Caleb Hattingh wrote:
>> Yes, you can even write
>>
>> f = open("data.txt")
>> for line in f:
>> # do stuff with line
>> f.close()
>>
>> This has the additional benefit of not slurping in the entire file at
>> once.
>
> Is there disk access on every iteration? I'm guessing yes? It shouldn't
> be an issue in the vast majority of cases, but I'm naturally curious :)
Well, you will hardly find an OS that does no buffering of disk access --
but file.next() does some extra optimization as Steven already explained.
Here are some timings performed on the file that has the first-hand
information about Python's file buffering strategy :-)
$ python2.4 -m timeit 'for line in file("fileobject.c"): pass'
1000 loops, best of 3: 528 usec per loop
$ python2.4 -m timeit 'for line in file("fileobject.c").readlines(): pass'
1000 loops, best of 3: 635 usec per loop
$ python2.4 -m timeit 'for line in iter(file("fileobject.c").readline, ""):
pass'
1000 loops, best of 3: 1.59 msec per loop
$ python2.4 -m timeit 'f = file("fileobject.c")' 'while 1:' ' if not
f.readline(): break'
100 loops, best of 3: 2.08 msec per loop
So not only is
for line in file(...):
# do stuff
the most elegant, it is also the fastest. file.readlines() comes close, but
is only viable for "small" files.
Peter
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