Python: unpacking into array elements

Question

Why does the behavior of unpacking change when I try to make the destination an array element?

>>> def foobar(): return (1,2)
>>> a,b = foobar()
>>> (a,b)
 (1, 2)
>>> a = b = [0, 0] # Make a and b lists
>>> a[0], b[0] = foobar()
>>> (a, b)
 ([2, 0], [2, 0])

In the first case, I get the behavior I expect. In the second case, both assignments use the last value in the tuple that is returned (i.e. '2'). Why?

Answer 1

When you do a = b = [0, 0], you're making both a and b point to the same list. Because they are mutable, if you change either, you change both. Use this instead:

a, b = [0, 0], [0, 0]

Answer 2

a = b = [0, 0] # Makes a and b the same list