[Tutor] problem importing class

[Kent Johnson]

Danny Yoo wrote:
>>> This isn't the first time this kind of problem has hit people.  This
>>> problem is well known and is the subject of a Python Enhancement Proposal
>>> (PEP 328).  If you're using Python 2.5, you can use a new feature called
>>> 'absolute_import' to avoid the import problem.  See:
>>>
>>>      http://docs.python.org/whatsnew/pep-328.html
>>>
>>> for more details.
>> Hmm, I don't see how that would help since AFAICT shawn's site.py is not 
>> in a package. Or maybe I don't understand what you are suggesting?
> 
> Hi Kent,
> 
> I don't think I replied back to you on this one!  Python loads up 
> 'site.py' on startup if called without the '-S' option:
> 
>      http://docs.python.org/lib/module-site.html
> 
> So imagine if we're a program, and we run Python on it.  For some reason 
> --- at least on startup --- Python places precendence on the modules 
> located in Python's library directory.  So we pick up the Standard 
> Library's site.py, which is then cached in sys.modules like any other 
> module import.
> 
> Later on, if we try to 'import site', Python reuses the already-loaded 
> site.py (because it's already cached in sys.modules) , regardless if 
> there's a 'site.py' module in the current working directory.  The reason 
> PEP 328 can be a solution here is because, if the absolute_import feature 
> were on, we'd be able to do:
> 
>      from . import site
> 
> which would pick up the correct relative path.

This only works for code that is in a package, when you want to import a 
module in the same package. It doesn't work for code that is not in a 
package. For example,

F:\Tutor>cat site.py
print 'imported site.py'

F:\Tutor>cat siteimporter.py
from __future__ import absolute_import
from . import site

F:\Tutor>py siteimporter.py

F:\Tutor>C:\Python25\python siteimporter.py
Traceback (most recent call last):
   File "siteimporter.py", line 2, in <module>
     from . import site
ValueError: Attempted relative import in non-package

> 
> 
> Concretely, here's what probably happened with the original poster:
> 
> ##############################################################################
> mumak:~ dyoo$ echo "print 'hi'" > site.py
> mumak:~ dyoo$ python
> Python 2.3.5 (#1, Mar 20 2005, 20:38:20)
> [GCC 3.3 20030304 (Apple Computer, Inc. build 1809)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
>>>> import site
>>>> print site.__file__
> /System/Library/Frameworks/Python.framework/Versions/2.3/lib/python2.3/site.pyc
> ###############################################################################
> 
> Note that the 'site.py' in the current directory was not used.  I have no 
> idea why.  I suspect that this has some relation to the way that site.py 
> is handled by a special case in the Python interpreter, but I have really 
> no clue.

site.py is a special case just in being loaded automatically. Other than 
that this is standard behaviour. If you import a module that has already 
been imported, you will get a reference to the already-imported module, 
sys.path is not searched again. Since the library module site has 
already been imported when your program starts, 'import site' will 
always return a reference to the library module.

Kent