diff -r 021f3b32e11f Doc/whatsnew/3.7.rst
--- a/Doc/whatsnew/3.7.rst	Sun Nov 20 20:37:08 2016 +0200
+++ b/Doc/whatsnew/3.7.rst	Sun Nov 20 21:47:39 2016 +0200
@@ -99,6 +99,9 @@ Build and C API Changes
   of libffi is now required when building ``_ctypes`` on such platforms.
   Contributed by Zachary Ware in :issue:`27979`.
 
+* Private variable :c:data:`_Py_PackageContext` is now of type ``const char *``
+  rather of ``char *``.  (Contributed by Serhiy Storchaka in :issue:`28748`.)
+
 
 Deprecated
 ==========
diff -r 021f3b32e11f Include/modsupport.h
--- a/Include/modsupport.h	Sun Nov 20 20:37:08 2016 +0200
+++ b/Include/modsupport.h	Sun Nov 20 21:47:39 2016 +0200
@@ -176,7 +176,7 @@ PyAPI_FUNC(PyObject *) PyModule_FromDefA
 #endif /* New in 3.5 */
 
 #ifndef Py_LIMITED_API
-PyAPI_DATA(char *) _Py_PackageContext;
+PyAPI_DATA(const char *) _Py_PackageContext;
 #endif
 
 #ifdef __cplusplus
diff -r 021f3b32e11f Objects/moduleobject.c
--- a/Objects/moduleobject.c	Sun Nov 20 20:37:08 2016 +0200
+++ b/Objects/moduleobject.c	Sun Nov 20 21:47:39 2016 +0200
@@ -188,7 +188,7 @@ PyModule_Create2(struct PyModuleDef* mod
        (if the name actually matches).
     */
     if (_Py_PackageContext != NULL) {
-        char *p = strrchr(_Py_PackageContext, '.');
+        const char *p = strrchr(_Py_PackageContext, '.');
         if (p != NULL && strcmp(module->m_name, p+1) == 0) {
             name = _Py_PackageContext;
             _Py_PackageContext = NULL;
diff -r 021f3b32e11f Python/importdl.c
--- a/Python/importdl.c	Sun Nov 20 20:37:08 2016 +0200
+++ b/Python/importdl.c	Sun Nov 20 21:47:39 2016 +0200
@@ -94,7 +94,7 @@ PyObject *
 #endif
     PyObject *name_unicode = NULL, *name = NULL, *path = NULL, *m = NULL;
     const char *name_buf, *hook_prefix;
-    char *oldcontext;
+    const char *oldcontext;
     dl_funcptr exportfunc;
     PyModuleDef *def;
     PyObject *(*p0)(void);
diff -r 021f3b32e11f Python/modsupport.c
--- a/Python/modsupport.c	Sun Nov 20 20:37:08 2016 +0200
+++ b/Python/modsupport.c	Sun Nov 20 21:47:39 2016 +0200
@@ -9,7 +9,7 @@ typedef double va_double;
 static PyObject *va_build_value(const char *, va_list, int);
 
 /* Package context -- the full module name for package imports */
-char *_Py_PackageContext = NULL;
+const char *_Py_PackageContext = NULL;
 
 /* Helper for mkvalue() to scan the length of a format */