python: class attributes and instance attributes

Question

I\'m new to python and learned that class attributes are like static data members in C++. However, I got confused after trying the following code:

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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.