[Python-Dev] OSError.errno => exception hierarchy? (original) (raw)

Gustavo Carneiro gjcarneiro at gmail.com
Fri Apr 3 01:13:05 CEST 2009

(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 OSErrorENAME. 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.OSErrorENAME. > 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() except OSError: if os.path.exists(): 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 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", ignoreerrors=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/Errorhiding > > 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20090403/5db48395/attachment.htm>

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