Why instance's character could be changed by class and share the same memory with class?

Python Help
1

Plz look at this first:

class test:
    test_value = 10
    def __init__(self):
        pass

a = test()
b = test()
print(id(a.test_value))
print(id(b.test_value))
print(id(test.test_value))

The outcome turns to be these 3 varieties memory are the same.

Is this proper？

I think an instance should have an unique memory after born from a class, cause they two different objects.

After all, instance is instance and class is class according to python’s saying " everything is an object in python."

2

test_value is a class variable shared by all instances. If you don’t want this behavior you can use instance variables which you can initialize in the __init__ method. See the docs:

Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:

class Dog:

    kind = 'canine'         # class variable shared by all instances

    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

>>> d = Dog('Fido')
>>> e = Dog('Buddy')
>>> d.kind                  # shared by all dogs
'canine'
>>> e.kind                  # shared by all dogs
'canine'
>>> d.name                  # unique to d
'Fido'
>>> e.name                  # unique to e
'Buddy'
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3

these are all printing the id of 10. Because 10 is immutable, it is safe to just give all variables that have the value 10 a pointer to the place where Python stores the constant 10.

change 10 to 10.0 for further experiments.

so long as the class does not use slots, a.test_value first looks up a.__dict__["test_value"], then if that doesn’t exist it looks up type(a).test_value. That’s the case you’re seeing here. It is mostly safe, and more efficient than the alternative you’re proposing.

1 Like
4

thanks a lot for reply，have read the docs，mine is a wrong grammar indeed, class attribute’s name should not be the same as the instance attribute’s name, or the latter will cover the former.

5

(post deleted by author)

6

Not because it is immutable, so much as because small integers are pre-allocated, so you always get the same instance. @ayhanfuat 's answer is good. If you want a distinct variable for each instance then it is done like this:

class test:
    def __init__(self):
        self.test_value = 12.345

        
a = test()
b = test()
id(12.345), id(a.test_value), id(b.test_value)
(2706378293104, 2706378293168, 2706378293168)

The value 12.345 is not pre-allocated, but the instances a and b still point to one instance of it, because the code that was generated for __init__ only needs one copy of the constant.

@jackBang Here’s another one to think about:

class test:
    def __init__(self):
        self.test_value = bytearray()

a = test()
b = test()
id(bytearray()), id(a.test_value), id(b.test_value)
(2706384672368, 2706384670256, 2706384672176)
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