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executing python scripts that are symlinked

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  executing python scripts that are symlinked Charles Smith <cts.private.yahoo@gmail.com> - 2013-05-16 00:48 -0700
    Re: executing python scripts that are symlinked Andrew Berg <bahamutzero8825@gmail.com> - 2013-05-16 03:12 -0500
    Re: executing python scripts that are symlinked Dave Angel <davea@davea.name> - 2013-05-16 04:18 -0400
      Re: executing python scripts that are symlinked Charles Smith <cts.private.yahoo@gmail.com> - 2013-05-16 01:29 -0700
        Re: executing python scripts that are symlinked Dave Angel <davea@davea.name> - 2013-05-16 10:45 -0400
    Re: executing python scripts that are symlinked Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-16 09:04 +0000
      Re: executing python scripts that are symlinked Charles Smith <cts.private.yahoo@gmail.com> - 2013-05-16 02:23 -0700

#45410 — executing python scripts that are symlinked

Hi.

How can I say, from the cmd line, that python should take my CWD as my
CWD, and not the directory where the script actually is?


I have a python script that works fine when it sits in directory WC,
but if I move it out of WC to H and put a symlink from H/script to WC,
it doesn't find the packages that are in WC.  Also, if I use the
absolute path to H, it won't find them, but I guess I can understand
that.

Someone said on the net that python doesn't know whether a file is
real or a symlink, but I think that somehow, python is able to find
out where the real file is and treat that as its base of operations.

Charles.

----------------------------------------------------------
http://www.creative-telcom-solutions.de

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#45412

On 2013.05.16 02:48, Charles Smith wrote:
> Hi.
> 
> How can I say, from the cmd line, that python should take my CWD as my
> CWD, and not the directory where the script actually is?
> 
> 
> I have a python script that works fine when it sits in directory WC,
> but if I move it out of WC to H and put a symlink from H/script to WC,
> it doesn't find the packages that are in WC.  Also, if I use the
> absolute path to H, it won't find them, but I guess I can understand
> that.
Symlinks can find their targets, but targets have absolutely no way of knowing where symlinks to them are. It's one-way. It would work if
the actual file were in WC and you created a symlink inside H.

-- 
CPython 3.3.1 | Windows NT 6.2.9200 / FreeBSD 9.1

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#45413

On 05/16/2013 03:48 AM, Charles Smith wrote:
> Hi.
>
> How can I say, from the cmd line, that python should take my CWD as my
> CWD, and not the directory where the script actually is?
>
>
> I have a python script that works fine when it sits in directory WC,
> but if I move it out of WC to H and put a symlink from H/script to WC,
> it doesn't find the packages that are in WC.  Also, if I use the
> absolute path to H, it won't find them, but I guess I can understand
> that.
>
> Someone said on the net that python doesn't know whether a file is
> real or a symlink, but I think that somehow, python is able to find
> out where the real file is and treat that as its base of operations.
>

You'd really better specify your environment - exact OS and Python 
version.  symlink and cwd usually imply a Unix-type system, but cmd is a 
Windows thing.

Then give examples of what your cwd is, what string you're typing at the 
shell prompt, and what's happening.

For example windows is good at changing cwd long before the executable 
gets control, in some situations.

-- 
DaveA

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#45414

On 16 Mai, 10:18, Dave Angel <da...@davea.name> wrote:
> On 05/16/2013 03:48 AM, Charles Smith wrote:
>
> > Hi.
>
> > How can I say, from the cmd line, that python should take my CWD as my
> > CWD, and not the directory where the script actually is?
>
> > I have a python script that works fine when it sits in directory WC,
> > but if I move it out of WC to H and put a symlink from H/script to WC,
> > it doesn't find the packages that are in WC.  Also, if I use the
> > absolute path to H, it won't find them, but I guess I can understand
> > that.
>
> > Someone said on the net that python doesn't know whether a file is
> > real or a symlink, but I think that somehow, python is able to find
> > out where the real file is and treat that as its base of operations.
>
> You'd really better specify your environment - exact OS and Python
> version.  symlink and cwd usually imply a Unix-type system, but cmd is a
> Windows thing.
>
> Then give examples of what your cwd is, what string you're typing at the
> shell prompt, and what's happening.


Well, I'm on a ubuntu platform, running subversion, but I can't commit
this debugging tool into the working copy where I'm using it, so I
maintain it in my home directory.  The tool does use production
packages, though.

So, if I say,

  $ python fapi-test.py

and fapi-test.py really is there, then it works, using the codec
production package.  But if I use a symlink instead, it says

Traceback (most recent call last):
  File "test2", line 1, in <module>
    from codec.support import *
ImportError: No module named codec.support


Python tells me Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)



----------------------------------------------------------
http://www.creative-telcom-solutions.de

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#45427

On 05/16/2013 04:29 AM, Charles Smith wrote:
> On 16 Mai, 10:18, Dave Angel <da...@davea.name> wrote:
>> On 05/16/2013 03:48 AM, Charles Smith wrote:
>>
>>> Hi.
>>
>>> How can I say, from the cmd line, that python should take my CWD as my
>>> CWD, and not the directory where the script actually is?
>>
>>> I have a python script that works fine when it sits in directory WC,
>>> but if I move it out of WC to H and put a symlink from H/script to WC,
>>> it doesn't find the packages that are in WC.  Also, if I use the
>>> absolute path to H, it won't find them, but I guess I can understand
>>> that.
>>
>>> Someone said on the net that python doesn't know whether a file is
>>> real or a symlink, but I think that somehow, python is able to find
>>> out where the real file is and treat that as its base of operations.
>>
>> You'd really better specify your environment - exact OS and Python
>> version.  symlink and cwd usually imply a Unix-type system, but cmd is a
>> Windows thing.
>>
>> Then give examples of what your cwd is, what string you're typing at the
>> shell prompt, and what's happening.
>
>
> Well, I'm on a ubuntu platform,

Good.  Linux seldom gets in the way.

