File handling: The easy and the hard way

Jeremy Jones zanesdad at
Fri Oct 1 04:54:56 CEST 2004

Hopefully, I'm not belaboring this.  Just a few questions that need to 
be addressed in the development of any application:
1.  Who is the user?
2.  What is the application?  (duh)
3.  What problems/errors/exceptions are going to occur and what do you 
do with them when they do?  (this is highly contingent upon the answers 
to the first two questions)

We'll get back to this.

Steve Holden wrote:

> Jeremy Jones wrote:
>> Hans-Joachim Widmaier wrote:
> [...]
>>> Especially not when the error is not in the program itself, but rather
>>> just a mistyped filename. (Most of my helper scripts that we use to
>>> develop software handle files this way. And even my co-workers don't
>>> recognize 'file or directory not found' for what it is.) End users are
>>> entitled to error messages they can easily understand like "I could not
>>> open 'blaah' because there is no such file".
>> So, you're saying that dumping a raw traceback like:
>> IOError: [Errno 2] No such file or directory: '/foo/bar/bam'
>> to a logfile is a no-no?  Instead, it should say:
>> I'm sorry sir, but an error occurred when trying to write to file 
>> /foo/bar/bam because it wasn't there.
> A traceback is not an error message. When a non-programming user sees 
> a fifteen-line stack trace with python statements and line numbers in 
> it that's quite enough to stop most of them from even reading it to 
> see if there's anything they understand at all.

You're partially right.  Not all tracebacks are error messages.  Not all 
error messages are tracebacks.  Tracebacks are when an exception 
occurs.  An error could happen and no traceback may result.  In the 
example of the OP of a mistyped filename, you could catch that error 
before you try to open the file.  That may not be the best way.  I kinda 
like relying on exceptions to help guide the flow of a program.  
Nevertheless, they are different, but there can be overlap.  Since the 
OP was concerned with exceptions that may happed during opening and 
writing to files, I focused on that.

Here we begin to answer the questions I posed above.  Who is the user?  
BTW - I didn't say it was a good idea in every case to always throw a 
traceback in the user's face, nor do I think it's always advisable to 
necessarily throw an error message in the user's face when an error 
occurs.  What I did say was to use common sense in every case.  If you 
are developing an application for a more technical segment of the 
population, you may very well be justified in displaying an error 
message at times *when common sense dictates*.  Even for the technical 
crowd, don't splat them with a stack trace just because you can and they 
may be able to decipher it.  If you're developing for the more general 
public, use common sense there, too, and give them what may help them 
solve the problem themselves.  That's a good goal, eh?  Let the user 
solve their own problems to the extent that they can and ask for help 
from the developer only when they absolutely need it.  I think this is 
the laziness virtue of a good programmer.

Another question to look at here is, what error occurred?  Again, if the 
user mistyped a filename, you should be able to gently tell the user 
that the file they specified was not found and not bother them 
(technical or not) with a traceback.  If you lost the filesystem that 
your data files live're pretty outa luck there.  
Stack trace isn't going to hurt anything depending on your audience.

>> I think the traceback is perfectly understandable.  I think that even 
>> an end-user would be able to comprehend that type of message.  Or, if 
>> you get an IOError, is this not sufficient:
>> IOError: [Errno 28] No space left on device
>> ?
> Again, that's not a traceback. It's an error message.
>> Chances are, end users aren't going to be particulary concerned with 
>> exceptions you log - unless they've got a problem that they can't 
>> figure out.  And if they've got a problem they can't figure out, 
>> you'd be better off giving them as much information as you can give 
>> them, or they'll come to you for help.  And when they do come to you 
>> for help, you'd better make sure you've given yourself the most 
>> informatino you can to solve the problem.  So logging a traceback is 
>> a great idea IMHO.  Now, in areas where you're dead sure that an 
>> exception is nothing to be concerned with, don't bother.  So, a good 
>> approach may be: handle the specific exceptions that you know may 
>> occur, let other unexpected (or expected in worst case scenarios) 
>> exceptions filter up to a higher level, log them there, and if need 
>> be, terminate program execution.
> This isn't about not terminating the program, it's about reporting the 
> reasons in a manner acceptable to average users. 

??  Did you take from what I said that I wasn't concerned with reporting 
the reasons back to the user in an acceptable fashion?  That's part of 
what I said above with "handle the specific exceptions that you know may 
occur."  A good developer should try to figure out what can break and 
address it before it does (within the bounds of common sense, of 
course).  Again, WRT common sense, quoting myself from below:
Do your best to try to figure out what that reason is, deal with it, 
figure out the most reasonable thing to do with _that_ exception, and 
move on.  Sometimes that'll mean throwing a traceback to a log file, 
sometimes it will mean handling it gracefully and "prettying up" the 
message for logging or display to the end user, sometimes it will mean 
totally ignoring it, other times you may need to just stop the program.

