iglob performance no better than glob

Cameron Simpson cs at zip.com.au
Sun Feb 14 07:12:06 CET 2010


On 31Jan2010 16:23, Kyp <kyp at stsci.edu> wrote:
| On Jan 31, 2:44 pm, Peter Otten <__pete... at web.de> wrote:
| > Kyp wrote:
| > > I have a dir with a large # of files that I need to perform operations
| > > on, but only needing to access a subset of the files, i.e. the first
| > > 100 files.
| > > Using glob is very slow, so I ran across iglob, which returns an
| > > iterator, which seemed just like what I wanted. I could iterate over
| > > the files that I wanted, not having to read the entire dir.
[...]
| > > So the iglob was faster, but accessing the first file took about the
| > > same time as glob.glob.
| >
| > > Here's some code to compare glob vs. iglob performance,  it outputs
| > > the time before/after a glob.iglob('*.*') files.next() sequence and a
| > > glob.glob('*.*') sequence.
| >
| > > #!/usr/bin/env python
| >
| > > import glob,time
| > > print '\nTest of glob.iglob'
| > > print 'before       iglob:', time.asctime()
| > > files = glob.iglob('*.*')
| > > print 'after        iglob:',time.asctime()
| > > print files.next()
| > > print 'after files.next():', time.asctime()
| >
| > > print '\nTest of glob.glob'
| > > print 'before        glob:', time.asctime()
| > > files = glob.glob('*.*')
| > > print 'after         glob:',time.asctime()
| >
| > > Here are the results:
| >
| > > Test of glob.iglob
| > > before       iglob: Sun Jan 31 11:09:08 2010
| > > after        iglob: Sun Jan 31 11:09:08 2010
| > > foo.bar
| > > after files.next(): Sun Jan 31 11:09:59 2010
| >
| > > Test of glob.glob
| > > before        glob: Sun Jan 31 11:09:59 2010
| > > after         glob: Sun Jan 31 11:10:51 2010
| >
| > > The results are about the same for the 2 approaches, both took about
| > > 51 seconds. Am I doing something wrong with iglob?
| >
| > No, but iglob() being lazy is pointless in your case because it uses
| > os.listdir() and fnmatch.filter() underneath which both read the whole
| > directory before returning anything.
| >
| > > Is there a way to get the first X # of files from a dir with lots of
| > > files, that does not take a long time to run?
| >
| > Here's my attempt. [...open directory and read native format...]

I'd be inclined first to time os.listdir('.') versus glob.lgo('*.*').

Glob routines tend to lstat() every matching name to ensure the path
exists. That's very slow. If you just do os.listdir() and choose your
100 nmaes, you only need to stat (or just try to open) them.

So time glob.glob("*.*") versus os.listdir(".") first.

Generally, with a large directory, stat time will change performance
immensely.
-- 
Cameron Simpson <cs at zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

Usenet is essentially a HUGE group of people passing notes in class. --R. Kadel



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