list of lists, object doesn't support item assignment?

[Manfred Bartz]

>>> xy = (1,2),(2,2)
>>> print xy, xy[0]
((1, 2), (2, 2)) (1, 2)
>>> xy[1]=(3,4)
TypeError: object doesn't support item assignment

Is there a way to assign the list (3,4) to an element of xy?
What am I missing?

-- 
Manfred
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