Set the __module__ of UnsupportedOperation even if io is not imported

python · JelleZijlstra · Apr 28, 2025 · Apr 25, 2025 · Apr 26, 2025 · Apr 26, 2025
commit c3cfa671f1ff319e01e6bf07d18ac419ff846aa4
diff --git a/Include/internal/pycore_global_objects_fini_generated.h b/Include/internal/pycore_global_objects_fini_generated.h
diff --git a/Include/internal/pycore_global_strings.h b/Include/internal/pycore_global_strings.h
@@ -514,6 +514,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(intern)
         STRUCT_FOR_ID(intersection)
         STRUCT_FOR_ID(interval)
+        STRUCT_FOR_ID(io)
         STRUCT_FOR_ID(is_running)
         STRUCT_FOR_ID(is_struct)
         STRUCT_FOR_ID(isatty)

diff --git a/Include/internal/pycore_runtime_init_generated.h b/Include/internal/pycore_runtime_init_generated.h
diff --git a/Include/internal/pycore_unicodeobject_generated.h b/Include/internal/pycore_unicodeobject_generated.h
diff --git a/Lib/io.py b/Lib/io.py
@@ -60,9 +60,6 @@
                  IncrementalNewlineDecoder, text_encoding, TextIOWrapper)
 
 
-# Pretend this exception was created here.
-UnsupportedOperation.__module__ = "io"
-
 # for seek()
 SEEK_SET = 0
 SEEK_CUR = 1

diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
@@ -445,6 +445,25 @@ def test_invalid_operations(self):
             self.assertRaises(exc, fp.seek, 1, self.SEEK_CUR)
             self.assertRaises(exc, fp.seek, -1, self.SEEK_END)
 
+    @support.cpython_only
+    def test_startup_optimization(self):
+        # gh-132952: Test that `io` is not imported at startup and that the
+        # __module__ of UnsupportedOperation is set to "io".
+        assert_python_ok("-S", "-c", textwrap.dedent(
+            """
+            import sys
+            assert "io" not in sys.modules
+            try:
+                sys.stdin.truncate()
+            except Exception as e:
+                typ = type(e)
+                assert typ.__module__ == "io", (typ, typ.__module__)
+                assert typ.__name__ == "UnsupportedOperation", (typ, typ.__name__)
+            else:
+                raise AssertionError("Expected UnsupportedOperation")
+            """
+        ))
+
     @unittest.skipUnless(hasattr(os, "pipe"), "requires os.pipe()")
     def test_optional_abilities(self):
         # Test for OSError when optional APIs are not supported

diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
@@ -661,6 +661,11 @@ iomodule_exec(PyObject *m)
         "UnsupportedOperation", PyExc_OSError, PyExc_ValueError);
     if (state->unsupported_operation == NULL)
         return -1;
+    if (PyObject_SetAttrString(state->unsupported_operation,
+                               "__module__", &_Py_ID(io)) < 0)
+    {
+        return -1;
+    }
     if (PyModule_AddObjectRef(m, "UnsupportedOperation",
                               state->unsupported_operation) < 0)
     {