On Sat, 09 Jul 2005 22:32:22 -0600, Steven Bethard <[EMAIL PROTECTED]> wrote:
>Raymond Hettinger wrote:
>> [Steven Bethard]
>>
>>>I would hope that in Python 3.0 list comprehensions and generator
>>>expressions would be able to share a large amount of implementation, and
>>>thus that the speed differences would be much smaller. But maybe not...
>>
>> Looking under the hood, you would see that the implementations are
>> necessarily as different as night and day. Only the API is similar.
>
>Necessarily? It seems like list comprehensions *could* be implemented
>as a generator expression passed to the list constructor. They're not
>now, and at the moment, changing them to work this way seems like a bad
>idea because list comprehensions would take a performance hit. But I
>don't understand why the implementations are *necessarily* different.
>Could you explain?
>
>STeVe
>
>P.S. The dis.dis output for list comprehensions makes what they're doing
>pretty clear. But dis.dis doesn't seem to give me as much information
>when looking at a generator expression:
>
>py> def ge(items):
>... return (item for item in items if item)
>...
>py> dis.dis(ge)
> 2 0 LOAD_CONST 1 (<code object <generator
>expression> at 0116FD20, file "<interactive input>", line 2>)
> 3 MAKE_FUNCTION 0
> 6 LOAD_FAST 0 (items)
> 9 GET_ITER
> 10 CALL_FUNCTION 1
> 13 RETURN_VALUE
>
>I tried to grep through the dist\src directories for what a generator
>expression code object looks like, but without any luck. Any chance you
>could point me in the right direction?
>>> import dis
>>> g = ge([1,2,0,3,'',4])
>>> dis.dis(g)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "d:\python-2.4b1\lib\dis.py", line 46, in dis
raise TypeError, \
TypeError: don't know how to disassemble generator objects
but:
>>> dis.dis(ge)
2 0 LOAD_CONST 1 (<code object <generator
expression> at 02EE4FA0, file "<stdin>", line 2>)
3 MAKE_FUNCTION 0
6 LOAD_FAST 0 (items)
9 GET_ITER
10 CALL_FUNCTION 1
13 RETURN_VALUE
>>> ge.func_code
<code object ge at 02EE4E60, file "<stdin>", line 1>
>>> ge.func_code.co_consts
(None, <code object <generator expression> at 02EE4FA0, file "<stdin>", line
2>)
>>> ge.func_code.co_consts[1]
<code object <generator expression> at 02EE4FA0, file "<stdin>", line 2>
>>> dis.dis(ge.func_code.co_consts[1])
2 0 SETUP_LOOP 28 (to 31)
3 LOAD_FAST 0 ([outmost-iterable])
>> 6 FOR_ITER 21 (to 30)
9 STORE_FAST 1 (item)
12 LOAD_FAST 1 (item)
15 JUMP_IF_FALSE 8 (to 26)
18 POP_TOP
19 LOAD_FAST 1 (item)
22 YIELD_VALUE
23 JUMP_ABSOLUTE 6
>> 26 POP_TOP
27 JUMP_ABSOLUTE 6
>> 30 POP_BLOCK
>> 31 LOAD_CONST 0 (None)
34 RETURN_VALUE
A little more info, anyway. HTH.
Regards,
Bengt Richter
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