[3.9] Fix 'gather' rules in the python parser generator (GH-22021) (GH-22080)

Currently, empty sequences in gather rules make the conditional for
gather rules fail as empty sequences evaluate as "False". We need to
explicitly check for "None" (the failure condition) to avoid false
negatives.
(cherry picked from commit e55a0e9)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

2 files changed, 18 insertions, 1 deletions

diff --git a/Lib/test/test_peg_generator/test_pegen.py b/Lib/test/test_peg_generator/test_pegen.py
index 5b4e964..bcfee3f 100644
--- a/Lib/test/test_peg_generator/test_pegen.py
+++ b/Lib/test/test_peg_generator/test_pegen.py
@@ -74,7 +74,7 @@ class TestPegen(unittest.TestCase):
             "Rule('term', 'int', Rhs([Alt([NamedItem(None, NameLeaf('NUMBER'))])]))"
         )
 
-    def test_repeat_with_separator_rules(self) -> None:
+    def test_gather(self) -> None:
         grammar = """
         start: ','.thing+ NEWLINE
         thing: NUMBER
@@ -85,6 +85,20 @@ class TestPegen(unittest.TestCase):
             "Rule('start', None, Rhs([Alt([NamedItem(None, Gather(StringLeaf(\"','\"), NameLeaf('thing'"
         ))
         self.assertEqual(str(rules["thing"]), "thing: NUMBER")
+        parser_class = make_parser(grammar)
+        node = parse_string("42\n", parser_class)
+        assert node == [
+            [[TokenInfo(NUMBER, string="42", start=(1, 0), end=(1, 2), line="42\n")]],
+            TokenInfo(NEWLINE, string="\n", start=(1, 2), end=(1, 3), line="42\n"),
+        ]
+        node = parse_string("1, 2\n", parser_class)
+        assert node == [
+            [
+                [TokenInfo(NUMBER, string="1", start=(1, 0), end=(1, 1), line="1, 2\n")],
+                [TokenInfo(NUMBER, string="2", start=(1, 3), end=(1, 4), line="1, 2\n")],
+            ],
+            TokenInfo(NEWLINE, string="\n", start=(1, 4), end=(1, 5), line="1, 2\n"),
+        ]
 
     def test_expr_grammar(self) -> None:
         grammar = """
diff --git a/Tools/peg_generator/pegen/python_generator.py b/Tools/peg_generator/pegen/python_generator.py
index 45a7597..b786de7 100644
--- a/Tools/peg_generator/pegen/python_generator.py
+++ b/Tools/peg_generator/pegen/python_generator.py
@@ -217,6 +217,9 @@ class PythonParserGenerator(ParserGenerator, GrammarVisitor):
                     else:
                         self.print("and")
                     self.visit(item)
+                    if is_gather:
+                        self.print("is not None")
+
             self.print("):")
             with self.indent():
                 action = node.action