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

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>
python · pablogsal · Sep 3, 2020 · Sep 3, 2020 · Sep 3, 2020 · a0eb9e0d4677b65191dfaf4eb99c2923d4882d65
commit a0eb9e0d4677b65191dfaf4eb99c2923d4882d65
@@ -74,7 +74,7 @@ def test_typed_rules(self) -> None:
            "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 @@ def test_repeat_with_separator_rules(self) -> None:
            "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 = """

@@ -217,6 +217,9 @@ def visit_Alt(self, node: Alt, is_loop: bool, is_gather: bool) -> None:
                    else:
                        self.print("and")
                    self.visit(item)
+                    if is_gather:
+                        self.print("is not None")
+
            self.print("):")
            with self.indent():
                action = node.action