[Python-Dev] Issue 11406: adding os.scandir(), a directory iterator returning stat-like info (original) (raw)

Christian Heimes christian at python.org
Mon May 13 16:49:19 CEST 2013

Am 13.05.2013 02:21, schrieb Ben Hoyt:

Are you suggesting just accessing .cachedlstat could call os.lstat()? That seems very bad to me. It's a property access -- it looks cheap, therefore people will expect it to be. From PEP 8 "Avoid using properties for computationally expensive operations; the attribute notation makes the caller believe that access is (relatively) cheap."

Even worse is error handling -- I'd expect the expression "entry.cachedlstat" to only ever raise AttributeError, not OSError in the case it calls stat under the covers. Calling code would have to have a try/except around what looked like a simple attribute access. For these two reasons I think lstat() should definitely be a function.

OK, you got me! I'm now convinced that a property is a bad idea.

I still like to annotate that the function may return a cached value. Perhaps lstat() could require an argument?

def lstat(self, cached):
    if not cached or self._lstat is None:
        self._lstat = os.lstat(...)
    return self._lstat

True. My isdir/isfile/islink implementations should catch any OSError from the lstat() and return False (like os.path.isdir etc do). But then calling code still doesn't need try/excepts around the isdir() calls. This is how os.walk() is implemented -- there's no extra error handling around the isdir() call.

You could take the opportunity and take the 'file was deleted' case into account. I admit it has a very low priority. Please regard the case for bonus points only. ;)

Sure. I'm primarily a Windows dev, so not too familiar with all the fancy stat* functions. But what you're saying makes sense.

I'm glad to be of assistance! The feature is new (added in 3.3) and is available on most POSIX platforms. http://docs.python.org/3/library/os.html#dir-fd

If you need any help or testing please feel free to ask me. I really like to get this feature into 3.4.

Christian

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