difference in binding between strings and tuples?

[Iwan van der Kleyn]

I tried to explain a Java minded collegue the concept of name binding or 
variable assignement, straight from Python in a Nutshell. (which by the 
way is, together with the Cookbook a very niece piece of work). But then 
it proved that perhaps I really did not fully grasp all aspects of it 
myself.  Check out:

 >>> a = 1
 >>> b = 1
 >>> a == b
1
 >>> a is b
1
 >>> a += 1
 >>> b += 1
 >>> a == b
1
 >>> a is b
1
 >>> x = 'test'
 >>> y = 'test'
 >>> x == y
1
 >>> x is y
1
 >>> x += 's'
 >>> x == y
1
 >>> x is y
0
 >>> q = (0, 1)
 >>> r = (0, 1)
 >>> q == r
1
 >>> q is r
0


Ok? Testing for equality gives no suprises, but testing for identity 
does, especially considering the differences between strings and tuples 
(both of which are immutable)
I had expected  (a is b) == True, even after the addidtion. But how do I 
explain the results for the strings and tuples?

Can I presume x and y to refer to the same string  object before the 
append ("addition" ) but to two seperate objects afterwards? Why isn't 
that true for the tuples? In other words: why the difference in binding 
between strings and tuples?

Regards,

iwan