What does dis tell us:
Python 3.4.1 (default, May 19 2014, 13:10:29)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from dis import dis
>>> dis("""x = [1,2,3,4,5]
... for x in x:
... print(x)
... print(x)""")
1 0 LOAD_CONST 0 (1)
3 LOAD_CONST 1 (2)
6 LOAD_CONST 2 (3)
9 LOAD_CONST 3 (4)
12 LOAD_CONST 4 (5)
15 BUILD_LIST 5
18 STORE_NAME 0 (x)
2 21 SETUP_LOOP 24 (to 48)
24 LOAD_NAME 0 (x)
27 GET_ITER
>> 28 FOR_ITER 16 (to 47)
31 STORE_NAME 0 (x)
3 34 LOAD_NAME 1 (print)
37 LOAD_NAME 0 (x)
40 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
43 POP_TOP
44 JUMP_ABSOLUTE 28
>> 47 POP_BLOCK
4 >> 48 LOAD_NAME 1 (print)
51 LOAD_NAME 0 (x)
54 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
57 POP_TOP
58 LOAD_CONST 5 (None)
61 RETURN_VALUE
The key bits are sections 2 and 3 – we load the value out of x (24 LOAD_NAME 0 (x)) and then we get its iterator (27 GET_ITER) and start iterating over it (28 FOR_ITER). Python never goes back to load the iterator again.
Aside: It wouldn’t make any sense to do so, since it already has the iterator, and as Abhijit points out in his answer, Section 7.3 of Python’s specification actually requires this behavior).
When the name x gets overwritten to point at each value inside of the list formerly known as x Python doesn’t have any problems finding the iterator because it never needs to look at the name x again to finish the iteration protocol.