Re: FileNotFoundError thrown due to file name in file, rather than file itself

Left Right <olegsivokon@gmail.com> writes:

Poor error reporting is a very common problem in programming.  Python
is not anything special in this case.  Of course, it would've been
better if the error reported what file wasn't found.  But, usually
these problems are stacking, like in your code.  Unfortunately, it's
your duty, as the language user, to anticipate those problems and act
accordingly. Now you've learned that the one file you believe that
could be the source for the error isn't the only one--well, adjust
your code to differentiate between those two (and potentially other?)
cases.  There's very little else you can do beside that.
>
NB. On the system level, the error has no information about what file
wasn't found.  It simply returns some numeric value (the famous
ENOENT) in case when the system call to open a file fails.  Python
could've been more helpful by figuring out what path caused the
problem and printing that in the error message, but it doesn't...
That's why I, myself, never use the vanilla FileNotFoundError, I
always re-rise it with a customized version that incorporates the
information about the missing file in the error message.

That sounds like a good idea.

NB2. It's always a bad idea to print logs to files.  Any sysadmin /
ops / infra person worth their salt will tell you that.  The only
place the logs should go to is the standard error.  There are true and
tried tools that can pick up logs from that point on, and do with them
whatever your heart desires.  That is, of course, unless you are
creating system tools for universal log management (in which case, I'd
question the choice of Python as a suitable language for such a task).
Unfortunately, even though this has been common knowledge for decades,
it's still elusive in the world of application development :|

I am not entirely convinced by NB2.  I am, in fact, a sort of sysadmin
person and most of my programs write to a log file.  The programs are
also moderately complex, so a single program might access a database,
query an LDAP server, send email etc., so potentially quite a lot can go
wrong.  They are also not programs whose output I would pipe to another
command.  What would be the advantage of logging to stderr?  Quite apart
from that, I find having a log file a useful for debugging when I am
developing.

Cheers,

Loris

On Mon, Nov 11, 2024 at 4:00 PM Loris Bennett via Python-list
<python-list@python.org> wrote:

>
Hi,
>
I have the following in my program:
>
    try:
        logging.config.fileConfig(args.config_file)
        config = configparser.ConfigParser()
        config.read(args.config_file)
        if args.verbose:
            print(f"Configuration file: {args.config_file}")
    except FileNotFoundError:
        print(f"Error: configuration file {args.config_file} not found.  Exiting.")
        sys.exit(0)
>
and when I ran the program I got the error
>
  Error: configuration file /usr/local/etc/sc_mailer not found.  Exiting.
>
However, this file *does* exist and *can* be read.  By checking the
'filename' attribute of the exception I discovered that the problem was
the log file defined *in* the config file, namely
>
  [handler_fileHandler]
  class=FileHandler
  level=DEBUG
  formatter=defaultFormatter
  args=('/var/log/my_prog.log', 'a')
>
This log file did not exist.  The exception is thrown by
>
  logging.config.fileConfig(args.config_file)
>
My questions are:
>
1. Should I be surprised by this behaviour?
2. In terms of generating a helpful error message, how should one
   distinguish between the config file not existing and the log file not
   existing?
>
Cheers,
>
Loris
>
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Dr. Loris Bennett (Herr/Mr)
FUB-IT, Freie Universität Berlin