Index: Lib/test/test_subprocess.py
===================================================================
--- Lib/test/test_subprocess.py	(révision 81227)
+++ Lib/test/test_subprocess.py	(copie de travail)
@@ -825,7 +825,14 @@
             stdout = stdout.rstrip(b'\n\r')
             self.assertEquals(stdout.decode('ascii'), repr(value))
 
+    def test_canonicalize_env(self):
+        env = os.environ.copy()
+        env['TEST'] = '1'
+        env[b'TEST'] = b'2'
+        self.assertRaises(ValueError,
+            subprocess.call, [sys.executable], env=env)
 
+
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):
 
Index: Lib/subprocess.py
===================================================================
--- Lib/subprocess.py	(révision 81227)
+++ Lib/subprocess.py	(copie de travail)
@@ -1077,6 +1077,17 @@
             if executable is None:
                 executable = args[0]
 
+            if env is not None:
+                envb = dict()
+                for key, value in env.items():
+                    key = os.fsencode(key)
+                    if key in envb:
+                        raise ValueError(
+                            "env contains two variables with the same "
+                            "name ({!r}) but different types".format(key))
+                    envb[key] = os.fsencode(value)
+                env = envb
+
             # For transferring possible exec failure from child to parent.
             # Data format: "exception name:hex errno:description"
             # Pickle is not used; it is complex and involves memory allocation.
@@ -1092,8 +1103,7 @@
                         # and pass it to fork_exec()
 
                         if env:
-                            env_list = [os.fsencode(k) + b'=' + os.fsencode(v)
-                                        for k, v in env.items()]
+                            env_list = [(k + b'=' + v) for k, v in env.items()]
                         else:
                             env_list = None  # Use execv instead of execve.
                         if os.path.dirname(executable):