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

Terry Reedy tjreedy at udel.edu
Thu May 19 03:44:24 CEST 2011

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

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