Interesting. I didn't know that. The .py file is always installed automatically next to the .so file by distutils.

Here's what strace gives me:

Python 2.7:

stat("my_test_package", {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
stat("my_test_package/__init__.py", {st_mode=S_IFREG|0664, st_size=557, ...}) = 0
stat("my_test_package/__init__", 0x7fffdc9dd320) = -1 ENOENT (No such file or directory)
open("my_test_package/__init__.so", O_RDONLY) = 3
open("my_test_package/__init__.so", O_RDONLY|O_CLOEXEC) = 4

Python 3.2:

stat("my_test_package", {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
stat("my_test_package/__init__.py", {st_mode=S_IFREG|0664, st_size=557, ...}) = 0
stat("my_test_package/__init__", 0x7fff9d99d700) = -1 ENOENT (No such file or directory)
stat("my_test_package/__init__.cpython-32mu.so", {st_mode=S_IFREG|0775, st_size=82517, ...}) = 0
open("my_test_package/__init__.cpython-32mu.so", O_RDONLY) = 3
open("my_test_package/__init__.cpython-32mu.so", O_RDONLY|O_CLOEXEC) = 4

Python 3.3:

stat("./my_test_package", {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
stat("./my_test_package/__init__.cpython-33dm.so", {st_mode=S_IFREG|0775, st_size=36119, ...}) = 0
open("./my_test_package/__init__.cpython-33dm.so", O_RDONLY|O_CLOEXEC) = 3

That's the difference then.

Ok, I think we'll have to emulate this for older CPython versions as well for the case that the .py file is not there. So it's likely best to let Cython register the package in sys.modules at init time, right after calling AddModule().

Still - can this be implemented in CPython for 3.3? Or 3.4, given that it might appear like a new feature? There shouldn't be all that much to change.