diff -r 20f18e5e8bfc Doc/library/string.rst
--- a/Doc/library/string.rst	Sat Jan 07 00:08:29 2017 +0100
+++ b/Doc/library/string.rst	Sat Jan 07 13:22:47 2017 +0200
@@ -95,9 +95,9 @@ implementation as the built-in :meth:`~s
       an arbitrary set of positional and keyword arguments.
       It is just a wrapper that calls :meth:`vformat`.
 
-      .. deprecated:: 3.5
-         Passing a format string as keyword argument *format_string* has been
-         deprecated.
+      .. versionchanged:: 3.7
+         A format string is :ref:`positional-only parameter
+         <positional-only_parameter>`.
 
    .. method:: vformat(format_string, args, kwargs)
 
diff -r 20f18e5e8bfc Lib/string.py
--- a/Lib/string.py	Sat Jan 07 00:08:29 2017 +0100
+++ b/Lib/string.py	Sat Jan 07 13:22:47 2017 +0200
@@ -175,14 +175,8 @@ class Formatter:
         try:
             format_string, *args = args # allow the "format_string" keyword be passed
         except ValueError:
-            if 'format_string' in kwargs:
-                format_string = kwargs.pop('format_string')
-                import warnings
-                warnings.warn("Passing 'format_string' as keyword argument is "
-                              "deprecated", DeprecationWarning, stacklevel=2)
-            else:
-                raise TypeError("format() missing 1 required positional "
-                                "argument: 'format_string'") from None
+            raise TypeError("format() missing 1 required positional "
+                            "argument: 'format_string'") from None
         return self.vformat(format_string, args, kwargs)
 
     def vformat(self, format_string, args, kwargs):
diff -r 20f18e5e8bfc Lib/test/test_string.py
--- a/Lib/test/test_string.py	Sat Jan 07 00:08:29 2017 +0100
+++ b/Lib/test/test_string.py	Sat Jan 07 13:22:47 2017 +0200
@@ -48,9 +48,8 @@ class ModuleTest(unittest.TestCase):
         self.assertEqual(fmt.format("-{format_string}-", format_string='test'),
                          '-test-')
         self.assertRaises(KeyError, fmt.format, "-{format_string}-")
-        with self.assertWarnsRegex(DeprecationWarning, "format_string"):
-            self.assertEqual(fmt.format(arg='test', format_string="-{arg}-"),
-                             '-test-')
+        with self.assertRaisesRegex(TypeError, "format_string"):
+            fmt.format(arg='test', format_string="-{arg}-")
 
     def test_auto_numbering(self):
         fmt = string.Formatter()