Question

I learned that Python class attributes are like static data members in C++. However, I got confused after trying the following code:

>>> class Foo:
...     a=1
... 
>>> f1=Foo();
>>> f2=Foo()
>>> f1.a
1
>>> f1.a=5
>>> f1.a
5
>>> f2.a
1

Shouldn’t f2.a also equal 5?

If a is defined as a list instead of an integer, the behavior is expected:

>>> class Foo:
...     a=[]
... 
>>> f1=Foo();
>>> f2=Foo()
>>> f1.a
[]
>>> f1.a.append(5)
>>> f1.a
[5]
>>> f2.a
[5]

I looked at
What is the difference between class and instance attributes?, but it doesn’t answer my question.

Can anyone explain why the difference?

Answer 1

You’re not doing the same thing in your second example. In you first example, you are assigning f1.a a new value:

f1.a = 5

In your second example, you are simply extending a list:

f1.a.append(5)

This doesn’t change what f1.a is pointing to. If you were instead to do this:

f1.a = [5]

You would find that this behaves the same as your first example.

But consider this example:

>>> f1=Foo()
>>> f2=Foo()
>>> Foo.a = 5
>>> f1.a
5
>>> f2.a
5

In this example, we’re actually changing the value of the class attribute,
and the change is visible in all instances of the class. When you
type:

f1.a = 5

You’re overriding the class attribute with an instance attribute.