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

Ben Hoyt benhoyt at gmail.com
Mon Jul 14 14:27:39 CEST 2014

First, just to clarify a couple of points.

You forgot one of my argument: we must have exactly the same API than os.path.isdir() and pathlib.Path.isdir(), because it would be very confusing (source of bugs) to have a different behaviour.

Actually, I specifically included that argument. It's item (b) in the list in my original message yesterday. :-)

Since these functions don't have any parameter (there is no such followsymlink(s) parameter), I'm opposed to the idea of adding such parameter.

If you really want to add a followsymlink optional parameter, IMO you should modify all os.path.is*() functions and all pathlib.Path.is*() methods to add it there too. Maybe if nobody asked for this feature before, it's because it's not useful in practice. You can simply test explicitly issymlink() before checking isdir().

Yeah, this is fair enough.

Well, let's imagine DirEntry.isdir() does not follow symlinks. How do you test isdir() and follow symlinks? "stat.SISDIR(entry.stat().stmode)" ? You have to import the stat module, and use the ugly C macro SISDIR().

No, you don't actually need stat/S_ISDIR in that case -- if DirEntry.is_dir() does not follow symlinks, you just say:

entry.is_symlink() and os.path.isdir(entry.full_name)

Or for the full test:

(entry.is_symlink() and os.path.isdir(entry.full_name)) or entry.is_dir()

On the other hand, if DirEntry.is_dir() does follow symlinks per your proposal, then to do is_dir without following symlinks you need to use DirEntry. lstat() like so:

stat.S_ISDIR(entry.lstat().st_mode)

So from this perspective it's somewhat nicer to have DirEntry.is_X() not follow links and use DirEntry.is_symlink() and os.path.isX() to supplement that if you want to follow links.

I think Victor has a good point re 92% of the stdlib calls that use listdir and isX do follow links.

However, I think Tim Delaney makes some good points above about the (not so) safety of scandir following symlinks by default -- symlinks to network file systems, nonexist files, or huge directory trees. In that light, this kind of thing should be opt-in.

I guess I'm still slightly on the DirEntry-does-not-follow-links side of the fence, due to the fact that it's a method on the directory entry object, due to simplicity of implementation, and due to Tim Delaney's "it should be safe by default" point above.

However, we're almost bikeshedding at this point, and I think we just need to pick one way or the other. It's straight forward to implement one in terms of the other in each case.

-Ben

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