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How to effectively use exception
Carl Banksidot at vt.edu
Sat Apr 14 15:39:49 EDT 2001
Steven Haryanto <steven at haryan.to> wrote:> So far I have understood one rule of thumb about exceptions:> "Catch as few of them as possible in the inner blocks of the> program. The uncaught exceptions will creep up to the upper> blocks until, at one level, it will be caught by the code that> know how to handle it. The main block should catch all the> rest and act accordingly.I prefer this rule of thumb: "The exeption should be handled whereeverthe program can logically proceed after the exception is raised."So if the exception is a fatal error, the only place I would handle itis at the very top.Sometimes a program can proceed logically from the point where the theexception occured; there is no reason at all to artificially move upthe try block in this case. For example, suppose a program logic isthis: if a file exists, open and parse it; if not, do nothing. Insuch a case, I would have no guilt at all about doing this:try:f = open (filename, 'r')parse_file (f)f.close ()except IOError:passNow, suppose that the file parser involves deep recursion, and if itencounters a syntax error (in the file, not in the Python program), itcompletely abandons the parse by raising UserSyntaxError. Whereshould UserSyntaxError be caught? Well, since the parse wasabandoned, the exception should be handled by the caller ofparse_file. So I would add the following except block to the tryblock above:except UserSyntaxError, v:f.close()error ("Syntax Error", v.line, v.column)Of course, it's possible that, for some other reason, the programmight be not able to proceed logically here. For example, the syntaxerror could turn out to be fatal, in which case the only place tohandle it would be at the very top.> There are two things that I seek advice on:>> First, should we use exceptions whenever possible? For example,> should a smallish function that do some testing return 1/0 to> indicate success/failure or should it raise ValueError if the> test fails? Is the try-except-else style more Pythonic/> recommended than the if-else-then style? If not, what are the> considerations to balance the two?You describe your smallish function as testing for success/failure.To me, "failure" means that something happens which wouldn't havehappened in the ideal universe. Failure means something isinterfering with your program. If that's what your function testsfor, then I advise using an exception.If your smallish function tests some condition, where either result islegal and doesn't interfere with the program, then I would normallyreturn 1/0.If your smallish test function does not normally test for failure, butin a particular context, a certain result does indicate failure, Iwould raise the exception in the calling function, not in the smallishtest function.I suppose a good rule of thumb here is to ask yourself, simply, "Is itan exception?"> Second, when do we create our own specific Exception classes,> and when do we just use the existing standard Exception (I use> ValueError in a lot of my methods & functions, accompanied by> the specific message string. So far I haven't faced with the> need to create a custom Exception class).It seems to me that if you use the builtin exception exactly as it isused by the language, it's ok. You might want to review how theexception is used by the language to make sure you're using it thesame way.Then again, it seldom hurts to use own. I would say, "When inreasonable doubt, use your own."> Lastly, can someone recommend some readings on this matter?Hacker and philosopher, myself. Can't help you too much here.-- CARL BANKS"What is life but a series of inspired follies?" -- Henry Higgins in _Pygmalion_, by George Bernard Shaw
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