[Python-Dev] Remaining decisions on PEP 471 -- os.scandir() (original) (raw)

Akira Li 4kir4.1i at gmail.com
Mon Jul 14 07:51:24 CEST 2014

Nick Coghlan <ncoghlan at gmail.com> writes:

On 13 Jul 2014 20:54, "Tim Delaney" <timothy.c.delaney at gmail.com> wrote:

On 14 July 2014 10:33, Ben Hoyt <benhoyt at gmail.com> wrote:

If we go with Victor's link-following .isdir() and .isfile(), then we probably need to add his suggestion of a followsymlinks=False parameter (defaults to True). Either that or you have to say "stat.SISDIR(entry.lstat().stmode)" instead, which is a little bit less nice. Absolutely agreed that followsymlinks is the way to go, disagree on the default value. Given the above arguments for symlink-following isdir()/isfile() methods (have I missed any, Victor?), what do others think? I would say whichever way you go, someone will assume the opposite. IMO not following symlinks by default is safer. If you follow symlinks by default then everyone has the following issues: 1. Crossing filesystems (including onto network filesystems); 2. Recursive directory structures (symlink to a parent directory); 3. Symlinks to non-existent files/directories; 4. Symlink to an absolutely huge directory somewhere else (very annoying if you just wanted to do a directory sizer ...). If followsymlinks=False by default, only those who opt-in have to deal with the above. Or the ever popular symlink to "." (or a directory higher in the tree). I think os.walk() is a good source of inspiration here: call the flag "followlink" and default it to False.

Let's not multiply entities beyond necessity.

There is well-defined follow_symlinks parameter https://docs.python.org/3/library/os.html#follow-symlinks e.g., os.access, os.chown, os.link, os.stat, os.utime and many other functions in os module support follow_symlinks parameter, see os.supports_follow_symlinks.

os.walk is an exception that uses followlinks. It might be because it is an old function e.g., newer os.fwalk uses follow_symlinks.

As it has been said: os.path.isdir, pathlib.Path.is_dir in Python File.directory? in Ruby, System.Directory.doesDirectoryExist in Haskell, test -d in shell do follow symlinks i.e., follow_symlinks=True as default is more familiar for .is_dir method.

cd path in shell, os.chdir(path), ls path, os.listdir(path), and os.scandir(path) itself follow symlinks (even on Windows: http://bugs.python.org/issue13772 ). GUI file managers such as nautilus also treat symlinks to directories as directories -- you may click on them to open corresponding directories.

Only recursive functions such as os.walk, os.fwalk do not follow symlinks by default, to avoid symlink loops. Note: the behavior is consistent with coreutils commands such as cp that follows symlinks for non-recursive actions but e.g., du utility that is inherently recursive doesn't follow symlinks by default.

follow_symlinks=True as default for DirEntry.is_dir method allows to avoid easy-to-introduce bugs while replacing old os.listdir/os.path.isdir code or writing a new code using the same mental model.

-- Akira

More information about the Python-Dev mailing list