Why mesh python code slower than decomposed one?

Question

I\'ve discovered surprising python behaviour while had investigating thread Why is reading lines from stdin much slower in C++ than Python?.

If I run simple python code

Answer 1

Watching into bytecode helped me to answer this question. Byte code for working part of the first script is:

 10          58 SETUP_LOOP              27 (to 88)
             61 LOAD_NAME                3 (sys)
             64 LOAD_ATTR                6 (stdin)
             67 GET_ITER         
        >>   68 FOR_ITER                16 (to 87)
             71 STORE_NAME               7 (line)
 11          74 LOAD_NAME                4 (count)
             77 LOAD_CONST               4 (1)
             80 INPLACE_ADD      
             81 STORE_NAME               4 (count)
             84 JUMP_ABSOLUTE           68
        >>   87 POP_BLOCK

And byte code for corresponding part of second script is:

 12          18 SETUP_LOOP              24 (to 45)
             21 LOAD_FAST                0 (input)
             24 GET_ITER
        >>   25 FOR_ITER                16 (to 44)
             28 STORE_FAST               3 (line)
 13          31 LOAD_FAST                1 (count)
             34 LOAD_CONST               2 (1)
             37 INPLACE_ADD
             38 STORE_FAST               1 (count)
             41 JUMP_ABSOLUTE           25
        >>   44 POP_BLOCK

I see that actual difference between this codes is LOAD_NAME vs LOAD_FAST and STORE_NAME vs STORE_FAST opcodes using. Documentation http://docs.python.org/2.7/library/dis.html#opcode-LOAD_FAST says that LOAD_FAST makes lookup using only indexes, while LOAD_NAME lookups variable by string name. And the first approach is two times faster.