(sorry for the delayed reply; vacation) On Aug 14, 2005, at 12:27 PM, Guido van Rossum wrote:
On 8/14/05, Michael Hudson <[EMAIL PROTECTED]> wrote:Wilfredo Sánchez Vega <[EMAIL PROTECTED]> writes:I'm curious about why Python lacks FileNotFoundError, PermissionError and the like as subclasses of IOError.Good question. Lack of effort/inertia?Well, I wonder how often it's needed. My typical use is this: try: f = open(filename) except IOError, err: print "Can't open %s: %s" % (filename, err) return and the error printed contains all the necessary details (in fact it even repeats the filename, so I could probably just say "print err").
That's fine for log output, but weak for handling the error.
Why do you need to know the exact reason for the failure? If you simply want to know whether the file exists, I'd use os.path.exists() or isfile(). (Never mind that this is the sometimes-frowned-upon look-before-you-leap; I think it's often fine.)
If you're going to wave off the look-before-you-leap argument, I guess you're right in that case, but I think it's a pretty valid argument for a lot of applications. os.path.exists() also has a race condition in the case where the file is deleted between your test for is and the subsequent attempt to access it, so you'd still need to handle that error in the exception handling if you care about correctness.
I agree that in many (most?) cases, this can be fudged, but if it's easier to do the correct thing, more people would do the correct thing.
In any case, os.path.exists() also doesn't catch permissions errors, and I often find myself wanting to handle those errors specially as well.
An example where both are useful is in an HTTP server, where a different status code should be returned to the client depending on success, file not found, permission denied, and other cases. Presently, I (and twisted.web) have to check errno in the IOError exception handler, which is really clunky, and I have to do it fairly often.
Also note that providing the right detail can be very OS specific. Python doesn't just run on Unix and Windows.
File not found is a detectable error case on all platforms, I think. On an OS that doesn't have permissions errors, I wouldn't expect the existence of an exception that isn't used to be a huge portability problem. I can't imagine that checking errno is a more portable solution.
These two exist and are quite useful in Java, for whatever that's worth.
-wsv
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