stupid question about os.listdir

[Jason Kratz]

OK. I've search on google groups and around the web for this and I
haven't found an answer. I'm a Python newbie and have what I assume is
a basic question. os.listdir takes a pathname as an arg but it doesn't
actually list the contents of the dir I pass in. it always operates on
the current dir (wherever the script is run) and I have to chdir
beforehand. Is that how its supposed to work? If so what is the point
in passing in a pathname?

thanks,

Jason

 

[Jason Kratz]

OK. I've search on google groups and around the web for this and I
haven't found an answer. I'm a Python newbie and have what I assume is
a basic question. os.listdir takes a pathname as an arg but it doesn't
actually list the contents of the dir I pass in. it always operates on
the current dir (wherever the script is run) and I have to chdir
beforehand. Is that how its supposed to work? If so what is the point
in passing in a pathname?

thanks,

Jason

oops. almost forgot. if I run interactively in the python interpreter
it works as I expect. its when doing 'python script.py' from the
command line that it only uses the current directory.

 

[Ben Finney]

os.listdir takes a pathname as an arg but it doesn't actually list the
contents of the dir I pass in.

Please reduce the problem to a simple script that others can examine,
and post it here. If the behaviour is as you say, it should be only a
few lines long:

import os
os.listdir( 'somethingyouthinkshouldwork' )

 

[John Hunter]

Jason> oops. almost forgot. if I run interactively in the python
Jason> interpreter it works as I expect. its when doing 'python
Jason> script.py' from the command line that it only uses the
Jason> current directory.

Code, we need more code. Please post an example, your platform, and
python version.

The following works for me on linux w/ python2.2 called as

> python scriptname.py

import os
print os.listdir('/home/jdhunter')
print os.listdir('/home/jdhunter/python')

and the same script (w/ different test paths) works on win32 w/ python
2.2.

JDH

 

[Erik Max Francis]

oops. almost forgot. if I run interactively in the python
interpreter
it works as I expect. its when doing 'python script.py' from the
command line that it only uses the current directory.

Never heard of any such thing. It's likely that your script is not
quite doing what you expect it to.

 

[Ben Finney]

Here is more clarification. Here is my script called backup.py

Again, I'll ask you to reduce this to a single, isolated incident of
os.listdir() that doesn't act as you expect.

If the failure *depends* on the rest of the script, then it's more
complex than "os.listdir() doesn't list the current directory". It may,
in fact, have nothing to do with os.listdir() at all.

Reducing the test case to a single os.listdir() instance will aid you as
well as us, since you'll be able to have a much better understanding of
what's going on.

 

[Jason Kratz]

Here is more clarification. Here is my script called backup.py

import os.path
import os

def getdirs(path):

dirs = []

os.chdir(path)
for entry in os.listdir(path):
if os.path.isdir(entry):
dirs.append(entry)

return dirs

print getdirs('/')


if I run this from the command line on linux with 'python backup.py' it
works *if* I have os.chdir in there. if I comment it out it doesnt list
starting from the root dir...it starts in my home dir.

If go into the interactive command mode by just typing 'python' at the
prompt and do:

import os
os.listdir('/') then it prints out the dirs under root.

incidentally this happens on windows as well

 

[Jason Kratz]

Please reduce the problem to a simple script that others can examine,
and post it here. If the behaviour is as you say, it should be only a
few lines long:

import os
os.listdir( 'somethingyouthinkshouldwork' )

Ben...I tried the above in a new script file (with print os.listdir) and
it works as I thought my other should. Which means i'm doing something
wrong when passing the path in to my function but I'm not sure what. ugh.

 

[Jason Kratz]

Congratulations! You've learned an immensely valuable debugging
technique: Reduce the problem behaviour to the *minimum necessary code*
to reproduce the problem; otherwise, you're searhing in code that, it
turns out, has absolutely no bearing on the problem.

(This leads, in turn, to the principle that writing less code in the
first place leads to fewer bugs -- but that will come naturally as you
learn Python

aha! I found it! its the call to os.path.isdir. I'm not passing it
a real pathname....just a string. I need to set my entries in my dir
list as real pathnames (ie: with the slashes)...not just the text.
question is how

 

[Peter Hansen]

def getdirs(path):
os.chdir(path)
for entry in os.listdir(path):
if os.path.isdir(entry):
dirs.append(entry)

if I run this from the command line on linux with 'python backup.py' it
works *if* I have os.chdir in there. if I comment it out it doesnt list
starting from the root dir...it starts in my home dir.

This might mean you are not passing it an absolute path, but
instead a relative one. Absolute paths (on Linux) are those
which start with a / (forward slash). Anything without that
will start from the current directory only.

But without actual examples of which paths are failing, as
Ben has asked for, we know nothing for certain.

Is it also possible that you are not having a problem with listdir()
at all, but with the values you are passing to os.path.isdir()?
You realize that os.listdir() returns names that are *relative*
to the path parameter you give it? So that if you then pass those
to isdir() you will get nothing useful if you don't first do
this instead? :

entry = os.path.join(path, entry)
if os.path.isdir(entry):
dirs.append(entry)

-Peter

 

[Jason Kratz]

This might mean you are not passing it an absolute path, but
instead a relative one. Absolute paths (on Linux) are those
which start with a / (forward slash). Anything without that
will start from the current directory only.

But without actual examples of which paths are failing, as
Ben has asked for, we know nothing for certain.

Is it also possible that you are not having a problem with listdir()
at all, but with the values you are passing to os.path.isdir()?
You realize that os.listdir() returns names that are *relative*
to the path parameter you give it? So that if you then pass those
to isdir() you will get nothing useful if you don't first do
this instead? :

entry = os.path.join(path, entry)
if os.path.isdir(entry):
dirs.append(entry)

-Peter

Peter-

Thanks much for the reply. You are completely correct and I managed to
work thru it myself (just finished right before I read your posting) and
I actually had the exact code (aside from using a new var instead of
reusing entry) you have just above (with the join). Was going to post
it and find out if that was the "correct" way of doing it

This is my code:

import os
import os.path

def getdirs(path):
dirs=[]

for entry in os.listdir(path):
fullpath=os.path.join(path,entry)
if os.path.isdir(fullpath):
dirs.append(fullpath)
return dirs

print getdirs('/')

 

[Erik Max Francis]

aha! I found it! its the call to os.path.isdir. I'm not passing
it
a real pathname....just a string. I need to set my entries in my dir
list as real pathnames (ie: with the slashes)...not just the text.
question is how

A pathname _is_ just a string. Presumably the problem is that you're
getting the results of os.listdir, which are just the names of the
contents of the directory, not the relative paths to them:

max@oxygen:~/tmp% mkdir subdir
max@oxygen:~/tmp% touch subdir/a subdir/b subdir/c
max@oxygen:~/tmp% ls subdir
a b c
max@oxygen:~/tmp% python
Python 2.2.3 (#1, May 31 2003, 21:31:33)
[GCC egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)] on linux2
Type "help", "copyright", "credits" or "license" for more information.['a', 'b', 'c']

So just join the paths:
.... p = os.path.join('subdir', f)
.... print p, os.path.isfile(p)
....
subdir/a 1
subdir/b 1
subdir/c 1