diff -r f256bd5b8418 Doc/howto/pyporting.rst
--- a/Doc/howto/pyporting.rst	Thu Sep 29 22:33:02 2016 +0200
+++ b/Doc/howto/pyporting.rst	Tue Oct 04 17:06:55 2016 -0700
@@ -298,12 +298,12 @@
     from __future__ import print_function
     from __future__ import unicode_literals
 
-You can also run Python 2 with the ``-3`` flag to be warned about various
+You can also run Python 2 with the ``-3`` flag (or by setting the environment
+variable ``PYTHON3WARNINGS`` to a non empty string) to be warned about various
 compatibility issues your code triggers during execution. If you turn warnings
 into errors with ``-Werror`` then you can make sure that you don't accidentally
 miss a warning.
 
-
 You can also use the Pylint_ project and its ``--py3k`` flag to lint your code
 to receive warnings when your code begins to deviate from Python 3
 compatibility. This also prevents you from having to run Modernize_ or Futurize_
diff -r f256bd5b8418 Doc/using/cmdline.rst
--- a/Doc/using/cmdline.rst	Thu Sep 29 22:33:02 2016 +0200
+++ b/Doc/using/cmdline.rst	Tue Oct 04 17:06:55 2016 -0700
@@ -405,6 +405,8 @@
    :exc:`DeprecationWarning` for features that are removed or significantly
    changed in Python 3.
 
+   See also :envvar:`PYTHON3WARNINGS`.
+
    .. versionadded:: 2.6
 
 Options you shouldn't use
@@ -612,6 +614,12 @@
    separated string, it is equivalent to specifying :option:`-W` multiple
    times.
 
+.. envvar:: PYTHON3WARNINGS
+
+   If this is set to a non-empty string it is equivalent to specifying the
+   :option:`-3` option.
+
+   .. versionadded:: 2.7.13
 
 .. envvar:: PYTHONHTTPSVERIFY
 
diff -r f256bd5b8418 Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Thu Sep 29 22:33:02 2016 +0200
+++ b/Lib/test/test_warnings.py	Tue Oct 04 17:06:55 2016 -0700
@@ -835,6 +835,24 @@
                 "['ignore::UnicodeWarning', 'ignore::DeprecationWarning']")
         self.assertEqual(p.wait(), 0)
 
+    def test_python3_warnings(self):
+        newenv = os.environ.copy()
+        newenv["PYTHON3WARNINGS"] = "1"
+        p = subprocess.Popen([sys.executable,
+                "-c", "import sys; sys.stdout.write(str(sys.py3kwarning))"],
+                stdout=subprocess.PIPE, env=newenv)
+        self.assertEqual(p.communicate()[0], "True")
+        self.assertEqual(p.wait(), 0)
+
+    def test_python3_warnings_inactive(self):
+        newenv = os.environ.copy()
+        newenv["PYTHON3WARNINGS"] = ""
+        p = subprocess.Popen([sys.executable,
+                "-c", "import sys; sys.stdout.write(str(sys.py3kwarning))"],
+                stdout=subprocess.PIPE, env=newenv)
+        self.assertEqual(p.communicate()[0], "False")
+        self.assertEqual(p.wait(), 0)
+
 class CEnvironmentVariableTests(EnvironmentVariableTests):
     module = c_warnings
 
diff -r f256bd5b8418 Misc/ACKS
--- a/Misc/ACKS	Thu Sep 29 22:33:02 2016 +0200
+++ b/Misc/ACKS	Tue Oct 04 17:06:55 2016 -0700
@@ -1564,3 +1564,4 @@
 Jelle Zijlstra
 Gennadiy Zlobin
 Peter Åstrand
+Roy Williams
diff -r f256bd5b8418 Misc/NEWS
--- a/Misc/NEWS	Thu Sep 29 22:33:02 2016 +0200
+++ b/Misc/NEWS	Tue Oct 04 17:06:55 2016 -0700
@@ -39,6 +39,10 @@
 - Issue #27514: Make having too many statically nested blocks a SyntaxError
   instead of SystemError.
 
+- Issue #28288: Allow Python 3 warnings to be enabled by setting the environment
+  variable PYTHON3WARNINGS to a non-emtpy string.
+
+
 Library
 -------
 
@@ -12092,4 +12096,3 @@
 ----
 
 **(For information about older versions, consult the HISTORY file.)**
-
diff -r f256bd5b8418 Modules/main.c
--- a/Modules/main.c	Thu Sep 29 22:33:02 2016 +0200
+++ b/Modules/main.c	Tue Oct 04 17:06:55 2016 -0700
@@ -328,8 +328,6 @@
 
         case '3':
             Py_Py3kWarningFlag++;
-            if (!Py_DivisionWarningFlag)
-                Py_DivisionWarningFlag = 1;
             break;
 
         case 'Q':
@@ -450,6 +448,12 @@
     if (Py_Py3kWarningFlag && !Py_TabcheckFlag)
         /* -3 implies -t (but not -tt) */
         Py_TabcheckFlag = 1;
+    if (!Py_Py3kWarningFlag &&
+        (p = Py_GETENV("PYTHON3WARNINGS")) && *p != '\0')
+        Py_Py3kWarningFlag++;
+
+    if (Py_Py3kWarningFlag && !Py_DivisionWarningFlag)
+      Py_DivisionWarningFlag = 1;
 
     if (!Py_InspectFlag &&
         (p = Py_GETENV("PYTHONINSPECT")) && *p != '\0')
@@ -701,4 +705,3 @@
 #ifdef __cplusplus
 }
 #endif
-