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From: Kyp on 31 Jan 2010 11:22
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?

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?

thanx, mark
From: Skip Montanaro on 31 Jan 2010 12:59
> So the iglob was faster, but accessing the first file took about the
> same time as glob.glob.

I'll wager most of the time required to access the first file is due
to filesystem overhead, not any inherent limitation in Python.

Skip Montanaro


From: John Bokma on 31 Jan 2010 13:06
Kyp <kyp(a)stsci.edu> writes:

> 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?

Assuming Linux: what does time

ls thedir | head

give?

with thedir the name of the actual dir

Also how many is many files?

--
John Bokma j3b

Hacking & Hiking in Mexico - http://johnbokma.com/
http://castleamber.com/ - Perl & Python Development
From: Peter Otten on 31 Jan 2010 14:44
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. It turned out to be more work than expected, so I cut a
few corners. It's Linux-only "works on my machine" code, but may give you
some hints on how to proceed.

from ctypes import *
import fnmatch
import glob
import os
import re
from itertools import ifilter, imap

class dirent(Structure):
"works on my machine ;)"
_fields_ = [
("d_ino", c_long),
("d_off", c_long),
("d_reclen", c_ushort),
("d_type", c_ubyte),
("d_name", c_char*256)]


direntp = POINTER(dirent)

LIBC = "libc.so.6"
cdll.LoadLibrary(LIBC)
libc = CDLL(LIBC)
libc.readdir.restype = direntp


def diriter(dir):
"lazy partial replacement for os.listdir()"
# errors? what errors?
dirp = libc.opendir(dir)
if not dirp:
return
try:
while True:
ep = libc.readdir(dirp)
if not ep:
break
yield ep.contents.d_name
finally:
libc.closedir(dirp)


def filter(names, pattern):
"lazy partial replacement for fnmatch.filter()"
import posixpath

pattern = os.path.normcase(pattern)
r = fnmatch.translate(pattern)
r = re.compile(r)

if os.path is not posixpath:
names = imap(os.path.normcase, names)

return ifilter(r.match, names)

def globiter(path):
"lazy partial replacement for glob.glob()"
dir, filename = os.path.split(path)
if glob.has_magic(dir):
raise ValueError("wildcards in directory not supported")
return filter(diriter(dir), filename)


if __name__ == "__main__":
import sys
[pattern] = sys.argv[1:]
for name in globiter(pattern):
print name

Peter
From: Benjamin Peterson on 31 Jan 2010 16:30
Kyp <kyp <at> stsci.edu> writes:

> So the iglob was faster, but accessing the first file took about the
> same time as glob.glob.

That would be because glob is implemented in terms of iglob.




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