Pythonic way for handling file errors

danielx danielwong at berkeley.edu
Thu Oct 11 00:16:17 CEST 2007


On Oct 10, 12:28 pm, Paul Hankin <paul.han... at gmail.com> wrote:
> On Oct 10, 7:41 pm, wink <w... at saville.com> wrote:
>
> > I would like to know what would be considered the most
> > Pythonic way of handling errors when dealing with files,
> > solutions that seem reasonable using 2.5:
>
> The best way to handle errors is to catch the exceptions that are
> raised by the code that handles the error. Your examples push the
> problem elsewhere: you're setting an error code which has to be tested
> for. But perhaps your application needs to do this for some reason
> (judging from your error codes, this is some sort of web script).
>
> > ...
> > try:
> >    with open('afile', 'r') as f:
> >        content = f.read()
> >    error = 200
> > except Exception:
> >    error = 404
>
> Of all your examples, this is the best. But the catch-all exception
> handler is bad: it's better to catch just file io exceptions. Also, I
> think it's better to put the OK case (error 200) in an else clause to
> make it clearer that it's only set when no error occurs. It's also
> better to use constants in place of magic numbers.
>
> import httplib
>
> try:
>     with open('afile', 'r') as f:
>         content = f.read()
> except IOError:
>     error = httplib.NOT_FOUND
> else:
>     error = httplib.OK
>
> --
> Paul Hankin

Wink,

One of the problems your facing is knowing whether you managed to open
the file before reaching the finally block where you close your file.
To avoid this, I open my files right before try/finally. For me, the
only purpose in having the try/finally is to make sure my files are
properly closed, although when I'm lazy, I just let the file close it
self during garbage collection. This is what it looks like:

file = open('filename')
try:
  # read and/or process file
finally:
  file.close()

Of course, this example does not handle any exceptions. In many cases,
you want these errors to propogate upward so the users of your
functions/methods can decide what needs to be done. On the other hand,
you could wrap this code with try/except/else if you wanted to handle
the exception "at the source".




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