Honestly, Steve, I know I may not be the sharpest knife in the drawer, 
but I'm getting the impression you're trying to make me out to be dumber 
than I am....

I'm not saying that we need to just spew tracebacks everywhere.  I'm 
saying you need to handle them the best you can.  _Sometimes_ that means 
you know that an exception may possibly occur and the best thing you can 
do is just let it hit the fan.

>>> Graceful error handling is
>>> even more important when a program isn't just run on a command line but
>>> with a GUI.
>> Maybe so.  But if you hit an "Oh, crap, what do I do now?" exception, 
>> you may want to throw up a dialog box with a traceback or something 
>> and when the user clicks OK on it, terminate program execution.  That 
>> gives them a chance to (unlikely) figure out what they can do to 
>> remedy the situation, otherwise call for help.
> I'm all for LOGGING tracebacks. Indeed WingIDE is a beauty in this 
> respect, since it's also prepared to send feedback to Wing if you ask 
> it to, as is Mozilla and (nowadays) Internet Explorer.
> Given this, there's little excuse for showing the traceback in the 
> regular case, though I don't object to allowing users to look for it 
> if they want.

Tell that to the anal retentive security guy who's got the application 
server totally cordoned off from the internet so that's an 
impossibility.  I know, you said, *regular case.*  All I'm saying is, be 
discerning.  Know the user and what kind of error you may be getting.  
Provide the information that the user and developer may need to solve 
the problem that occurred.  If that means sending it back via a feedback 
agent, great.  If that's dumping a stack trace to a log file, great.  
Sometimes, if that means showing the exception to the user, great.

>>> Which means? Which means that all this convenient file handling that
>>> Python offers really should not be used in programs you give away. 
>>> When I
>>> asked for a canonical file access pattern some months ago, this was the
>>> result:
>>> Now I have some programs that read and write several files at once. And
>>> the reading and writing is done all over the place. If I really 
>>> wanted to
>>> do it "right", my once clear and readily understandable code turns 
>>> into a
>>> nightmare. This doesn't look like the language I love for its 
>>> clarity and
>>> expressivness any more. Python, being a very high level language, 
>>> needs a
>>> higher level file type, IMHO. This is, of course, much easier said than
>>> done. And renown dimwits like me aren't expected to come up with 
>>> solutions.
>>> I've thought about subclassing file, but to me it looks like it 
>>> wouldn't
>>> help much. With all this try/except framing you need to insert a call
>>> level anyway (wondering if this new decorator stuff might help?). 
>>> The best
>>> I've come up so far is a vague idea for an error callback (if there 
>>> isn't
>>> one, the well known exceptions might be raised) that gets called for
>>> whatever error occured, like:
>>> class File:
>>>    ...
>>>    def write(self, data):
>>>        while True:
>>>            try:
>>>                self._write(data)
>>>            except IOError, e:
>>>                if self.errorcallback:
>>>                    ret, dat = self.errorcallback(self, F_WRITE, e, 
>>> data)
>>>                    if ret == F_RETURN:
>>>                        return dat
>>>                else:
>>>                    raise
>>> The callback could then write a nice error message, abort the program,
>>> maybe retry the operation (that's what the 'while True'-loop is for) or
>>> return whatever value to the original caller. Although the callback
>>> function will usually be more than a few lines, it can be reused. It 
>>> can
>>> even be packed into your own file-error-handling module, something the
>>> original usage pattern can't.
>> Hmmm....interesting.  Shouldn't you put a break after your 
>> self._write(data)?  This is probably not a bad way of going about 
>> things, but what types of files are we talking about here?  Log 
>> files?  I think you're probably better off using the builtin logging 
>> and just dump raw tracebacks in there.  Data files?  Then you've 
>> probaby got that wrapped in code to write formatted data to the data 
>> file anyway in which case, this type of specialized class is probably 
>> not a bad thing.  If you're trying to write data to a data file, you 
>> don't want litter it with error messages.  You want to log it and, 
>> maybe even unlink the data file and do something special.
>>> If you still bear with me, you might as well sacrifice a few more 
>>> seconds
>>> and tell me what you think about my rant. Is everything just fine as 
>>> it is
>>> now? Or do I have a point? I always felt it most important to handle 
>>> all
>>> errors a program may encounter gracefully, and the easier this is to 
>>> do,
>>> the less likely it is a programmer will just sneak around the issue and
>>> let the interpreter/run time system/operating system handle it. (And 
>>> yes,
>>> I'm guilty of not obeying it myself, as it can double or triple the 
>>> time
>>> needed to write the whole program; just because its so cumbersome.)
>> I dunno - something just doesn't feel right here.  I kinda feel like 
>> you're wanting to create an over-generalized solution.  Your File 
>> class is interesting and may be a good start for a lot of general 
>> solutions and having a callback mechanism helps specialize it, 
>> but....something just doesn't sit totally right here with me.  This 
>> may work totally perfectly and may be an excellent piece of code to 
>> handle all of your file writing activities.  I dunno....
>> You're not going to be able to catch every exception - not 
>> meaningfully, anyway.  You could do something like:
>> if __name__ == "__main__":
>>    try:
>>        main()
>>    except Exception, e:
>>        log(e)
>> But that isn't handling all errors....
> It's certainly catching all subclasses of Exception, though, which in 
> modern Python should be everything not handled inside (string 
> exceptions are a throwback, retained for compatibility reasons). As to 
> whether they are bing "handled", I guess that's a matter of opinion.
>> Production quality code doesn't necessarily mean never terminating 
>> because of an exception.  You want to reduce the frequency of program 
>> termination due to exceptions.  I can appreciate your desire to make 
>> sure you've got good solid software, and not encumber the end user 
>> with ever little exception you hit, but sometimes it's OK to log/show 
>> exceptions.  Like I said earlier, when you hit an exception, you hit 
>> it for a reason.  Do your best to try to figure out what that reason 
>> is, deal with it, figure out the most reasonable thing to do with 
>> _that_ exception, and move on.  Sometimes that'll mean throwing a 
>> traceback to a log file, sometimes it will mean handling it 
>> gracefully and "prettying up" the message for logging or display to 
>> the end user, sometimes it will mean totally ignoring it, other times 
>> you may need to just stop the program.  All of these resolutions can 
>> be part of a production quality piece of software.  The discerning 
>> programmer has to decide which solution is appropriate for which 
>> situation.  Like Steve Holden mentioned, it's really good that you're 
>> concerned with such things, but make sure you apply common sense to 
>> each scenario.
>>> Waiting-for-you-to-jump-on-me'ly yours,
>>> Hans-Joachim
>> Hope I didn't jump too hard.
> Possibly.

