyClass()
y = MyClass()
x.my_var is y.my_var # This will still be True!

This is because the two strings actually reference the same object in Python. You can see this by typing the following in a Python interpretor:

id('test')
id('test') # This will be the same as the one above!

edited Sep 09 '18 at 18:48

answered Sep 09 '18 at 18:34

Kirollos Morkos

2,503
16
22

I'm learning the syntax of python; so variables declared just below the class name are considered static? how do you set the variable as part of the instance of the class? – Thomas Sep 09 '18 at 18:37
3

It is not a static method, but a "class attribute". – wim Sep 09 '18 at 18:38
Whoops, good call @wim – Kirollos Morkos Sep 09 '18 at 18:40
@Thomas you would want to declare instance variables in the `__init__` method. I'll update my answer to provide an example. – Kirollos Morkos Sep 09 '18 at 18:41
1

@Thomas probably important to note, but *Python doesn't have variable declarations* – juanpa.arrivillaga Sep 09 '18 at 18:50

kur ag · Answer 2 · 2018-09-09T18:40:55.277

0

Because "variable" is class or static variable. If you define it inside method, you will be correct. Please look here: Are static class variables possible?

edited Sep 09 '18 at 18:40

answered Sep 09 '18 at 18:33

kur ag

591
8
19

1

"Static variable" is barely a good name in C; it has no meaning whatsoever in Python. It's a class attribute. – chepner Sep 09 '18 at 18:51

Why is 'is' returning true for a different instance in Python

2 Answers2