> running subversion, but I can't commit
> this debugging tool into the working copy where I'm using it, so I
> maintain it in my home directory.  The tool does use production
> packages, though.
>
> So, if I say,
>
>    $ python fapi-test.py
>
> and fapi-test.py really is there,

where?  Be explicit.  When copying the python invocation line above, you 
never bothered to include the current directory that's generally 
displayed right in front of that $ prompt.

> then it works, using the codec
> production package.

What codec production package.  And where is it installed?  And how do 
you intend that Python find it?

   But if I use a symlink instead,

So you delete the pfapi-tst.py out of the current directory, and create 
instead a symlink in the current directory that points to its real 
location  And without changing current directory, you run the same 
command and it runs till it gets to the import line, at which point it 
gives a traceback that has nothing to do with the file you're running??? 
  The traceback thinks you're running test2.py.

  it says
>
> Traceback (most recent call last):
>    File "test2", line 1, in <module>
>      from codec.support import *
> ImportError: No module named codec.support
>
>
> Python tells me Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56)
>
>

Take a look at Stephen's example.  He runs some things, shows you 
exactly what he's running, and what happens.  The things he runs may not 
match what you did, but we cannot tell.

 From within your script, you can display os.getcwd(), you can ...


import sys, os
print "cwd is", os.getcwd()
print "file is",  __file__
print "PATH is", os.getenv("PATH")
print "python-path is", os.getenv("PYTHONPATH")
print "sys.path is", sys.path

import codec
print "codec file is", codec.__file__

if you run that both as a regular file and on a symlink pointing to that 
file, comparing the two runs could give you a clue as to what's 
different.  My suspicion is that you have a different cwd when you test 
it the second way.

-- 
DaveA

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#45416

On Thu, 16 May 2013 00:48:45 -0700, Charles Smith wrote:

> Hi.
> 
> How can I say, from the cmd line, that python should take my CWD as my
> CWD, and not the directory where the script actually is?

*scratches head*

Python does use your current working directory as your current working 
directory. I think you are misdiagnosing the problem.

Here's a demonstration:

steve@runes:~$ cat test.py
import os
print os.getcwd()


steve@runes:~$ ln -s ~/test.py /tmp/test
steve@runes:~$ ls -l /tmp/test 
lrwxrwxrwx 1 steve steve 19 May 16 18:58 /tmp/test -> /home/steve/test.py
steve@runes:~$ cd /etc/
steve@runes:/etc$ python /tmp/test 
/etc


> I have a python script that works fine when it sits in directory WC, but
> if I move it out of WC to H and put a symlink from H/script to WC, it
> doesn't find the packages that are in WC.  Also, if I use the absolute
> path to H, it won't find them, but I guess I can understand that.

The obvious solution is to make sure that WC is in the Python path. You 
can do that by adding it the environment variable PYTHONPATH, or by 
adding it to sys.path at the start of your script. I think you can also 
use a .pth file as well.

Another solution may be to add this to the beginning of your script:

os.setcwd('path/to/WC')

but that's a crappy solution, you generally don't want to be changing the 
working directory from inside scripts if you can avoid it.



-- 
Steven

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#45417

On 16 Mai, 11:04, Steven D'Aprano <steve
+comp.lang.pyt...@pearwood.info> wrote:
> Python does use your current working directory as your current working
> directory. I think you are misdiagnosing the problem.


That's usually how it ends up ...



>
> Here's a demonstration:
>
> steve@runes:~$ cat test.py
> import os
> print os.getcwd()
>
> steve@runes:~$ ln -s ~/test.py /tmp/test
> steve@runes:~$ ls -l /tmp/test
> lrwxrwxrwx 1 steve steve 19 May 16 18:58 /tmp/test -> /home/steve/test.py
> steve@runes:~$ cd /etc/
> steve@runes:/etc$ python /tmp/test
> /etc


You're right.  I can believe that python doesn't really change it's
notion of the CWD.

It seems like when I'm in WC and invoke ./script (which is symlinked
to H/script), python says, "I have a specification somewhere that says
I can look for modules where my script is, and since I'm really smart,
I happen to know that I'm in WC but the script is not, but is really
in H, where the stupid operator better have his packages set up"


>
> > I have a python script that works fine when it sits in directory WC, but
> > if I move it out of WC to H and put a symlink from H/script to WC, it
> > doesn't find the packages that are in WC.  Also, if I use the absolute
> > path to H, it won't find them, but I guess I can understand that.
>
> The obvious solution is to make sure that WC is in the Python path.
...

> but that's a crappy solution,

Right.  Other tools don't try to figure out what I really want to do
with a symlink, but just accept that it's a file.  Python should do
the same ...

But, as you say, I'm surely misdiagnosing the problem.

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