Scanning a file

[Alex Martelli]

Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:
   ...
> I should also point out that for really serious work, the idiom:
> 
> f = file("parrot")
> handle(f)
> f.close()
> 
> is insufficiently robust for production level code. That was a detail I
> didn't think I needed to drop on the original newbie poster, but depending
> on how paranoid you are, or how many exceptions you want to insulate the
> user from, something like this might be needed:
> 
> try:
>     f = file("parrot")
>     try:
>         handle(f)
>     finally:
>         try:
>             f.close()
>         except:
>             print "The file could not be closed; see your sys admin."
> except:
>     print "The file could not be opened."

The inner try/finally is fine, but both the try/except are total, utter,
unmitigated disasters: they will hide a lot of information about
problems, let the program continue in a totally erroneous state, give
mistaken messages if handle(f) causes any kind of error totally
unrelated to opening the file (or if the user hits control-C during a
lengthy run of handle(f)), emit messages that can erroneously end up in
the redirected stdout of your program... VERY, VERY bad things.

Don't ever catch and ``handle'' exceptions in such ways.  In particular,
each time you're thinking of writing a bare 'except:' clause, think
again, and you'll most likely find a much better approach.


Alex