Revert PEP-489 behaviour, but keep error raising code to make it easy to try both.

Cython/Compiler/ModuleNode.py

@@ -2326,14 +2326,19 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         code.put_declare_refcount_context()
         code.putln("#if CYTHON_PEP489_MULTI_PHASE_INIT")
         # Hack: enforce single initialisation.
-        # 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
-        code.putln("if (%s) {" % Naming.module_cname)
-        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("if (%s && %s == %s) return 0;" % (
+            Naming.module_cname,
+            Naming.module_cname,
+            Naming.pymodinit_module_arg,
+        ))
+        # # 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
+        # code.putln("if (%s) {" % Naming.module_cname)
+        # 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).
         code.putln("if (%s) return __Pyx_NewRef(%s);" % (

Cython/Utility/ModuleSetupCode.c

@@ -178,7 +178,9 @@
     #define CYTHON_FAST_PYCALL 1
   #endif
   #ifndef CYTHON_PEP489_MULTI_PHASE_INIT
-    #define CYTHON_PEP489_MULTI_PHASE_INIT (PY_VERSION_HEX >= 0x03050000)
+    // 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)
   #endif
   #ifndef CYTHON_USE_TP_FINALIZE
     #define CYTHON_USE_TP_FINALIZE (PY_VERSION_HEX >= 0x030400a1)