[Python-Dev] __file__ is not always an absolute path

[exarkun at twistedmatrix.com]

On 10:29 pm, guido at python.org wrote:
>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/@test_1234_cwd/).
>>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 "the_package = __import__(abstest,
>>globals(), locals(), [])" in runtest_inner (in regrtest.py), and a 
>>"print
>>__import__('test').__file__" there returns 'test/__init__.pyc'.
>>This can be reproduced quite easily:
>[snip]
>
>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.)

Assuming you mean os.getcwd():

exarkun at boson:~$ python -m timeit -s 'def f(): pass' 'f()'
10000000 loops, best of 3: 0.132 usec per loop
exarkun at boson:~$ python -m timeit -s 'from os import getcwd' 'getcwd()'
1000000 loops, best of 3: 1.02 usec per loop
exarkun at boson:~$
So it's about 7x more expensive than a no-op function call.  I'd call 
this pretty quick.  Compared to everything else that happens during an 
import, I'm not convinced this wouldn't be lost in the noise.  I think 
it's at least worth implementing and measuring.

Jean-Paul