The imported .sip file is parsed. %ExportedHeaderCode and %ExportedDoc sections are extracted. %HeaderCode, %C++Code, %ExposeFunction, %Copying, %PrePyCode, %PyCode, %Doc and %Makefile blocks are ignored.
The wrapping python file imports the dynamic lib libimport_module_namec (as .pyd or .dll) before libmodule_namec.
This does not mean that the imported lib is available as a wrapped module as well. To achieve this, you have to import import_module_name.py too.