[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

R. David Murray added the comment:

Is your patch backward?

"Treat the path naturally" assume everyone agrees with you as to what is 
natural.  It would be better, I think, to be explicit about it.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Nick Coghlan]

Changes by Nick Coghlan :


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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Nick Coghlan]

Changes by Nick Coghlan :


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nosy: +ncoghlan

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[W. Owen Parry]

W. Owen Parry added the comment:

Patch as described above. Comments appreciated.

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Added file: http://bugs.python.org/file30367/issue17545.patch

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[W. Owen Parry]

W. Owen Parry added the comment:

I suggest that this is a documentation issue.

I have seen three classes of functions is os and os.path
i - those which operate on path names only (os.path.join, os.path.dirname, etc) 
and do not depend on the state of the file system. Since these are string 
manipulation functions, they treat '' as the empty string.
ii - those which turn path names into valid paths via knowledge of the state of 
the file system (os.path.abspath, os.path.normpath, etc). These treat '' as an 
empty relative path and thus equivalent to the current directory.
iii - those which operate on valid paths only and thus depend on the state of 
the file system (os.listdir, os.stat, os.path.relpath, os.path.exists, etc). '' 
is not a valid path so these functions throw an exception.

The documentation should clearly state which class a function belongs to. I 
will provide a patch sometime this week.

os.walk is an odd duck - it has class iii behaviour except that it returns an 
empty iterator instead of raising an exception. Note that it still 
distinguishes between '' (invalid path) and '.' (current directory) and so is 
not a good reason to change os.listdir:

>>> [x for x in os.walk('')]
[]
>>> [x for x in os.walk('.')]
[('.', [], [])]

in an empty directory.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Thomas Fenzl]

Thomas Fenzl added the comment:

I also looked into creating a patch and now I'm not convinced about the 
solution. 

While os.path.join accepts an empty string, the functions is os (e.g. stat and 
friends, utime, chown don't. os.walk doesn't throw an Exception, but generates 
an empty iterator on ''.
So from a module perspective, things wouldn't get more consistent, but less. 

Any thoughts about how to proceed?

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[W. Owen Parry]

W. Owen Parry added the comment:

I started working on a patch for this, but the more I think about it the less I 
am convinced it is wanted.

The issue requests that os.listdir('') be equal to os.listdir('.')

The given example of os.path.join doesn't follow this:

>>> os.path.join('','aaa')
'aaa'
>>> os.path.join('.','aaa')
'.\\aaa'

This makes sense: prepending an empty path should be a no-op, while prepending 
the current directory has a different meaning.

It seems consistent in this case that listing an empty path and listing the 
current directory should have different meanings.

Is there any other traction/use case for this change?

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

R. David Murray added the comment:

I don't know if the docs team replies to messages or not.  I know that issues 
sometimes get opened up here that refer to emails to that alias.  You can open 
an issue here yourself for the doc bug, if you want to.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Bernard Lang]

Bernard Lang added the comment:

Thank you, David.

BTW, I sent a message on april 20 to d...@python.org about a bug in
the documentation regarding os.readlink(path)
on page  http://docs.python.org/2/library/os.html

and proposing an alternative text.

I got no reply.

This was not long ago  ... should I just wait ?

Sorry for asking you ... I know no one else.

Regards

Bernard

* R. David Murray , le 20-04-13, a écrit:
> 
> Changes by R. David Murray :
> 
> 
> --
> keywords: +easy
> stage:  -> needs patch
> 
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[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

Changes by R. David Murray :


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stage:  -> needs patch

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

R. David Murray added the comment:

I'm inclined to agree with you.  I believe '' represents the current directory 
in sys.path, for example.  It ought to be easy to add this since issue 6095 is 
already fixed.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Bernard Lang]

Bernard Lang added the comment:

Reply to R.David.Murray

> See also issue 6095.

You are right. I goofed, this is the issue I meant to point to.

> $ ls ''
> ls: cannot access : No such file or directory
> So, the behavior is consistent with the shell.

This is a fair remark.
But still, giving a meaning to something that has none in the shell would not 
be an inconsistency either. There are lots of other differences in the Unix 
shell design, and strings are often used as a syntactic device. You do not have 
to quote file names or paths, unless they raise syntactic problems.

Now if you look at path manipulation commands in the shell, you have :

$ basename aaa
aaa
$ dirname aaa
.

This is a fair choice for the shell since an empty path would print as nothing. 
Furthermore, string manipulation is not as convenient with the shell as it is 
with Python. So the shell is altogether ignoring the empty path, and string 
manipulations (possibly using the empty string) to build a path representation 
are not part of the path system.

Python has already made a different choice.

In [4]: os.path.basename('aaa')
Out[4]: 'aaa'

In [5]: os.path.dirname('aaa')
Out[5]: ''

These are the two results of os.path.split('aaa'), which is somewhat the 
inverse of os.path.join(...) which I initially considered.

So os.path.dirname in Python does return the empty string '' where the shell 
dirname returns a dot. This could also be seen as an inconsistency between Unix 
shell and Python.

However, the Unix shell is internally consistent, while taking into account its 
own specific constraints.

It seems that it is more important for Python to similarly have its own 
internal consistency, especially when considering that Python is already 
departing from Unix shell in some minor ways, which are related to the internal 
consistency issue that was raised.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

R. David Murray added the comment:

By the way, you could write this as:

  localist = os.listdir(path if path else '.')

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[R. David Murray]

R. David Murray added the comment:

See also issue 6095.

However:

rdmurray@hey:~>ls ''
ls: cannot access : No such file or directory

So, the behavior is consistent with the shell.  The shell has no equivalent to 
os.path.join.

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[issue17545] os.listdir and os.path.join inconsistent on empty path

[Bernard Lang]

New submission from Bernard Lang:

The empty path '' is considered as an acceptable path in os.path.join, and 
works as a neutral prefix:
print os.path.join('','aaa')  ===>  aaa
which seems rather natural.
But it raises an exception when used as a parameter to os.listdir.
Logically, it should then list the current directory.
(the alternative would be to raise an exception when os.path.join gets an empty 
argument).

The inconsistency became clear to me when I had to write the following function 
:

def listdirs(path,flist): # Appends to "flist" all paths to files that
# start with "path". Argument "path" can be empty string ''.
if path=='' : localist=os.listdir('.')
else : localist=os.listdir(path)
for f in localist :
p=os.path.join(path,f)
flist.append(p)
if os.path.isdir(p) :
listdirs(p,flist)
return flist

The conditionnal is needed only to avoid the exception, while the code is 
unique afterwards.

This is related to Issue818059, but I did not see how that issue was resolved.

Furthermore, the case of the empty path as argument to os.listdir should be 
documented in http://docs.python.org/2/library/os.html

--
components: Library (Lib)
messages: 185207
nosy: babou
priority: normal
severity: normal
status: open
title: os.listdir and os.path.join inconsistent on empty path
type: enhancement
versions: Python 2.7

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