(cross-posting back to python-dev to finalize discussions)
2009/4/2 Guido van Rossum
<guido@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.
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.