bpo-40334: PEP 617 implementation: New PEG parser for CPython (GH-19503)

Co-authored-by: Guido van Rossum <guido@python.org>
Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Tools/peg_generator/pegen/grammar_visualizer.py

@@ -0,0 +1,65 @@
+import argparse
+import sys
+
+from typing import Any, Iterator, Iterable, Callable
+
+from pegen.build import build_parser
+from pegen.grammar import Grammar, Rule
+
+argparser = argparse.ArgumentParser(
+    prog="pegen", description="Pretty print the AST for a given PEG grammar"
+)
+argparser.add_argument("filename", help="Grammar description")
+
+
+class ASTGrammarPrinter:
+    def children(self, node: Rule) -> Iterator[Any]:
+        for value in node:
+            if isinstance(value, list):
+                yield from value
+            else:
+                yield value
+
+    def name(self, node: Rule) -> str:
+        if not list(self.children(node)):
+            return repr(node)
+        return node.__class__.__name__
+
+    def print_grammar_ast(self, grammar: Grammar, printer: Callable[..., None] = print) -> None:
+        for rule in grammar.rules.values():
+            printer(self.print_nodes_recursively(rule))
+
+    def print_nodes_recursively(self, node: Rule, prefix: str = "", istail: bool = True) -> str:
+
+        children = list(self.children(node))
+        value = self.name(node)
+
+        line = prefix + ("└──" if istail else "├──") + value + "\n"
+        sufix = "   " if istail else "│  "
+
+        if not children:
+            return line
+
+        *children, last = children
+        for child in children:
+            line += self.print_nodes_recursively(child, prefix + sufix, False)
+        line += self.print_nodes_recursively(last, prefix + sufix, True)
+
+        return line
+
+
+def main() -> None:
+    args = argparser.parse_args()
+
+    try:
+        grammar, parser, tokenizer = build_parser(args.filename)
+    except Exception as err:
+        print("ERROR: Failed to parse grammar file", file=sys.stderr)
+        sys.exit(1)
+
+    visitor = ASTGrammarPrinter()
+    visitor.print_grammar_ast(grammar)
+
+
+if __name__ == "__main__":
+    main()