[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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status: open -> closed

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

Fabien Dubosson added the comment:

> I'm going to close this issue. 

I was doing to do so, here was my message:



> See shells sources. According to the manpage `pwd -L` just uses PWD from 
> environment.

I looked directly at `pwd` sources, and indeed it is using $PWD [1].

[1] 
http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/pwd.c?id=509152bdd47a278dc00ddaf3200ac65044b836b1#n305

So it looks like `cd -L` is just some bash internal dealing with $PWD. Probably 
not something useful enough to be in the standard library, closing it then.

Sorry for the incovenience

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[R. David Murray]

R. David Murray added the comment:

Ah, that should all be implementable from your python shell, then, no need for 
support in the os module.

For reference, when I said 'cd man page', this is what I was referring to:

http://www.unix.com/man-page/posix/1posix/cd/

I'm going to close this issue.  If you find you do need access to some system 
interface the os module doesn't currently provide, you can open a specific 
issue for that.

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resolution:  -> rejected
stage:  -> resolved

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

Fabien Dubosson added the comment:

> Most os functions are thin wrappers around system calls. There is no system 
> call that works as 'cd' without '-P'.

I would like to believe in this, but then if `cd` is some bash internal, how 
does `/usr/bin/pwd -L` find it back?

> Since yours is the itch, I'm afraid you are going to have to be the one to 
> figure out how this could be implemented :)

Once I'll have figured out how all this is working, maybe. Not sure it would me 
in my competencies though.

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> Just wondering, do you know what is the function called by `cd`/`cd -L` then? 
> It doesn't seems to be a bash internal tweak because `/usr/bin/pwd -L` is 
> able to get the symlink path, and this binary is not part of bash.

See shells sources. According to the manpage `pwd -L` just uses PWD from 
environment.

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[R. David Murray]

R. David Murray added the comment:

Since yours is the itch, I'm afraid you are going to have to be the one to 
figure out how this could be implemented :)

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

Fabien Dubosson added the comment:

Thanks for the additional information!

> The 'cd' man page says that 'cd -P' should "perform actions equivalent to the 
> chdir() function".

Just wondering, do you know what is the function called by `cd`/`cd -L` then? 
It doesn't seems to be a bash internal tweak because `/usr/bin/pwd -L` is able 
to get the symlink path, and this binary is not part of bash.

> You'll have to make a case for it being useful enough to add.

The use case is for shells written in Python, like for instance `xonsh` [1,2]. 
Being powered by Python, the only way to change directory is by using 
`os.chdir()`. It is then not possible to mimic the bash `cd` function without 
using workarounds, like for instance storing the symlink path in a variable, 
and even this doesn't allow to use `/usr/bin/pwd -L` in scripts because it 
would always returns the physical location.

Having such described functions (yet to be named) would permit python shells to 
offer users differentiated `cd` and `cd -P` commands, as well as having `pwd 
-L` in scripts behaving the same than in traditional shells.

[1] http://xon.sh/
[2] https://github.com/xonsh/xonsh

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

'cd' is not an external command and has no manpage. It is a shell builtin 
command.

Most os functions are thin wrappers around system calls. There is no system 
call that works as 'cd' without '-P'.

If you implement a shell in Python, you perhaps need an implementation of the 
'cd' command. But I have doubts that this is needed in the stdlib.

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nosy: +serhiy.storchaka

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[R. David Murray]

Changes by R. David Murray :


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type: behavior -> enhancement

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[R. David Murray]

R. David Murray added the comment:

The function is chdir, not cd.  The 'cd' man page says that 'cd -P' should 
"perform actions equivalent to the chdir() function".

So, you are asking for a different function, which should *not* be named 
'os.chdir'.  You'll have to make a case for it being useful enough to add.

The same applies to getcwd, which is also a wrapper for the posix function, not 
an emulation of the shell's pwd.

--
components: +Library (Lib) -Interpreter Core
nosy: +r.david.murray
versions:  -Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

Fabien Dubosson added the comment:

The previous example (sorry, first time using this platform, I'm a little bit 
messy here), shows that `os.chdir` resolves symlinks by default, as opposed to 
what `cd` does in bash. This means it is not possible to change the directory 
to a symlink folder.

A solution would be a keyword argument in `os.chdir` and `os.getcwd` that would 
mimic the `-P` and `-L` arguments of `cd` and `pwd`, like:

os.chdir(logical=False)  # or follow_symlinks=True maybe
os.getcwd(logical=False)

I don't know what should be the default values for these args, but at least 
having an option to change the behavior would be nice.

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

New submission from Fabien Dubosson:

When using bash, the `cd` function does not follow symlinks by default, but `cd 
-P` does. The `os.chdir` function behaves like `cd -P` preventing to be able to 
change directory to a symlink folder.

Initial setup (make a `/tmp/to/dst` symlink pointing to `/tmp/from/src`):

cd /tmp
mkdir from from/src to
ln -s ../from/src to/dst

Here is an example, when using python's `os.chdir` function:

[fabien@asus ~]$ python
>>> import os
>>> os.chdir('/tmp/to/dst')
>>> os.system('/usr/bin/pwd')
/tmp/from/src
0
>>> os.system('/usr/bin/pwd -P')
/tmp/from/src
0
>>> os.system('/usr/bin/pwd -L')
/tmp/from/src
0
>>> os.getcwd()
'/tmp/from/src'
>>> 

And here is an example when the folder is first changed with bash:


[fabien@asus ~]$ cd /tmp/to/dst/
[fabien@asus dst]$ python
>>> import os
>>> os.system('/usr/bin/pwd')
/tmp/from/src
0
>>> os.system('/usr/bin/pwd -P')
/tmp/from/src
0
>>> os.system('/usr/bin/pwd -L')
/tmp/to/dst
0
>>> os.getcwd()
'/tmp/from/src'
>>>

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[issue29635] os.chdir() acts like `cd -P` by default, it should offer an option to not follow symlinks

[Fabien Dubosson]

Changes by Fabien Dubosson :


--
components: Interpreter Core
nosy: StreakyCobra
priority: normal
severity: normal
status: open
title: os.chdir() acts like `cd -P` by default, it should offer an option to 
not follow symlinks
type: behavior
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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