Regression for no attribute '__reduce_cython__' in Cython 0.29

Platform: Windows 10 Python Version: 3.6.7

When running a program built with PyInstaller, cdef class types built with Cython 0.29 or later fail during extension module initialization with AttributeError: type object '<type>' has no attribute '__reduce_cython__'.

I can confirm that the fix in Cython 0.28.3-2.8.6 still works.

The program works without issue when run from Python instead of the PyInstaller bundle.

[carljm]

I have a pretty simple pure-Python and Cython repro of this problem. Using Cython 0.29.13, create this file cytest.pyx:

import doesnotexist

cdef class Foo:
    pass

Then after compiling it, run:

try:
    import cytest
except ImportError:
    pass

import cytest

Expected behavior is to see an ImportError from the attempt to import a nonexistent module. Actual behavior is that we see the __reduce_cython__ AttributeError instead, hiding the real error.

This is not just a toy example, it was simplified from a real-world Django application. When importing your INSTALLED_APPS, Django has code to catch ImportError and try to import again, treating the final component of the dotted path as an attribute instead of a submodule. We had an ImportError in unrelated code several layers deeper in the import stack from our Cython module, and Cython (in combination with Django’s re-importing behavior) replaced the ImportError with the __reduce_cython__ AttributeError on the second try, totally hiding the real ImportError and making it very difficult to debug. (Note that the failing import doesn’t have to be directly in the Cython module, as in the simplest-case above, it can be anywhere deeper in the import stack.)

[scoder]

Closing as duplicate of #3545.