gh-151408: Re-register submodules in sys.modules on lazy re-import

Include/internal/pycore_import.h

@@ -32,6 +32,12 @@ extern int _PyImport_FixupBuiltin(
     PyObject *modules
     );
 
+extern int _PyImport_EnsureSubmoduleRegistered(
+    PyThreadState *tstate,
+    PyObject *parent,
+    PyObject *name,
+    PyObject *submodule);
+
 extern PyObject * _PyImport_ResolveName(
     PyThreadState *tstate, PyObject *name, PyObject *globals, int level);
 extern PyObject * _PyImport_GetAbsName(

Lib/test/test_lazy_import/__init__.py

@@ -469,6 +469,44 @@ def test_lazy_import_pkg_cross_import(self):
         self.assertEqual(type(g["x"]), int)
         self.assertEqual(type(g["b"]), types.LazyImportType)
 
+    def test_lazy_reimport_after_sys_modules_delete(self):
+        """gh-151408: re-resolving a lazy import restores sys.modules."""
+        modname = "test.test_lazy_import.data.pkg.bar"
+        import test.test_lazy_import.data.pkg.bar
+        del sys.modules[modname]
+
+        exec(
+            "lazy from test.test_lazy_import.data.pkg import bar\nbar.f()",
+            globals(),
+        )
+        self.assertIn(modname, sys.modules)
+
+    def test_lazy_import_as_reimport_after_sys_modules_delete(self):
+        """gh-151408: lazy import with alias restores sys.modules."""
+        modname = "test.test_lazy_import.data.pkg.bar"
+        import test.test_lazy_import.data.pkg.bar
+        del sys.modules[modname]
+
+        ns = {}
+        exec(
+            "lazy import test.test_lazy_import.data.pkg.bar as bar\nbar.f()",
+            ns,
+        )
+        self.assertIn(modname, sys.modules)
+
+    def test_lazy_import_dotted_reimport_after_sys_modules_delete(self):
+        """gh-151408: dotted lazy import access restores sys.modules."""
+        modname = "test.test_lazy_import.data.pkg.bar"
+        import test.test_lazy_import.data.pkg.bar
+        del sys.modules[modname]
+
+        exec(
+            "lazy import test.test_lazy_import.data.pkg.bar\n"
+            "test.test_lazy_import.data.pkg.bar.f()",
+            globals(),
+        )
+        self.assertIn(modname, sys.modules)
+
     @support.requires_subprocess()
     def test_lazy_from_import_does_not_pollute_parent(self):
         """Lazy from import should not add the name to the parent module's dict."""

Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-21-15-00.gh-issue-151408.pR3kLm.rst

@@ -0,0 +1,2 @@
+Fix lazy import reification to restore submodules removed from
+:mod:`sys.modules` but still cached on the parent package.

Objects/moduleobject.c

@@ -1326,6 +1326,20 @@ try_load_lazy_submodule(PyModuleObject *m, PyObject *name)
     return result;
 }
 
+static PyObject *
+ensure_submodule_and_return(PyModuleObject *m, PyObject *name, PyObject *attr)
+{
+    if (attr != NULL && PyModule_Check(attr)) {
+        if (_PyImport_EnsureSubmoduleRegistered(
+                PyThreadState_GET(), (PyObject *)m, name, attr) < 0)
+        {
+            Py_DECREF(attr);
+            return NULL;
+        }
+    }
+    return attr;
+}
+
 PyObject*
 _Py_module_getattro_impl(PyModuleObject *m, PyObject *name, int suppress)
 {
@@ -1372,9 +1386,9 @@ _Py_module_getattro_impl(PyModuleObject *m, PyObject *name, int suppress)
                 Py_CLEAR(new_value);
             }
             Py_DECREF(attr);
-            return new_value;
+            return ensure_submodule_and_return(m, name, new_value);
         }
-        return attr;
+        return ensure_submodule_and_return(m, name, attr);
     }
     if (suppress == 1) {
         if (PyErr_Occurred()) {
@@ -1392,7 +1406,7 @@ _Py_module_getattro_impl(PyModuleObject *m, PyObject *name, int suppress)
     assert(m->md_dict != NULL);
     attr = try_load_lazy_submodule(m, name);
     if (attr != NULL) {
-        return attr;
+        return ensure_submodule_and_return(m, name, attr);
     }
     if (PyErr_Occurred()) {
         return NULL;

Python/ceval.c

@@ -3130,6 +3130,12 @@ _PyEval_ImportFrom(PyThreadState *tstate, PyObject *v, PyObject *name)
     PyObject *fullmodname, *mod_name, *origin, *mod_name_or_unknown, *errmsg, *spec;
 
     if (PyObject_GetOptionalAttr(v, name, &x) != 0) {
+        if (x != NULL &&
+            _PyImport_EnsureSubmoduleRegistered(tstate, v, name, x) < 0)
+        {
+            Py_DECREF(x);
+            return NULL;
+        }
         return x;
     }
     /* Issue #17636: in case this failed because of a circular relative
@@ -3311,6 +3317,13 @@ _PyEval_LazyImportFrom(PyThreadState *tstate, _PyInterpreterFrame *frame, PyObje
                     return NULL;
                 }
                 if (ret != NULL) {
+                    if (_PyImport_EnsureSubmoduleRegistered(
+                            tstate, mod, name, ret) < 0)
+                    {
+                        Py_DECREF(ret);
+                        Py_DECREF(mod);
+                        return NULL;
+                    }
                     Py_DECREF(mod);
                     return ret;
                 }

Python/import.c

@@ -255,6 +255,59 @@ _PyImport_SetModuleString(const char *name, PyObject *m)
     return PyMapping_SetItemString(modules, name, m);
 }
 
+int
+_PyImport_EnsureSubmoduleRegistered(PyThreadState *tstate, PyObject *parent,
+                                    PyObject *name, PyObject *submodule)
+{
+    if (!PyModule_Check(submodule) || !PyModule_Check(parent)) {
+        return 0;
+    }
+
+    PyObject *parent_name;
+    if (PyObject_GetOptionalAttr(parent, &_Py_ID(__name__), &parent_name) < 0) {
+        return -1;
+    }
+    if (parent_name == NULL || !PyUnicode_Check(parent_name)) {
+        Py_XDECREF(parent_name);
+        return 0;
+    }
+
+    PyObject *full_name = PyUnicode_FromFormat("%U.%U", parent_name, name);
+    Py_DECREF(parent_name);
+    if (full_name == NULL) {
+        return -1;
+    }
+
+    PyObject *sub_name;
+    if (PyObject_GetOptionalAttr(submodule, &_Py_ID(__name__), &sub_name) < 0) {
+        Py_DECREF(full_name);
+        return -1;
+    }
+    if (sub_name == NULL || !PyUnicode_Check(sub_name) ||
+        PyUnicode_Compare(sub_name, full_name) != 0)
+    {
+        Py_XDECREF(sub_name);
+        Py_DECREF(full_name);
+        return 0;
+    }
+    Py_DECREF(sub_name);
+
+    PyObject *existing = PyImport_GetModule(full_name);
+    if (existing != NULL) {
+        Py_DECREF(existing);
+        Py_DECREF(full_name);
+        return 0;
+    }
+    if (_PyErr_Occurred(tstate)) {
+        Py_DECREF(full_name);
+        return -1;
+    }
+
+    int res = _PyImport_SetModule(full_name, submodule);
+    Py_DECREF(full_name);
+    return res;
+}
+
 static PyObject *
 import_get_module(PyThreadState *tstate, PyObject *name)
 {