gh-145059: Record lazy modules without submodules in sys.lazy_modules (#146081)

Record simple lazy modules as well

Lib/test/test_lazy_import/__init__.py

@@ -969,6 +969,21 @@ class SysLazyModulesTrackingTests(unittest.TestCase):
         # Basic test that sys.lazy_modules exists and is a dict
         self.assertIsInstance(sys.lazy_modules, dict)
 
+    def test_lazy_module_without_children_is_tracked(self):
+        code = textwrap.dedent("""
+            import sys
+            lazy import json
+            assert "json" in sys.lazy_modules, (
+                f"expected 'json' in sys.lazy_modules, got {set(sys.lazy_modules)}"
+            )
+            assert sys.lazy_modules["json"] == set(), (
+                f"expected empty set for sys.lazy_modules['json'], "
+                f"got {sys.lazy_modules['json']!r}"
+            )
+            print("OK")
+        """)
+        assert_python_ok("-c", code)
+
 
 @support.requires_subprocess()
 class CommandLineAndEnvVarTests(unittest.TestCase):

Misc/NEWS.d/next/Core_and_Builtins/2026-03-17-14-20-56.gh-issue-145059.aB3xKm.rst

@@ -0,0 +1 @@
+Fixed ``sys.lazy_modules`` to include lazy modules without submodules. Patch by Bartosz Sławecki.

Python/import.c

@@ -4377,6 +4377,12 @@ register_lazy_on_parent(PyThreadState *tstate, PyObject *name,
         Py_ssize_t dot = PyUnicode_FindChar(name, '.', 0,
                                             PyUnicode_GET_LENGTH(name), -1);
         if (dot < 0) {
+            PyObject *lazy_submodules = ensure_lazy_submodules(
+                (PyDictObject *)lazy_modules, name);
+            if (lazy_submodules == NULL) {
+                goto done;
+            }
+            Py_DECREF(lazy_submodules);
             ret = 0;
             goto done;
         }