[C++-sig] Inconsistency with Boost.Python code

Stefan Seefeld seefeld at sympatico.ca
Thu Oct 9 19:06:50 CEST 2008

Hi Robert,

I believe the reason for this behavior is that an implicit cast from 
object to dict and list will cause a copy, so you don't actually insert 
the path into the original (global) path object, but a local copy, which 
obviously isn't seen by the rest of the application.

Robert wrote:
> Hi,
> Suppose the following C++ Boost.Python code below. Note that I am 
> embedding the python interpreter to execute python scripts from C++:
>     using namespace boost::python;
>     object imported( import( "sys" ) );
>     dict sysdict( imported.attr( "__dict__" ) );

The call to 'attr()' returns an object, and you assign it to a dict. 
This causes a copy. Try this instead:

object dict_object = imported.attr("__dict__");
dict sysdict = extract<dict>(dict_object);

>     list syspath( sysdict["path"] );

And likewise here:

object list_object = sysdict["path"];
list syspath = extract<list>(list_object);

>     syspath.append( "C:\testing" );

However, as you have discovered, there are ways to fold these lines to 
make it more compact. I only spelled the individual steps out to 
illustrate what's going on underneath.



      ...ich hab' noch einen Koffer in Berlin...

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