python · ericsnowcurrently · Oct 7, 2016 · Oct 10, 2016 · Oct 7, 2016 · Sep 4, 2017
diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
@@ -485,6 +485,9 @@ Changes in the Python API
   and module are affected by this change. (Contributed by INADA Naoki and
   Eugene Toder in :issue:`29463`.)
 
+* ``PyInterpreterState`` no longer has a ``modules`` field.  Instead use
+  ``sys.modules``.
+
 * The *mode* argument of :func:`os.makedirs` no longer affects the file
   permission bits of newly-created intermediate-level directories.
   To set their file permission bits you can set the umask before invoking

@@ -53,7 +53,6 @@ typedef struct _is {
 
     int64_t id;
 
-    PyObject *modules;
     PyObject *modules_by_index;
     PyObject *sysdict;
     PyObject *builtins;

diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-04-10-46-09.bpo-28411.IU9rQL.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-04-10-46-09.bpo-28411.IU9rQL.rst
@@ -0,0 +1,4 @@
+``PyInterpreterState`` has a "modules" field that is copied into
+``sys.modules`` during interpreter startup.  This causes problems if a
+program replaces ``sys.modules`` with something else.  To solve this we
+eliminate ``PyInterpreterState.modules``.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
@@ -1684,7 +1684,6 @@ whichmodule(PyObject *global, PyObject *dotted_path)
     Py_ssize_t i;
     PyObject *modules;
     _Py_IDENTIFIER(__module__);
-    _Py_IDENTIFIER(modules);
     _Py_IDENTIFIER(__main__);
 
     module_name = _PyObject_GetAttrId(global, &PyId___module__);
@@ -1705,7 +1704,7 @@ whichmodule(PyObject *global, PyObject *dotted_path)
     assert(module_name == NULL);
 
     /* Fallback on walking sys.modules */
-    modules = _PySys_GetObjectId(&PyId_modules);
+    modules = PyImport_GetModuleDict();
     if (modules == NULL) {
         PyErr_SetString(PyExc_RuntimeError, "unable to get sys.modules");
         return NULL;

@@ -290,11 +290,17 @@ _PyImport_Fini(void)
 PyObject *
 PyImport_GetModuleDict(void)
 {
-    PyInterpreterState *interp = PyThreadState_GET()->interp;
-    if (interp->modules == NULL) {
-        Py_FatalError("PyImport_GetModuleDict: no module dictionary!");
+    PyObject *sysdict = PyThreadState_GET()->interp->sysdict;
+    if (sysdict == NULL) {
+        Py_FatalError("PyImport_GetModuleDict: no sys module!");
+    }
+
+    _Py_IDENTIFIER(modules);
+    PyObject *modules = _PyDict_GetItemId(sysdict, &PyId_modules);
+    if (modules == NULL) {
+        Py_FatalError("lost sys.modules");
     }
-    return interp->modules;
+    return modules;
 }
 
 /* In some corner cases it is important to be sure that the import
@@ -304,7 +310,11 @@ PyImport_GetModuleDict(void)
 int
 _PyImport_IsInitialized(PyInterpreterState *interp)
 {
-    if (interp->modules == NULL)
+    if (interp->sysdict == NULL)
+        return 0;
+    _Py_IDENTIFIER(modules);
+    PyObject *modules = _PyDict_GetItemId(interp->sysdict, &PyId_modules);
+    if (modules == NULL)
         return 0;
     return 1;
 }
@@ -543,7 +553,6 @@ PyImport_Cleanup(void)
     /* Clear and delete the modules directory.  Actual modules will
        still be there only if imported during the execution of some
        destructor. */
-    interp->modules = NULL;
     Py_DECREF(modules);
 
     /* Once more */

@@ -674,7 +674,6 @@ void _Py_InitializeCore(const _PyCoreConfig *config)
     PyObject *modules = PyDict_New();
     if (modules == NULL)
         Py_FatalError("Py_InitializeCore: can't make modules dictionary");
-    interp->modules = modules;
 
     sysmod = _PySys_BeginInit();
     if (sysmod == NULL)
@@ -1210,7 +1209,6 @@ Py_NewInterpreter(void)
     PyObject *modules = PyDict_New();
     if (modules == NULL)
         Py_FatalError("Py_NewInterpreter: can't make modules dictionary");
-    interp->modules = modules;
 
     sysmod = _PyImport_FindBuiltin("sys", modules);
     if (sysmod != NULL) {

@@ -90,7 +90,6 @@ PyInterpreterState_New(void)
                                  PyMem_RawMalloc(sizeof(PyInterpreterState));
 
     if (interp != NULL) {
-        interp->modules = NULL;
         interp->modules_by_index = NULL;
         interp->sysdict = NULL;
         interp->builtins = NULL;
@@ -154,7 +153,6 @@ PyInterpreterState_Clear(PyInterpreterState *interp)
     Py_CLEAR(interp->codec_search_path);
     Py_CLEAR(interp->codec_search_cache);
     Py_CLEAR(interp->codec_error_registry);
-    Py_CLEAR(interp->modules);
     Py_CLEAR(interp->modules_by_index);
     Py_CLEAR(interp->sysdict);
     Py_CLEAR(interp->builtins);