Consequences of importing the same module multiple times in C++?

[Robert Dailey]

Hi,

I'm currently using boost::python:: import() to import Python modules,
so I'm not sure exactly which Python API function it is calling to
import these files. I posted to the Boost.Python mailing list with
this question and they said I'd probably get a better answer here, so
here it goes...

If I do the following:

using namespace boost::python;
import( "__main__" ).attr( "new_global " ) = 40.0f;
import( "__main__" ).attr( "another_global " ) = 100.0f:

Notice that I'm importing twice. What would be the performance
consequences of this? Do both import operations query the disk for the
module and load it into memory? Will the second call simply reference
a cached version of the module loaded at the first import() call?

Thanks.

[Lie Ryan]

Re: Consequences of importing the same module multiple times in C++?

On Fri, 24 Oct 2008 12:23:18 -0700, Robert Dailey wrote:

Hi,
>
I'm currently using boost::python:: import() to import Python modules, so
I'm not sure exactly which Python API function it is calling to import
these files. I posted to the Boost.Python mailing list with this
question and they said I'd probably get a better answer here, so here it
goes...
>
If I do the following:
>
using namespace boost::python;
import( "__main__" ).attr( "new_global " ) = 40.0f; import( "__main__"
).attr( "another_global " ) = 100.0f:
>
Notice that I'm importing twice. What would be the performance
consequences of this? Do both import operations query the disk for the
module and load it into memory? Will the second call simply reference a
cached version of the module loaded at the first import() call?
>
Thanks.

I think it does not reload the module. Running python with verbose mode:

blah@blah-laptop:~$ python -v
(snip)

>>import xml

import xml # directory /usr/local/lib/python2.6/xml
# /usr/local/lib/python2.6/xml/__init__.pyc matches /usr/local/lib/
python2.6/xml/__init__.py
import xml # precompiled from /usr/local/lib/python2.6/xml/__init__.pyc

>>import xml
>>>

It's also mentioned in the docs: (paraphrased to clarify the points)
'''
The system maintains a table of modules that have been ...
initialized.... When a module name is found..., step (1) is finished. If
not, a search for a module ... . When ... found, it is loaded.
'''

http://www.python.org/doc/2.5.2/ref/import.html

[Aaron Brady]

Re: Consequences of importing the same module multiple times in C++?

On Oct 24, 2:23 pm, Robert Dailey <rcdai...@gmail .comwrote:

Hi,
>
I'm currently using boost::python:: import() to import Python modules,
so I'm not sure exactly which Python API function it is calling to
import these files. I posted to the Boost.Python mailing list with
this question and they said I'd probably get a better answer here, so
here it goes...
>
If I do the following:
>
using namespace boost::python;
import( "__main__" ).attr( "new_global " ) = 40.0f;
import( "__main__" ).attr( "another_global " ) = 100.0f:
>
Notice that I'm importing twice. What would be the performance
consequences of this? Do both import operations query the disk for the
module and load it into memory? Will the second call simply reference
a cached version of the module loaded at the first import() call?
>
Thanks.

Docs:

Note
For efficiency reasons, each module is only imported once per
interpreter session. Therefore, if you change your modules, you must
restart the interpreter – or, if it’s just one module you want to test
interactively, use reload(), e.g. reload(modulena me).