[C++-sig] [Boost.Python] Importing twice causes error in MSVSwith Boost.Python extension?

[English, Mark]

> -----Original Message-----
> Behalf Of David Abrahams
> Sent: 20 August 2007 15:33
> Subject: Re: [C++-sig] [Boost.Python] Importing twice causes error in
> MSVSwith Boost.Python extension?
> 
> on Wed Aug 15 2007, Lawrence Spector 
> <Lawrence.Spector-AT-CanfieldSci.com> wrote:
> 
> > In one case I use “PyImport_ImportModule”.  Later on, I do 
> “PyRun_String”.  Either way, same
> > deal.  I import the module and the first time it succeeds.  
> That means the same type converters are being registered twice.
> Python shouldn't let you actually load the same module twice (i.e. run
> its init function) unless you do Py_Finalize(), which isn't supported
> by Boost.Python.
FWIW confusingly it is possible to import the same module file twice using different relative/absolute paths.
This can be avoided by always using absolute import paths.
In a sense though it's not the same module due to the different import paths so six of one == half dozen other.

For example (not actually tried verbatim but should make the idea clear despite potential typos)
assuming there isn't a mod01 or sub01 anywhere on PYTHONPATH:
md sub01
touch sub01/__init__.py
echo print 'Importing mod01' > sub01/mod01.py
python
>>> import sys, os
>>> sys.path.append(os.path.join(os.getcwd(), 'sub01'))
>>> os.chdir('sub01')
>>> import mod01
Importing mod01
>>> import mod01 #<--- Note nothing happens here as expected
>>> import sub01.mod01 
Importing mod01 #<--- Note second import
>>> import sub01.mod01 #<-- Nothing again

This happens at least on Windows 2000 with Python 2.3.6 up to Python 2.5.1,
and I'd expect that at least on all other Windows platforms.

If you're uncertain what's importing what, it's also sometimes helpful
to run python with the -v flag which shows what imports are occurring.

Hope that helps,
Mark
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