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

Cameron Simpson cs at zip.com.au
Wed Jul 16 05:40:00 CEST 2014

I was going to stay out of this one...

On 14Jul2014 10:25, Victor Stinner <victor.stinner at gmail.com> wrote:

2014-07-14 4:17 GMT+02:00 Nick Coghlan <ncoghlan at gmail.com>:

Or the ever popular symlink to "." (or a directory higher in the tree). "." and ".." are explicitly ignored by os.listdir() an os.scandir().

I think os.walk() is a good source of inspiration here: call the flag "followlink" and default it to False.

I also think followslinks should be spelt like os.walk, and also default to False.

IMO the specific function os.walk() is not a good example. It includes symlinks to directories in the dirs list and then it does not follow symlink,

I agree that is a bad mix.

it is a recursive function and has a followlinks optional parameter (default: False).

Which I think is desirable.

Moreover, in 92% of cases, functions using os.listdir() and os.path.isdir() follow symlinks: https://mail.python.org/pipermail/python-dev/2014-July/135435.html

Sigh.

This is a historic artifact, a convenience, and a side effect of bring symlinks into UNIX in the first place.

The objective was that symlinks should largely be transparent to users for naive operation. So the UNIX calls open/cd/listdir all follow symlinks so that things work transparently and a million C programs do not break.

However, so do chmod/chgrp/chown, for the same reasons and with generally less desirable effects.

Conversely, the find command, for example, does not follow symlinks and this is generally a good thing. "ls" is the same. Like os.walk, they are for inspecting stuff, and shouldn't indirect unless asked.

I think following symlinks, especially for something like os.walk and os.scandir, should default to False. I DO NOT want to quietly wander to remote parts of the file space because someone has stuck a symlink somewhere unfortunate, lurking like a little bomb (or perhaps trapdoor, waiting to suck me down into an unexpected dark place).

It is also slower to follow symlinks by default.

I am also against flag parameters that default to True, on the whole; they are a failure of ergonomic design. Leaving off a flag should usually be like setting it to False. A missing flag is an "off" flag.

For these reasons (and others I have not yet thought through:-) I am voting for a:

followlinks=False

optional parameter.

If you want to follow links, it is hardly difficult.

Cheers, Cameron Simpson <cs at zip.com.au>

Our job is to make the questions so painful that the only way to make the pain go away is by thinking. - Fred Friendly

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