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To: python-list@python.org
From: Frank Millman <frank@chagford.com>
Subject: Re: Best way to find starting directory
Date: Tue, 19 Mar 2013 16:29:36 +0200
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On 19/03/2013 14:46, Dave Angel wrote:
> On 03/19/2013 04:21 AM, Frank Millman wrote:
>> On 19/03/2013 09:55, Peter Otten wrote:
>>> Frank Millman wrote:
>>>
>>>> I want to locate a file relative to the directory from which the main
>>>> program was launched.
>>>>
>>>> I have found two ways of finding the starting directory -
>>>>
>>>> 1.
>>>> import os
>>>> dir = os.getcwd()
>>>
>>> This gives the current working directory...
>>>
>>>> 2.
>>>> import os.path
>>>> import __main__
>>>> dir = os.path.dirname(__main__.__file__)
>>>
>>> ... and this gives the location of your main script.
>>>
[...]
>>
>> That makes sense. I usually launch the script from its own directory,
>> but that is not guaranteed.
>>
>> Therefore option 2 is the way to go.
>
> You might want to reconsider.  There are really two different kinds of
> data files you might want to access from your script. The first is
> constant data that gets initialized when the script is installed.  And
> the second is user data that he's thinking about right now.
>
> For example, if a script uses a saved cache of prime numbers to make
> calculations a bit faster, it might keep that file in with its own
> source code, or relative to it.
>
> And if I wanted to calculate md5 sums for a directory tree, I'd usually
> make that my cwd before starting the script.
>
> Config files are somewhere in between.  In Linux, get them relative to
> the $HOME environment variable.
>
> FWIW, I try to keep all the first kind of files on a separate partition,
> and except where other programs force me, never let them leak onto the
> OS+program partition.  That way, I'm not likely to lose an important
> jpeg when the OS trashes its partition.  This particular paranoia is
> left over from my Windows days, but I stick to it anyway.  It also makes
> it easier to migrate to a new OS.  Just format the OS partition and
> install the OS and all the apps.  The data is already separate.
>

Thanks Dave, good advice.

I learnt an important lesson from Peter's response, as I did not fully 
understand the difference before. Now that I do, I am better equipped to 
make the correct decision for a given situation.

As you say, there is a variety of types of data that one might to store 
externally. My current scenario is that, in my business/accounting 
application, I use xml to store form definitions, report definitions, 
etc, which are kept in the database (compressed). I am now constructing 
some xml schemas to validate the xml files. I need to store the schemas 
somewhere, so I have created a directory called 'schemas' under the main 
directory. I need to access them from various parts of the application, 
so I need a reliable way to locate the 'schemas' directory.

In theory I could store them somewhere different, and use a parameter to 
provide the path. But they are only used within the context of the 
application, so I think it makes sense to keep them alongside the 'py' 
files that make up the application.

Frank