The Python module import logic has changed to stop obfuscating real ImportError problems.

Only one import of the low-level C/C++ module from the pure Python module is
attempted now. Previously a second import of the low-level C/C++ module was attempted
after an ImportError occurred and was done to support 'split modules'. A 'split module' is
a configuration where the pure Python module is a module within a Python package and the
low-level C/C++ module is a global Python module. Now a 'split module' configuration is
no longer supported by default. This configuration can be supported with a simple
customization, such as:

  %module(package="mypackage", moduleimport="import $module") foo

or if using -builtin:

  %module(package="mypackage", moduleimport="from $module import *") foo

instead of

  %module(package="mypackage") foo

See the updated Python chapter titled "Location of modules" in the documentation.

Closes #848 #1343

Examples/python/import_packages/split_modules/README

@@ -2,6 +2,9 @@
 between two packages.  Specifically the pure python part is part of a package
 and the C/C++ part is not in any package at all.  Historically SWIG has
 supported this sort of thing.
+From SWIG 4.0.0 onwards, split modules are not supported by default.
+The %module directive needs to be customised with the moduleimport attribute
+in order to import the a global C/C++ module.
 
 vanilla        # "plane Jane" module both halves in pkg1
 vanilla_split  # python 1/2 in pkg1 C 1/2 in global namespace

Examples/python/import_packages/split_modules/vanilla_split/foo.i

@@ -1,4 +1,9 @@
-%module(package="pkg1") foo
+#if defined(SWIGPYTHON_BUILTIN) /* defined when using -builtin */
+%module(package="pkg1", moduleimport="from $module import *") foo
+#else
+%module(package="pkg1", moduleimport="import $module") foo
+#endif
+
 %{
 static unsigned count(void)
 {