[Python-Dev] file is not always an absolute path (original) (raw)

Guido van Rossum guido at python.org
Sat Feb 6 23:29:38 CET 2010

On Sat, Feb 6, 2010 at 12:49 PM, Ezio Melotti <ezio.melotti at gmail.com> wrote:

In #7712 I was trying to change regrtest to always run the tests in a temporary CWD (e.g. /tmp/@test1234cwd/). The patches attached to the issue add a context manager that changes the CWD, and it works fine when I run ./python -m test.regrtest from trunk/. However, when I try from trunk/Lib/ it fails with ImportErrors (note that the latest patch by Florent Xicluna already tries to workaround the problem). The traceback points to "thepackage = import(abstest, globals(), locals(), [])" in runtestinner (in regrtest.py), and a "print import('test').file" there returns 'test/init.pyc'. This can be reproduced quite easily:

trunk$ ./python Python 2.7a2+ (trunk:77941M, Feb  3 2010, 06:40:49) [GCC 4.4.1] on linux2 Type "help", "copyright", "credits" or "license" for more information.

import os, sys os.getcwd() '/home/wolf/dev/trunk' import test test.file  # absolute '/home/wolf/dev/trunk/Lib/test/init.pyc' os.chdir('/tmp') test.file '/home/wolf/dev/trunk/Lib/test/init.pyc' from test import testunicode  # works testunicode.file '/home/wolf/dev/trunk/Lib/test/testunicode.pyc'

[21]+  Stopped                 ./python trunk$ cd Lib/ trunk/Lib$ ../python Python 2.7a2+ (trunk:77941M, Feb  3 2010, 06:40:49) [GCC 4.4.1] on linux2 Type "help", "copyright", "credits" or "license" for more information. import os, sys os.getcwd() '/home/wolf/dev/trunk/Lib' import test test.file  # relative 'test/init.pyc' os.chdir('/tmp') from test import testunicode  # fails Traceback (most recent call last):  File "", line 1, in ImportError: cannot import name testunicode Is there a reason why in the second case test.file is relative?

I haven't tried to repro this particular example, but the reason is that we don't want to have to call getpwd() on every import nor do we want to have some kind of in-process variable to cache the current directory. (getpwd() is relatively slow and can sometimes fail outright, and trying to cache it has a certain risk of being wrong.)

What we do instead, is code in site.py that walks over the elements of sys.path and turns them into absolute paths. However this code runs before '' is inserted in the front of sys.path, so that the initial value of sys.path is ''.

You may want to print the value of sys.path at various points to see for yourself.

-- --Guido van Rossum (python.org/~guido)

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