gh-122575: gh-142349: fix sys.flags tuple size (it unintentionally increased) (GH-145988)

the lazy imports PEP initial implementation (3.15 alpha) inadvertently incremented the length of the sys.flags tuple. In a way that did not do anything useful or related to the lazy imports setting (it exposed sys.flags.gil in the tuple). This fixes that to hard code the length to the 3.13 & 3.14 released length of 18 and have our tests and code comments make it clear that we've since stopped making new sys.flags attributes available via sequence index.

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