is this a bug?

[Vassilis Virvilis]

I am using python 2.1 in Redhat Linux.
Do you find anything irrational to the following session?
>>> a='+='
>>> a is a
1
>>> a is '+='
0
^^^^^^^^^^^^^
>>> a = 'dsd'
>>> a is 'dsd'
1
>>> '+=' is '+='
1
>>>

I apologize if this has been brought to the attention of this group before.
Maybe I did misunderstand the use of the 'is' operator, but then how it is
supposed to work?

	.Bill