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.

2 files changed, 15 insertions, 8 deletions

diff --git a/Lib/test/test_peg_parser.py b/Lib/test/test_peg_parser.py
index ea4afa6..43d30ba 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 e1ec36e..59b80f9 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;
     }