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

Amaury Forgeot d'Arc amauryfa at gmail.com
Wed May 18 23:37:30 CEST 2011

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 mystring) would just return mystring? 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(joinwords.funccode.coconsts[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

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)).

-- Amaury Forgeot d'Arc

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