class object's attribute is also the instance's attribute?

[Ulrich Eckhardt]

Am 30.08.2012 12:55, schrieb 陈伟:
> class A(object):
>      d = 'it is a doc.'
>
> t = A()
>
> print t.__class__.d
> print t.d
>
> the output is same.

You could go even further:

print id(t.__class__.d)
print id(t.d)

which should show you that they are not just equal but identical.


> so it means class object's attribute is also the instance's
> attribute.is it right?

Yes. This is even useful sometimes:

class Point(object):
     x = 0
     y = 0

This will cause every Point to have two attributes x and y that have a 
default value 0.


Note that setting this attribute on an instance does not change the 
class' attribute, just in that that was what confused you. However, if 
the attribute references a mutable type (e.g. a list) this can cause 
problems because the instance (see id() above) is the same and thus 
modifications affect both the class and all instances.


Uli