diff -r b110dd3d6cea Doc/library/inspect.rst
--- a/Doc/library/inspect.rst	Thu Jun 02 10:20:16 2016 -0700
+++ b/Doc/library/inspect.rst	Thu Jun 02 12:15:07 2016 -0700
@@ -623,7 +623,8 @@
    .. attribute:: Parameter.name

       The name of the parameter as a string.  The name must be a valid
-      Python identifier.
+      Python identifier or an implicit parameters like ``.0``. Parameters of
+      this form are generated for comprehensions.

    .. attribute:: Parameter.default

diff -r b110dd3d6cea Lib/inspect.py
--- a/Lib/inspect.py	Thu Jun 02 10:20:16 2016 -0700
+++ b/Lib/inspect.py	Thu Jun 02 12:15:07 2016 -0700
@@ -2396,7 +2396,8 @@
         if not isinstance(name, str):
             raise TypeError("name must be a str, not a {!r}".format(name))

-        if not name.isidentifier():
+        # handle implicit arguments to set comprehensions
+        if not name.isidentifier() and not (name[0] == '.' and name[1:].isdigit()):
             raise ValueError('{!r} is not a valid parameter name'.format(name))

         self._name = name
diff -r b110dd3d6cea Lib/test/test_inspect.py
--- a/Lib/test/test_inspect.py	Thu Jun 02 10:20:16 2016 -0700
+++ b/Lib/test/test_inspect.py	Thu Jun 02 12:15:07 2016 -0700
@@ -2903,6 +2903,10 @@
                                     'is not a valid parameter name'):
             inspect.Parameter('$', kind=inspect.Parameter.VAR_KEYWORD)

+        with self.assertRaisesRegex(ValueError,
+                                    'is not a valid parameter name'):
+            inspect.Parameter('.a', kind=inspect.Parameter.VAR_KEYWORD)
+
         with self.assertRaisesRegex(ValueError, 'cannot have default values'):
             inspect.Parameter('a', default=42,
                               kind=inspect.Parameter.VAR_KEYWORD)
@@ -2919,6 +2923,9 @@
         self.assertTrue(repr(p).startswith('<Parameter'))
         self.assertTrue('"a=42"' in repr(p))

+        implicit_param = inspect.Parameter('.0', kind=inspect.Parameter.POSITIONAL_ONLY)
+        self.assertEqual(implicit_param.name, '.0')
+
     def test_signature_parameter_hashable(self):
         P = inspect.Parameter
         foo = P('foo', kind=P.POSITIONAL_ONLY)
@@ -3234,6 +3241,16 @@
         ba = sig.bind(args=1)
         self.assertEqual(ba.arguments, {'kwargs': {'args': 1}})

+    def test_signature_bind_implicit_arg(self):
+        # Issue #19611: getcallargs should work with set comprehensions
+        def make_set():
+            return {z * z for z in range(5)}
+        setcomp_code = make_set.__code__.co_consts[1]
+        setcomp_func = types.FunctionType(setcomp_code, {})
+
+        iterator = iter(range(5))
+        self.assertEqual(self.call(setcomp_func, iterator), {0, 1, 4, 9, 16})
+

 class TestBoundArguments(unittest.TestCase):
     def test_signature_bound_arguments_unhashable(self):