Yes, I may have jumped him a bit hard.  Here were my beefs:

1.  The OP comes in in rant mode and almost states that Python is flawed 
and inadequate to be released as production quality code - or that it is 
impossible to write production quality code in Python because it is 
flawed.  I know that's not what he said, but that's the impression I got 
from his overall tone and his comments:

While this is very handy for interactive usage or throw-away scripts, I'd
consider it a serious bug in a "production quality" software.

Python, being a very high level language, needs a
higher level file type, IMHO.

Honestly, they just kinda set me off a bit.  I apologize to the OP if I 
did jump a bit too hard, but when you waltz into c.l.p. and state what 
it felt like he was saying, it's kind of like someone waltzing into my 
living room and kicking my dog.  It's just not going to go over too 
well.  I don't think Python needs a higher level file class.  I still 
think he's got a lot of good ideas in there.  I think the thing that 
didn't sit well with me earlier was that it came across sounding like, 
"Python needs a new file class.  You can't claim that your code is 
production ready unless you do something similar to this."  Which is a 
nice segue into

2.  It seemed to me that the OP was positing a solution to be used in a 
broader context than necessary.  Whereas I have stated over and over and 
over that common sense must be applied to every situation and every 
situation must be evaluated differently, I haven't gotten the same vibe 
from the OP.  Sometimes his solution may work just fine and dandy.  May 
even make a great open source project.  I'm sure a lot of people would 
use it.  I might even use it some.  I just got the impression that he 
was speaking far too categorically for my tastes.

>> Jeremy Jones
> regards
>  Steve

I hope that I haven't come across as antagonistic as I fear I may have.  
I greatly respect you, Steve.  You're a great writer and a valuable 
contributor to this group.  I don't think we're too far off from what 
the other is saying.  I don't even think I'm too far off from the spirit 
of what the OP was trying to say.  I just had a problem with what I 
perceived the OP to be saying (WRT generalization of a solution) and 
didn't care for how it came across.  I feel better after getting that 
out.  Hope we can still be friends :-)

Mr. Holden, I remain sincerely and respectfully yours,

Jeremy Jones

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