gh-122575: gh-142349: fix sys.flags tuple size (it unintentionally increased)

Author: gpshead

Lib/test/test_sys.py

@@ -858,24 +858,35 @@ def test_subinterp_intern_singleton(self):
                     '''))
                 self.assertTrue(sys._is_interned(s))
 
-    def test_sys_flags(self):
+    def test_sys_flags_indexable_attributes(self):
         self.assertTrue(sys.flags)
-        attrs = ("debug",
+        # We've stopped assigning sequence indices to new sys.flags attributes:
+        # https://github.com/python/cpython/issues/122575#issuecomment-2416497086
+        indexable_attrs = ("debug",
                  "inspect", "interactive", "optimize",
                  "dont_write_bytecode", "no_user_site", "no_site",
                  "ignore_environment", "verbose", "bytes_warning", "quiet",
                  "hash_randomization", "isolated", "dev_mode", "utf8_mode",
-                 "warn_default_encoding", "safe_path", "int_max_str_digits",
-                 "lazy_imports")
-        for attr in attrs:
+                 "warn_default_encoding", "safe_path", "int_max_str_digits")
+        for attr_idx, attr in enumerate(indexable_attrs):
             self.assertHasAttr(sys.flags, attr)
             attr_type = bool if attr in ("dev_mode", "safe_path") else int
             self.assertEqual(type(getattr(sys.flags, attr)), attr_type, attr)
+            attr_value = getattr(sys.flags, attr)
+            self.assertEqual(sys.flags[attr_idx], attr_value,
+                             msg=f"sys.flags .{attr} vs [{attr_idx}]")
         self.assertTrue(repr(sys.flags))
-        self.assertEqual(len(sys.flags), len(attrs))
+        self.assertEqual(len(sys.flags), 18, msg="Do not increase, see GH-122575")
 
         self.assertIn(sys.flags.utf8_mode, {0, 1, 2})
 
+    def test_sys_flags_name_only_attributes(self):
+        # non-tuple sequence fields (name only sys.flags attributes)
+        self.assertIsInstance(sys.flags.gil, int|type(None))
+        self.assertIsInstance(sys.flags.thread_inherit_context, int|type(None))
+        self.assertIsInstance(sys.flags.context_aware_warnings, int|type(None))
+        self.assertIsInstance(sys.flags.lazy_imports, int|type(None))
+
     def assert_raise_on_new_sys_type(self, sys_attr):
         # Users are intentionally prevented from creating new instances of
         # sys.flags, sys.version_info, and sys.getwindowsversion.
@@ -1908,10 +1919,16 @@ def test_pythontypes(self):
         # symtable entry
         # XXX
         # sys.flags
-        # FIXME: The +3 is for the 'gil', 'thread_inherit_context' and
-        # 'context_aware_warnings' flags and will not be necessary once
-        # gh-122575 is fixed
-        check(sys.flags, vsize('') + self.P + self.P * (3 + len(sys.flags)))
+        # FIXME: The non_sequence_fields adjustment is for these flags:
+        # - 'gil'
+        # - 'thread_inherit_context'
+        # - 'context_aware_warnings'
+        # - 'lazy_imports'
+        # Not needing to increment this every time we add a new field
+        # per GH-122575 would be nice...
+        # Q: What is the actual point of this sys.flags C size derived from PyStructSequence_Field array assertion?
+        non_sequence_fields = 4
+        check(sys.flags, vsize('') + self.P + self.P * (non_sequence_fields + len(sys.flags)))
 
     def test_asyncgen_hooks(self):
         old = sys.get_asyncgen_hooks()

Python/sysmodule.c

@@ -3495,11 +3495,12 @@ static PyStructSequence_Field flags_fields[] = {
     {"dev_mode",                "-X dev"},
     {"utf8_mode",               "-X utf8"},
     {"warn_default_encoding",   "-X warn_default_encoding"},
-    {"safe_path", "-P"},
+    {"safe_path",               "-P"},
     {"int_max_str_digits",      "-X int_max_str_digits"},
+    // Fields below are only usable by sys.flags attribute name, not index:
     {"gil",                     "-X gil"},
     {"thread_inherit_context",  "-X thread_inherit_context"},
-    {"context_aware_warnings",    "-X context_aware_warnings"},
+    {"context_aware_warnings",  "-X context_aware_warnings"},
     {"lazy_imports",            "-X lazy_imports"},
     {0}
 };
@@ -3510,7 +3511,9 @@ static PyStructSequence_Desc flags_desc = {
     "sys.flags",        /* name */
     flags__doc__,       /* doc */
     flags_fields,       /* fields */
-    19
+    18  /* NB - do not increase beyond 3.13's value of 18. */
+    // New sys.flags fields should NOT be tuple addressable per
+    // https://github.com/python/cpython/issues/122575#issuecomment-2416497086
 };
 
 static void