Actually enable the error on PEP-489 module reinitialisation, and add it to the changelog.

CHANGES.rst

@@ -57,6 +57,10 @@ Bugs fixed
   exception stack traces.
   Patch by Jeroen Demeyer.  (Github issue #2492)
 
+* When PEP-489 support is enabled, reloading the module overwrote any static
+  module state. It now raises an exception instead, given that reloading is
+  not actually supported.
+
 Other changes
 -------------
 

Cython/Build/IpythonMagic.py

@@ -166,11 +166,15 @@ class CythonMagics(Magics):
             f.write(cell)
         if 'pyximport' not in sys.modules or not self._pyximport_installed:
             import pyximport
-            pyximport.install(reload_support=True)
+            pyximport.install()
             self._pyximport_installed = True
         if module_name in self._reloads:
             module = self._reloads[module_name]
-            reload(module)
+            # Note: reloading extension modules is not actually supported
+            # (requires PEP-489 reinitialisation support).
+            # Don't know why this should ever have worked as it reads here.
+            # All we really need to do is to update the globals below.
+            #reload(module)
         else:
             __import__(module_name)
             module = sys.modules[module_name]

Cython/Compiler/ModuleNode.py

@@ -2333,11 +2333,10 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             Naming.module_cname,
             Naming.pymodinit_module_arg,
         ))
-        # TODO: We should raise an exception here, as long as Cython cannot actually support reinitialisation.
-        # code.putln('PyErr_SetString(PyExc_RuntimeError,'
-        #            ' "Module \'%s\' has already been imported. Re-initialisation is not supported");' %
-        #            env.module_name)
-        # code.putln("return -1;")
+        code.putln('PyErr_SetString(PyExc_RuntimeError,'
+                   ' "Module \'%s\' has already been imported. Re-initialisation is not supported.");' %
+                   env.module_name)
+        code.putln("return -1;")
         code.putln("}")
         code.putln("#elif PY_MAJOR_VERSION >= 3")
         # Hack: enforce single initialisation also on reimports under different names on Python 3 (with PEP 3121/489).