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Python 2.7.3 (default, Apr 20 2012, 22:44:07) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = ['1']
>>> b = []
>>> c = a
>>> b.append(a)
>>> a
['1']
>>> b
[['1']]
>>> c
['1']
>>> a = ['2']
>>> c.extend(a)
>>> b
[['1', '2']]
>>> c
['1', '2']
>>> a
['2']
>>> 
Is this wrong? I think the "b" should not change.

This is correct behavior, because:
>>> b[0] is c
True
(read up on the meaning of the "is" operator, if this is not a convincing proof to you).

>>> a = ['1']
>>> b = []
>>> c = a
# now c and a refer to the same object
>>> b.append(a)
# this object is appended to b
>>> a
['1']
>>> b
[['1']]
>>> c
['1']
# a and c refer to the same object you have in b
# so all these ['1'] are actually the same object
>>> a = ['2']
# now a refers to another object
>>> c.extend(a)
# and here you are extending the object referred by c
# and the same object that you have in b
>>> b
[['1', '2']]
>>> c
['1', '2']
# so this is correct
>>> a
['2']
>>>
You can use id() to verify the identity of the objects, and read http://python.net/crew/mwh/hacks/objectthink.html for more information.

Thank u both.