[Tutor] What is the easiest way to ensure the current working directory is the same as the directory where the main program is saved?

[boB Stepp]

I have asked questions on this topic off and on in the past.  So far I
have been able to make my programs do what I intend, but I have a
feeling I am still not getting something fundamental.

On Fri, Jun 25, 2021 at 10:21 PM Cameron Simpson <cs at cskk.id.au> wrote:

> [...]
> >Is there an _easy_ way to have a program start with its current
> >working directory in the same directory where its source code resides
> >no matter from where the program is launched?  Or must I always use
> >the __file__ attribute to determine where the source code resides and
> >then change directories from there?
>
> The latter.
>
> But I recommend _not_ changing your working directory. It is a process
> global state which affects everything. When someone invokes your
> programme and supplies a filename on the command line, they usually
> expect that name to be resolved from the working directory where they
> invoked the programme.

I never considered this possibility!  Yes, must avoid doing that!!

> If the change directory you need to know where you were if you need to
> work with any caller suppplied relative filenames.
>
> >How do people deal with their
> >Python applications that in theory may be installed anywhere in a
> >user's file system?  There must be some standard way of dealing with
> >this that I am too dense to uncover.
>
> Well, broadly you don't care where the code is. You care about the data
> of the person using your programme, which they'll tell you. (Directly
> with filenames or the like, or also by envionment variables.)

<Scratch head>  But I *do* care about where my source code winds up,
don't I?  How else do I load data from its data folder if I don't know
where I am in the user's file system?

> When you do care there are two basic approaches that come to mind:
>
> - use __file__, get its dirname, and access some resources you know are
>   installed beside the source; this is generally awkward - you need
>   control during install, and it is possible to ship python source as a
>   zip file and the __file__ paradigm doesn't work there
>
> - have a config file specifying the location of resources

Maybe this is where my misunderstandings are occurring.  To date, I
have had no installation process.  Any programs that I need to be
elsewhere, I copy the program to its new living facilities, whether
elsewhere on my PC, somewhere else on a network, or to another PC.
Does an actual installation process make these issues go away?  As a
user of other people's programs that come with an installer one of the
first choices the user usually must make is accept the default
installation suggestion or choose a different location.  I suppose
this is where the needed information is fed into the program and
stored for later use?

I thought I could put off figuring out how to install Python packages
the "proper" way (if there is one) ...

> Maybe you could enumerate some circumstances giving you trouble. See
> what can be done.

A typical example from my earlier playing-around-with-this session --
needing to open and read in another file:

Play around code:
------------------------
import os

print("Testing!")
print("Current working directory: ", os.getcwd())
print("Source code file: ", __file__)
with open('test_text.txt') as f:
    print(f.read())
------------------------

Now if I run this from the same folder where the source code and data
file are, all is fine.  But if I start the program elsewhere then a
problem ensues:

PS C:\> Users\boB\Practical_Programming\testing.py    # NOT the
program directory.
Testing!
Current working directory:  C:\
Source code file:  C:\Users\boB\Practical_Programming\testing.py
Traceback (most recent call last):
  File "C:\Users\boB\Practical_Programming\testing.py", line 6, in <module>
    with open('test_text.txt') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'test_text.txt'
PS C:\> cd .\Users\boB\Practical_Programming\
PS C:\Users\boB\Practical_Programming> .\testing.py    # Now in the
program's folder.
Testing!
Current working directory:  C:\Users\boB\Practical_Programming
Source code file:  C:\Users\boB\Practical_Programming\testing.py
This is just a plain text file of no import.
This is the second line of a boring file.

Though a toy example, this is the basic problem I have been hacking
around.  My usual approach is when I am lazy I hard-code the "home"
location for wherever I place the program.  When I am more energetic I
make use of the __file__ attribute to automatically decide the "home"
location.

How should I be handling these situations?

Cheers!
boB Stepp