Enable PEP-489 multi-phase module initialisation, now that re-imports are handled with an error.

CHANGES.rst

@@ -8,6 +8,9 @@ Cython Changelog
 Features added
 --------------
 
+* PEP-489 multi-phase module initialisation has been enabled again.  Module
+  reloads raise an exception to prevent corruption of the static module state.
+
 * Raising exceptions from nogil code will automatically acquire the GIL, instead
   of requiring an explicit ``with gil`` block.
 

Cython/Utility/ModuleSetupCode.c

@@ -178,9 +178,7 @@
     #define CYTHON_FAST_PYCALL 1
   #endif
   #ifndef CYTHON_PEP489_MULTI_PHASE_INIT
-    // Disabled for now.  Most extension modules simply can't deal with it, and Cython isn't ready either.
-    // See issues listed here: https://docs.python.org/3/c-api/init.html#sub-interpreter-support
-    #define CYTHON_PEP489_MULTI_PHASE_INIT (0 && PY_VERSION_HEX >= 0x03050000)
+    #define CYTHON_PEP489_MULTI_PHASE_INIT (PY_VERSION_HEX >= 0x03050000)
   #endif
   #ifndef CYTHON_USE_TP_FINALIZE
     #define CYTHON_USE_TP_FINALIZE (PY_VERSION_HEX >= 0x030400a1)

tests/run/reimport_from_package.srctree

@@ -33,9 +33,8 @@ import atest.package.module as module
 assert module in sys.modules.values(), list(sys.modules)
 assert sys.modules['atest.package.module'] is module, list(sys.modules)
 
-# TODO: re-enable with PEP-489 support
-#if sys.version_info >= (3, 5):
-#    from . import pymodule
+if sys.version_info >= (3, 5):
+    from . import pymodule
 
 ######## atest/package/pymodule.py ########