[Python-Dev] Don't set local variable in a list comprehension or generator

[Terry Reedy]

On 5/18/2011 5:34 PM, Victor Stinner wrote:

You initial example gave me the impression that the issue has something 
to do with join in particular, or even comprehensions in particular. It 
is really about for loops.

>     squares = (x*x for x in range(10000))

 >>> dis('for x in range(3): y = x*x')
   1           0 SETUP_LOOP              30 (to 33)
               3 LOAD_NAME                0 (range)
               6 LOAD_CONST               0 (3)
               9 CALL_FUNCTION            1
              12 GET_ITER
         >>   13 FOR_ITER                16 (to 32)
              16 STORE_NAME               1 (x)
              19 LOAD_NAME                1 (x)
              22 LOAD_NAME                1 (x)
              25 BINARY_MULTIPLY
              26 STORE_NAME               2 (y)
              29 JUMP_ABSOLUTE           13
         >>   32 POP_BLOCK
         >>   33 LOAD_CONST               1 (None)
              36 RETURN_VALUE

> You don't really need the "x" variable, you just want the square.

It is nothing new that hand-crafted assembler (which mnemonic bytecode 
is) can sometimes beat a compiler. In this case, you want store, load, 
load before the multiply replaced with dup, and you cannot get that with 
Python code without a much smarter optimizer.

>

-- 
Terry Jan Reedy