bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)

miss-islington · corona10 · web-flow · commit 6ceab46a602c · 2018-07-10T01:00:35.000-07:00
(cherry picked from commit 445f1b3) Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>
diff --git a/Lib/functools.py b/Lib/functools.py
@@ -800,8 +800,13 @@ def register(cls, func=None):
         return func
 
     def wrapper(*args, **kw):
+        if not args:
+            raise TypeError(f'{funcname} requires at least '
+                            '1 positional argument')
+
         return dispatch(args[0].__class__)(*args, **kw)
 
+    funcname = getattr(func, '__name__', 'singledispatch function')
     registry[object] = func
     wrapper.register = register
     wrapper.dispatch = dispatch
diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
@@ -2078,6 +2078,13 @@ class X:
         self.assertEqual(len(td), 0)
         functools.WeakKeyDictionary = _orig_wkd
 
+    def test_invalid_positional_argument(self):
+        @functools.singledispatch
+        def f(*args):
+            pass
+        msg = 'f requires at least 1 positional argument'
+        with self.assertRaisesRegexp(TypeError, msg):
+            f()
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2018-07-08-18-49-41.bpo-33967.lhaAez.rst b/Misc/NEWS.d/next/Library/2018-07-08-18-49-41.bpo-33967.lhaAez.rst
@@ -0,0 +1,2 @@
+functools.singledispatch now raises TypeError instead of IndexError when no
+positional arguments are passed.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+functools.singledispatch now raises TypeError instead of IndexError when no`
	`2`	`+positional arguments are passed.`