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.

1 files changed, 3 insertions, 0 deletions

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