Method much slower than function?

Peter Otten __peter__ at web.de
Thu Jun 14 09:31:45 CEST 2007


Gabriel Genellina wrote:

> En Thu, 14 Jun 2007 01:39:29 -0300, sjdevnull at yahoo.com
> <sjdevnull at yahoo.com> escribió:
> 
>> Gabriel Genellina wrote:
>>> In addition, += is rather inefficient for strings; the usual idiom is
>>> using ''.join(items)
>>
>> Ehh.  Python 2.5 (and probably some earlier versions) optimize += on
>> strings pretty well.
>>
>> a=""
>> for i in xrange(100000):
>>     a+="a"
>>
>> and:
>>
>> a=[]
>> for i in xrange(100000):
>>     a.append("a")
>> a="".join(a)
>>
>> take virtually the same amount of time on my machine (2.5), and the
>> non-join version is clearer, IMO.  I'd still use join in case I wind
>> up running under an older Python, but it's probably not a big issue
>> here.
> 
> Yes, for concatenating a lot of a's, sure... Try again using strings
> around the size of your expected lines - and make sure they are all
> different too.
> 
> py> import timeit
> py>
> py> def f1():
> ...   a=""
> ...   for i in xrange(100000):
> ...       a+=str(i)*20
> ...
> py> def f2():
> ...   a=[]
> ...   for i in xrange(100000):
> ...       a.append(str(i)*20)
> ...   a="".join(a)
> ...
> py> print timeit.Timer("f2()", "from __main__ import f2").repeat(number=1)
> [0.42673663831576358, 0.42807591467630662, 0.44401481193838876]
> py> print timeit.Timer("f1()", "from __main__ import f1").repeat(number=1)
> 
> ...after a few minutes I aborted the process...

I can't confirm this.

$ cat join.py
def f1():
    a = ""
    for i in xrange(100000):
        a += str(i)*20

def f2():
    a = []
    for i in xrange(100000):
        a.append(str(i)*20)
    a = "".join(a)

def f3():
    a = []
    append = a.append
    for i in xrange(100000):
        append(str(i)*20)
    a = "".join(a)

$ python2.5 -m timeit -s 'from join import f1' 'f1()'
10 loops, best of 3: 212 msec per loop
$ python2.5 -m timeit -s 'from join import f2' 'f2()'
10 loops, best of 3: 259 msec per loop
$ python2.5 -m timeit -s 'from join import f3' 'f3()'
10 loops, best of 3: 236 msec per loop

Peter




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