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

[Terry Reedy]

On 5/18/2011 5:37 PM, Amaury Forgeot d'Arc wrote:
> Hi,
>
> 2011/5/18 Terry Reedy<tjreedy at udel.edu>:
>> On 5/18/2011 10:19 AM, Nadeem Vawda wrote:
>>
>>> I'm not sure why you would encounter code like that in the first place.
>>> Surely any code of the form:
>>>
>>>      ''.join(c for c in my_string)
>>>
>>> would just return my_string? Or am I missing something?
>>
>> Good question. Anything useful like "'-'.join(c for c in 'abc')" is the same
>> as "'-'.join('abc'). The same, as far as I can think of, for anything like
>> list() or set() taking an iterable arg.
>
> With a little imagination you can build something non trivial.
> For example, a join_words function:
>
> def join_words(words):
>      return ', '.join(w.strip() for w in words)
>
> Like Victor says, the code of the generator object contains a
> STORE_FAST followed by LOAD_FAST.
> This pair of opcodes could be removed, and the value left on the stack.
>
>>>> dis.dis(join_words.func_code.co_consts[2])
>    1           0 SETUP_LOOP              24 (to 27)
>                3 LOAD_FAST                0 (.0)
>          >>     6 FOR_ITER                17 (to 26)
>                9 STORE_FAST               1 (w)
>               12 LOAD_FAST                1 (w)
>               15 LOAD_ATTR                0 (strip)
>               18 CALL_FUNCTION            0
>               21 YIELD_VALUE
>               22 POP_TOP
>               23 JUMP_ABSOLUTE            6
>          >>    26 POP_BLOCK
>          >>    27 LOAD_CONST               0 (None)
>               30 RETURN_VALUE

As I pointed out in response to Victor, you get nearly the same with 
bytecode with regular old for loops; in particular, the store x/load x pair.

> It's probably not easy to do though.
> Think of expressions where the variable appears several times,
> or even where the variable is not the first object, like str(ord(x)).

Where first means first in left-to-right order rather than in innermost 
to outermost order. (OT: I think Python is a bit unusual in this way.)

-- 
Terry Jan Reedy