[Python-ideas] [Python-Dev] OSError.errno => exception hierarchy?

[Gustavo Carneiro]

(cross-posting back to python-dev to finalize discussions)

2009/4/2 Guido van Rossum <guido at python.org>
[...]

> > The problem you report:
> >>
> >>  try:
> >>    ...
> >>  except OSWinError:
> >>    ...
> >>  except OSLinError:
> >>    ...
> >>
> >
> > Would be solved if both OSWinError and OSLinError were always defined in
> > both Linux and Windows Python.  Programs could be written to catch both
> > OSWinError and OSLinError, except that on Linux OSWinError would never
> > actually be raised, and on Windows OSLinError would never occur.  Problem
> > solved.
>
> Yeah, but now you'd have to generate the list of exceptions (which
> would be enormously long) based on the union of all errno codes in the
> universe.
>
> Unless you only want to do it for some errno codes and not for others,
> which sounds like asking for trouble.
>
> Also you need a naming scheme that works for all errnos and doesn't
> require manual work. Frankly, the only scheme that I can think of that
> could be automated would be something like OSError_ENAME.
>
> And, while OSError is built-in, I think these exceptions (because
> there are so many) should not be built-in, and probably not even live
> in the 'os' namespace -- the best place for them would be the errno
> module, so errno.OSError_ENAME.
>
> > The downsides of this?  I can only see memory, at the moment, but I might
> be
> > missing something.
>
> It's an enormous amount of work to make it happen across all
> platforms. And it doesn't really solve an important problem.


I partially agree.  It will be a lot of work.  I think the problem is valid,
although not very important, I agree.


>
>
> > Now just one final word why I think this matters.  The currently correct
> way
> > to remove a directory tree and only ignore the error "it does not exist"
> is:
> >
> > try:
> >     shutil.rmtree("dirname")
> > except OSError, e:
> >     if errno.errorcode[e.errno] != 'ENOENT':
> >        raise
> >
> > However, only very experienced programmers will know to write that
> correct
> > code (apparently I am not experienced enought!).
>
> That doesn't strike me as correct at all, since it doesn't distinguish
> between ENOENT being raised for some file deep down in the tree vs.
> the root not existing. (This could happen if after you did
> os.listdir() some other process deleted some file.)


OK.  Maybe in a generic case this could happen, although I'm sure this won't
happen in my particular scenario.  This is about a build system, and I am
assuming there are no two concurrent builds (or else a lot of other things
would fail anyway).


> A better way might be
>
> try:
>  shutil.rmtree(<dir>)
> except OSError:
>  if os.path.exists(<dir>):
>   raise


Sure, this works, but at the cost of an extra system call.  I think it's
more elegant to check the errno (assuming the corner case you pointed out
above is not an issue).


> Though I don't know what you wish to happen of <dir> were a dangling
> symlink.
>
> > What I am proposing is that the simpler correct code would be something
> > like:
> >
> > try:
> >     shutil.rmtree("dirname")
> > except OSNoEntryError:
> >     pass
> >
> > Much simpler, no?
>
> And wrong.
>
> > Right now, developers are tempted to write code like:
> >
> >     shutil.rmtree("dirname", ignore_errors=True)
> >
> > Or:
> >
> > try:
> >     shutil.rmtree("dirname")
> > except OSError:
> >     pass
> >
> > Both of which follow the error hiding anti-pattern [1].
> >
> > [1] http://en.wikipedia.org/wiki/Error_hiding
> >
> > Thanks for reading this far.
>
> Thanks for not wasting any more of my time.


OK, I won't waste more time.  If this were an obvious improvement beyond
doubt to most people, I would pursue it, but since it's not, I can live with
it.

Thanks anyway,

-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert
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