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

Co-authored-by: Pablo Galindo <[email protected]>
(cherry picked from commit 55460ee)

Co-authored-by: Batuhan Taskaya <[email protected]>

Author: miss-islington

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: