On Wed, 31 Oct 2007 13:14:41 +0000, George Sakkis wrote:
> On Oct 31, 8:44 am, Neil Cerutti <[EMAIL PROTECTED]> wrote:
>> On 2007-10-30, George Sakkis <[EMAIL PROTECTED]> wrote:
>>
>>
>>
>> > On Oct 30, 11:25 am, Neil Cerutti <[EMAIL PROTECTED]> wrote:
>> >> On 2007-10-30, Eduardo O. Padoan <[EMAIL PROTECTED]> wrote:
>>
>> >> > This is a FAQ:
>> >> >http://effbot.org/pyfaq/why-does-python-use-methods-for-some-
function...
>>
>> >> Holy Airy Persiflage Batman!
>>
>> >> Python 2.5.1 (r251:54863, Apr 18 2007, 08:51:08) [MSC v.1310 32 bit
>> >> (Intel)] on win32
>> >> Type "help", "copyright", "credits" or "license" for more
>> >> information.>>> import timeit
>> >> >>> timeit.Timer('len(seq)', 'seq = range(100)').timeit()
>> >> 0.20332271187463391
>> >> >>> timeit.Timer('seq.__len__()', 'seq = range(100)').timeit()
>>
>> >> 0.48545737364457864
>>
>> > Common mistake; try this instead:
>>
>> > timeit.Timer('seqlen()',
>> > 'seq = range(100); seqlen=seq.__len__').timeit()
>>
>> Why would I want to do that?
>
>
> To realize that __len__ is actually faster than len in this case; your
> second timing is dominated by the time to do attribute lookup.
What Neil has done is the correct thing to measure in this case.
What you have measured is a local optimization that is only useful when
you have a tight loop with lots of calls to the same len():
Len = sequence.__len__
while Len() < 100000:
foo(sequence)
But what Neil is measuring is relevant for the more general case of
calling len() on arbitrary objects at arbitrary times:
x = len(sequence)
y = 42 + len("xyz")
if len(records) == 12:
print "twelve!"
To get the benefit of the micro-optimization you suggested, you would
need to write this:
Len = sequence.__len__
x = Len()
Len = "xyz".__len__
y = 42 + Len()
Len = records.__len__
if Len() == 12:
print "twelve!"
Not only is that an especially ugly, confusing piece of code, it is also
counter-productive as you still have to do the method lookup every single
time.
Taking the method lookup is an apple-and-oranges comparison. It is useful
to know for the times when you don't want an apple, you want an orange
(i.e. the tight loop example above).
This is an apples-and-apples comparison:
>>> import timeit
>>> timeit.Timer("Len = seq.__len__; Len()", "seq = range(12)").repeat()
[1.3109469413757324, 0.7687380313873291, 0.55280089378356934]
>>> timeit.Timer("len(seq)", "seq = range(12)").repeat()
[0.60111594200134277, 0.5977931022644043, 0.59935498237609863]
The time to do the lookup and call the method is about 0.6 microseconds
whether you do it manually or let len() do it.
The logic behind the micro-optimization trick is to recognize when you
can avoid the cost of repeated method lookups by doing it once only. That
is not relevant to the question of whether len() should be a function or
a method.
--
Steven.
--
http://mail.python.org/mailman/listinfo/python-list
