bpo-41044: Generate valid PEG python parsers for opt+seq rules (GH-20995)

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

Author: isidentical

Lib/test/test_peg_generator/test_pegen.py

@@ -493,6 +493,14 @@ def test_start_leader(self) -> None:
  # Would assert False without a special case in compute_left_recursives().
  make_parser(grammar)
 
+ def test_opt_sequence(self) -> None:
+ grammar = """
+ start: [NAME*]
+ """
+ # This case was failing because of a double trailing comma at the end
+ # of a line in the generated source. See bpo-41044
+ make_parser(grammar)
+
  def test_left_recursion_too_complex(self) -> None:
  grammar = """
  start: foo

Tools/peg_generator/pegen/python_generator.py

@@ -93,7 +93,13 @@ def visit_NegativeLookahead(self, node: NegativeLookahead) -> Tuple[None, str]:
 
  def visit_Opt(self, node: Opt) -> Tuple[str, str]:
  name, call = self.visit(node.node)
- return "opt", f"{call}," # Note trailing comma!
+ # Note trailing comma (the call may already have one comma
+ # at the end, for example when rules have both repeat0 and optional
+ # markers, e.g: [rule*])
+ if call.endswith(","):
+ return "opt", call
+ else:
+ return "opt", f"{call},"
 
  def visit_Repeat0(self, node: Repeat0) -> Tuple[str, str]:
  if node in self.cache: