From 69e802ed812e38cb68a4ab74af64b4f719b6cc78 Mon Sep 17 00:00:00 2001
From: Lysandros Nikolaou <lisandrosnik@gmail.com>
Date: Thu, 30 Apr 2020 01:53:30 +0300
Subject: [PATCH] bpo-40334: Fix test_peg_parser to actually use the old parser
 (GH-19778)

Now that the default parser is the new PEG parser, ast.parse uses it, which means that we don't actually test something in test_peg_parser. This commit introduces a new keyword argument (`oldparser`) for `_peg_parser.parse_string` for specifying that a string needs to be parsed with the old parser. This keyword argument is used in the tests to actually compare the ASTs the new parser generates with those generated by the old parser.
---
 Lib/test/test_peg_parser.py |  9 ++++-----
 Modules/_peg_parser.c       | 14 +++++++++++---
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/Lib/test/test_peg_parser.py b/Lib/test/test_peg_parser.py
index ea4afa6e179..43d30bae849 100644
--- a/Lib/test/test_peg_parser.py
+++ b/Lib/test/test_peg_parser.py
@@ -699,7 +699,7 @@ class ASTGenerationTest(unittest.TestCase):
         self.maxDiff = None
         for source in TEST_SOURCES:
             actual_ast = peg_parser.parse_string(source)
-            expected_ast = ast.parse(source)
+            expected_ast = peg_parser.parse_string(source, oldparser=True)
             self.assertEqual(
                 ast.dump(actual_ast, include_attributes=True),
                 ast.dump(expected_ast, include_attributes=True),
@@ -721,12 +721,11 @@ class ASTGenerationTest(unittest.TestCase):
                 f"Actual error message does not match expexted for {source}"
             )
 
-    @support.skip_if_new_parser("This tests nothing for now, since compile uses pegen as well")
     @unittest.expectedFailure
     def test_correct_but_known_to_fail_ast_generation_on_source_files(self) -> None:
         for source in GOOD_BUT_FAIL_SOURCES:
             actual_ast = peg_parser.parse_string(source)
-            expected_ast = ast.parse(source)
+            expected_ast = peg_parser.parse_string(source, oldparser=True)
             self.assertEqual(
                 ast.dump(actual_ast, include_attributes=True),
                 ast.dump(expected_ast, include_attributes=True),
@@ -736,7 +735,7 @@ class ASTGenerationTest(unittest.TestCase):
     def test_correct_ast_generation_without_pos_info(self) -> None:
         for source in GOOD_BUT_FAIL_SOURCES:
             actual_ast = peg_parser.parse_string(source)
-            expected_ast = ast.parse(source)
+            expected_ast = peg_parser.parse_string(source, oldparser=True)
             self.assertEqual(
                 ast.dump(actual_ast),
                 ast.dump(expected_ast),
@@ -752,7 +751,7 @@ class ASTGenerationTest(unittest.TestCase):
     def test_correct_ast_generatrion_eval(self) -> None:
         for source in EXPRESSIONS_TEST_SOURCES:
             actual_ast = peg_parser.parse_string(source, mode='eval')
-            expected_ast = ast.parse(source, mode='eval')
+            expected_ast = peg_parser.parse_string(source, mode='eval', oldparser=True)
             self.assertEqual(
                 ast.dump(actual_ast, include_attributes=True),
                 ast.dump(expected_ast, include_attributes=True),
diff --git a/Modules/_peg_parser.c b/Modules/_peg_parser.c
index e1ec36e07bd..59b80f9e06e 100644
--- a/Modules/_peg_parser.c
+++ b/Modules/_peg_parser.c
@@ -45,11 +45,13 @@ error:
 PyObject *
 _Py_parse_string(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *keywords[] = {"string", "mode", NULL};
+    static char *keywords[] = {"string", "mode", "oldparser", NULL};
     char *the_string;
     char *mode_str = "exec";
+    int oldparser = 0;
 
-    if (!PyArg_ParseTupleAndKeywords(args, kwds, "s|s", keywords, &the_string, &mode_str)) {
+    if (!PyArg_ParseTupleAndKeywords(args, kwds, "s|sp", keywords,
+            &the_string, &mode_str, &oldparser)) {
         return NULL;
     }
 
@@ -77,7 +79,13 @@ _Py_parse_string(PyObject *self, PyObject *args, PyObject *kwds)
     PyCompilerFlags flags = _PyCompilerFlags_INIT;
     flags.cf_flags = PyCF_IGNORE_COOKIE;
 
-    mod_ty res = PyPegen_ASTFromString(the_string, mode, &flags, arena);
+    mod_ty res;
+    if (oldparser) {
+        res = PyParser_ASTFromString(the_string, "<string>", mode, &flags, arena);
+    }
+    else {
+        res = PyPegen_ASTFromString(the_string, mode, &flags, arena);
+    }
     if (res == NULL) {
         goto error;
     }