Question

I defined such simple class as below:

>>> class myclass(object):
    name = "google"

And I am creating an instance to call name attribute

>>> a = myclass()
>>> a.name
'google'

and also calling it directly from the class itself.

>>> myclass.name
'google'

So far (as I am having background with C# and Java) I will call myclass.name static invocation and calling static members from instances are fine.

But what I am observing is till I set a.name with another value, changing myclass.name is also affecting a.name. Once I set a.name, changing myclass.name doesn’t affect the a.name any more and they begin to have different values.

You can see the whole experiment below:

>>> class myclass(object):
    name = "google"


>>> myclass.name
'google'
>>> a = myclass()
>>> a.name
'google'
>>> myclass.name = "yahoo"
>>> a.name
'yahoo'
>>> myclass.name = "hello world"
>>> a.name
'hello world'
>>> myclass.name
'hello world'
>>> myclass.name = "another trick"
>>> myclass.name
'another trick'
>>> a.name
'another trick'
>>> a.name = "I changed the value of a"
>>> a.name
'I changed the value of a'
>>> myclass.name
'another trick'
>>> myclass.name = "changed again"
>>> a.name
'I changed the value of a'

I would like someone to explain the reason behind this behavior.

Thanks.

Answer 1

I took a look at Python Data Model documentation. If you skip ahead to “Class instances” section, you will see the crucial explanation:

Attribute assignments and deletions update the instance’s dictionary,
never a class’s dictionary.

So in your example, when you do a.name = ..., you introduce a new key name into a’s __dict__. After that when you access name attribute, the standard attribute lookup mechanism looks into a’s __dict__ first.