[Python-Dev] Don't set local variable in a list comprehension or generator
tjreedy at udel.edu
Thu May 19 03:59:47 CEST 2011
On 5/18/2011 5:37 PM, Amaury Forgeot d'Arc wrote:
> 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.
> 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
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