importing yields unexpected results when initial script is a symbolic link

[jbeulich]

BPO	6386
Nosy	@brettcannon, @ncoghlan, @matejcik, @bitdancer, @ericsnowcurrently
Files	tutorial-symlink-syspath-note.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/brettcannon'
closed_at = <Date 2014-02-06.14:24:12.426>
created_at = <Date 2009-06-30.10:05:43.446>
labels = ['type-feature', 'docs']
title = 'importing yields unexpected results when initial script is a symbolic link'
updated_at = <Date 2014-02-06.14:24:12.425>
user = 'https://bugs.python.org/jbeulich'

bugs.python.org fields:

activity = <Date 2014-02-06.14:24:12.425>
actor = 'brett.cannon'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2014-02-06.14:24:12.426>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2009-06-30.10:05:43.446>
creator = 'jbeulich'
dependencies = []
files = ['30822']
hgrepos = []
issue_num = 6386
keywords = ['patch']
message_count = 13.0
messages = ['89914', '187280', '187281', '190145', '190146', '190158', '190167', '190171', '192456', '192476', '192514', '210384', '210385']
nosy_count = 10.0
nosy_names = ['brett.cannon', 'ncoghlan', 'matejcik', 'Arfrever', 'r.david.murray', 'jbeulich', 'python-dev', 'eric.snow', 'jamadagni', 'senko']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'needs patch'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue6386'
versions = ['Python 2.7', 'Python 3.3', 'Python 3.4']

[jbeulich]

Due to the way PySys_SetArgv() works, when the initial script is a
symbolic link, importing from normal files in the same directory does
not work. This is particularly surprising when the work tree is a
symlinked clone (cp -s) of an original (i.e. snapshot) tree with a few
modifications (patches) applied to one or more of the modules imported
from: Due the the erratum, the modifications made will appear to not
take effect until one realizes that the wrong module is being imported from.

The solution would in my opinion be to not only add the path left after
the readlink()/realpath() processing to the import search path list, but
also any intermediately encountered ones (in the order processed) as
long as the leaf component continues to be a symlink.

(Note: It seems pointless to use readlink() in the current [3.1 and
earlier] implementation, since the result gets passed to realpath()
anyway. Also, the documentation doesn't seem to mention this behavior,
and from the sources it remains unclear why this processing is needed at
all.)

[BreamoreBoy]

It will be difficult to take this forward owing to the problem description in msg89914 being limited to "importing from normal files in the same directory does not work". Plus any problems back then may well have been fixed due to the reworking of the import mechanism by somebody who apparently has spent 10 years as a Python core developer. Congratulations :)

[brettcannon]

In case someone wants to reproduce:

mkdir pkg
echo "import tester" > pkg/symlinked.py
ln -s pkg/symlinked.py linked.py
echo "print('HIT')" > tester.py

That fails because Python assumes you are in the pkg directory, not the directory you started execution. This makes sense to me. If you used a hard link then this isn't a problem. Python treats a symlink as a redirect, which means it works where the redirect tells it to and doesn't try to confuse things by considering 2 different locations to be the cwd for imports.

Closing as "won't fix" since I think it would be more confusing to support both a symlink directory and the cwd.

[jamadagni]

I'm sorry but I don't get why this is a WONTFIX. I reported what is (now) apparently a dup: bpo-18067. Just like the OP of this bug, I feel that in doing testing and such, one would naturally symlink and expect the library in the *current* directory to be imported.

And about the CWD, I have demonstrated in bpo-18067 how the CWD is in fact reported to be the directory of the *source* of the symlink (i.e. the dir containing the symlink inode) and not the *target* of the symlink. This is precisely what is frustrating about this bug: the fact that Python does not import something from a directory which it reports to be the current directory as per os.getcwd().

While I myself lack the internal CPython code knowledge to fix this, I can't imagine this would be too difficult to fix, given that os.getcwd() already reports the correct current directory -- in setting up the import path list, you just have to use that i.o. whatever else you are using now.

Thanks.

[bitdancer]

When a script is executed by python, it does *not* import from the CWD, it imports from the *location of the script*. From this, then, you can see that there are two possible interpretations of "the location of the script" when the script is a symlink, and we have chosen the more secure (and practical) one: the location of the real script file.

[ncoghlan]

The current behaviour is also needed to sanely support Python scripts
symlinked from Linux /bin directories.

[jamadagni]

The current behaviour is also needed to sanely support Python
scripts symlinked from Linux /bin directories.

OK that clinched it for me -- I can't argue against that! And obviously it is not meaningful to copy/symlink *all* the current-directory modules a particular script depends upon to the symlink directory as well. And searching both directories (containing the source and target of the symlink) is not good for security I guess.

And I also checked the contents of sys.path with my test case -- and sure enough the directory corresponding to the actual output was printed. Just that sys.path is different from os.getcwd() needs some effort to bring into mind.

So I think someone should please clearly mention this behaviour in the documentation under http://docs.python.org/3/tutorial/modules.html#the-module-search-path and the Py2 equivalent. Specifically this point needs clarification:

""the directory containing the input script (or the current directory).""

It would be best to remove the text in parantheses (which immediately makes one think of os.getcwd()) and add a clarification like:

Note that on filesystems that support symlinks, this means the directory containing the actual script file. Symlinks to the script may be present elsewhere and may be used to invoke the script, but the directories containing those symlinks will *not* be searched for dependency modules.

Thank you very much for these clarifications and for your work on Python! Please do add the above documentation clarification, though.

[ncoghlan]

That's fair - reopening this as a docs bug.