fix inspect.formatargspec on functions with keyword-only arguments without defaults #4959

Lib/inspect.py

@@ -894,7 +894,7 @@ def formatargspec(args, varargs=None, varkw=None, defaults=None,
     if kwonlyargs:
         for kwonlyarg in kwonlyargs:
             spec = formatargandannotation(kwonlyarg)
-            if kwonlyarg in kwonlydefaults:
+            if kwonlydefaults and kwonlyarg in kwonlydefaults:
                 spec += formatvalue(kwonlydefaults[kwonlyarg])
             specs.append(spec)
     if varkw is not None:

Lib/test/inspect_fodder2.py

@@ -105,3 +105,7 @@ def keyworded(*arg1, arg2=1):
 #line 105
 def annotated(arg1: list):
     pass
+
+#line 109
+def keyword_only_arg(*, arg):
+    pass

Lib/test/test_inspect.py

@@ -386,6 +386,9 @@ class TestClassesAndFunctions(unittest.TestCase):
 
         self.assertRaises(ValueError, self.assertArgSpecEquals,
                           mod2.annotated, [])
+        self.assertRaises(ValueError, self.assertArgSpecEquals,
+                          mod2.keyword_only_arg, [])
+
 
     def test_getfullargspec(self):
         self.assertFullArgSpecEquals(mod2.keyworded, [], varargs_e='arg1',
@@ -396,6 +399,10 @@ class TestClassesAndFunctions(unittest.TestCase):
         self.assertFullArgSpecEquals(mod2.annotated, ['arg1'],
                                      ann_e={'arg1' : list},
                                      formatted='(arg1: list)')
+        self.assertFullArgSpecEquals(mod2.keyword_only_arg, [],
+                                     kwonlyargs_e=['arg'],
+                                     formatted='(*, arg)')
+
 
     def test_getargspec_method(self):
         class A(object):

Misc/NEWS

@@ -126,6 +126,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #4959: inspect.formatargspec now works for keyword only arguments
+  without defaults.
+
 - Issue #3826 and #4791: The socket module now closes the underlying socket
   appropriately when it is being used via socket.makefile() objects
   rather than delaying the close by waiting for garbage collection to do it.