gh-145076: Validate type inputs in __lazy_import__

Lib/test/test_import/test_lazy_imports.py

@@ -409,6 +409,21 @@ def test_dunder_lazy_import_builtins(self):
         self.assertNotIn("test.test_import.data.lazy_imports.basic2", sys.modules)
         self.assertEqual(dunder_lazy_import_builtins.basic.basic2, 42)
 
+    def test_dunder_lazy_import_argument_validation(self):
+        """__lazy_import__ should strictly validate argument types to avoid SystemError."""
+        invalid_type_scenarios = [
+            (123, {}, {}, [], 0, "argument 1 must be str"),
+            ('os', 1, {}, [], 0, "argument 2 must be dict"),
+            ('os', {}, "not_a_dict", [], 0, "argument 3 must be dict"),
+            ('os', {}, {}, 42, 0, "argument 4 must be a list or tuple"),
+            ('os', {}, {}, "string_instead_of_list", 0, "argument 4 must be a list or tuple"),
+        ]
+
+        for name, glbs, lcls, flist, lvl, msg in invalid_type_scenarios:
+            with self.subTest(case=msg):
+                with self.assertRaisesRegex(TypeError, msg):
+                    __lazy_import__(name, glbs, lcls, flist, lvl)
+
 
 class SysLazyImportsAPITests(unittest.TestCase):
     """Tests for sys lazy imports API functions."""

Misc/NEWS.d/next/Core_and_Builtins/2026-02-21-16-13-57.gh-issue-145076.N3fQ-Q.rst

@@ -0,0 +1,3 @@
+Fixed a :exc:`SystemError` in :func:`__lazy_import__` caused by passing
+invalid argument types. Added proper type validation for *name*, *globals*,
+*locals*, and *fromlist* arguments.

Python/bltinmodule.c

@@ -311,6 +311,37 @@ builtin___lazy_import___impl(PyObject *module, PyObject *name,
 {
     PyObject *builtins;
     PyThreadState *tstate = PyThreadState_GET();
+
+    // Validate input arguments
+    if (!PyUnicode_Check(name)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__lazy_import__() argument 1 must be str, not %.200s",
+                     Py_TYPE(name)->tp_name);
+        return NULL;
+    }
+
+    if (globals != NULL && !PyDict_Check(globals)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__lazy_import__() argument 2 must be dict, not %.200s",
+                     Py_TYPE(globals)->tp_name);
+        return NULL;
+    }
+
+    if (locals != NULL && !PyDict_Check(locals)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__lazy_import__() argument 3 must be dict, not %.200s",
+                     Py_TYPE(locals)->tp_name);
+        return NULL;
+    }
+
+    if (fromlist != NULL && fromlist != Py_None &&
+        !PyList_Check(fromlist) && !PyTuple_Check(fromlist)) {
+        PyErr_Format(PyExc_TypeError,
+                     "__lazy_import__() argument 4 must be a list or tuple, not %.200s",
+                     Py_TYPE(fromlist)->tp_name);
+        return NULL;
+    }
+
     if (globals == NULL) {
         globals = PyEval_GetGlobals();
     }