Is behavior of += intentional for int?

[zaur]

Python 2.6.2 (r262:71600, Apr 16 2009, 09:17:39)
[GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin
Type "copyright", "credits" or "license()" for more information.
>>> a=1
>>> x=[a]
>>> id(a)==id(x[0])
True
>>> a+=1
>>> a
2
>>> x[0]
1

I thought that += should only change the value of the int object. But
+= create new.
Is this intentional?