gh-107155: Fix help() for lambda function with return annotation (GH-107401)

Author: Eclips4

Lib/pydoc.py

@@ -1144,7 +1144,8 @@ def docroutine(self, object, name=None, mod=None,
                 # XXX lambda's won't usually have func_annotations['return']
                 # since the syntax doesn't support but it is possible.
                 # So removing parentheses isn't truly safe.
-                argspec = argspec[1:-1] # remove parentheses
+                if not object.__annotations__:
+                    argspec = argspec[1:-1] # remove parentheses
         if not argspec:
             argspec = '(...)'
 
@@ -1586,7 +1587,8 @@ def docroutine(self, object, name=None, mod=None, cl=None, homecls=None):
                 # XXX lambda's won't usually have func_annotations['return']
                 # since the syntax doesn't support but it is possible.
                 # So removing parentheses isn't truly safe.
-                argspec = argspec[1:-1] # remove parentheses
+                if not object.__annotations__:
+                    argspec = argspec[1:-1]
         if not argspec:
             argspec = '(...)'
         decl = asyncqualifier + title + argspec + note

Lib/test/test_pydoc/test_pydoc.py

@@ -693,6 +693,30 @@ def test_help_output_redirect(self):
         finally:
             pydoc.getpager = getpager_old
 
+    def test_lambda_with_return_annotation(self):
+        func = lambda a, b, c: 1
+        func.__annotations__ = {"return": int}
+        with captured_output('stdout') as help_io:
+            pydoc.help(func)
+        helptext = help_io.getvalue()
+        self.assertIn("lambda (a, b, c) -> int", helptext)
+
+    def test_lambda_without_return_annotation(self):
+        func = lambda a, b, c: 1
+        func.__annotations__ = {"a": int, "b": int, "c": int}
+        with captured_output('stdout') as help_io:
+            pydoc.help(func)
+        helptext = help_io.getvalue()
+        self.assertIn("lambda (a: int, b: int, c: int)", helptext)
+
+    def test_lambda_with_return_and_params_annotation(self):
+        func = lambda a, b, c: 1
+        func.__annotations__ = {"a": int, "b": int, "c": int, "return": int}
+        with captured_output('stdout') as help_io:
+            pydoc.help(func)
+        helptext = help_io.getvalue()
+        self.assertIn("lambda (a: int, b: int, c: int) -> int", helptext)
+
     def test_namedtuple_fields(self):
         Person = namedtuple('Person', ['nickname', 'firstname'])
         with captured_stdout() as help_io:

Misc/NEWS.d/next/Library/2023-08-02-01-17-32.gh-issue-107155.Mj1K9L.rst

@@ -0,0 +1,3 @@
+Fix incorrect output of ``help(x)`` where ``x`` is a :keyword:`lambda`
+function, which has an ``__annotations__`` dictionary attribute with a
+``"return"`` key.