gh-132952: Import _io instead of io at startup

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)

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

Lib/site.py

@@ -73,7 +73,7 @@
 import os
 import builtins
 import _sitebuiltins
-import io
+import _io as io
 import stat
 
 # Prefixes for site-packages; add additional prefixes like /usr/local here

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

Lib/test/test_site.py

@@ -8,6 +8,7 @@
 import test.support
 from test import support
 from test.support.script_helper import assert_python_ok
+from test.support import import_helper
 from test.support import os_helper
 from test.support import socket_helper
 from test.support import captured_stderr
@@ -574,6 +575,17 @@ def test_license_exists_at_url(self):
             code = e.code
         self.assertEqual(code, 200, msg="Can't find " + url)
 
+    @support.cpython_only
+    def test_lazy_imports(self):
+        import_helper.ensure_lazy_imports("site", [
+            "io",
+            "locale",
+            "traceback",
+            "atexit",
+            "warnings",
+            "textwrap",
+        ])
+
 
 class StartupImportTests(unittest.TestCase):
 

Misc/NEWS.d/next/Core_and_Builtins/2025-04-26-08-49-05.gh-issue-132952.ifvP10.rst

@@ -0,0 +1,2 @@
+Speed up startup with the ``-S`` argument by about 19% by importing the
+private ``_io`` module instead of :mod:`io`.

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)
     {

Python/pylifecycle.c

@@ -2762,7 +2762,7 @@ init_set_builtins_open(void)
         goto error;
     }
 
-    if (!(wrapper = PyImport_ImportModuleAttrString("io", "open"))) {
+    if (!(wrapper = PyImport_ImportModuleAttrString("_io", "open"))) {
         goto error;
     }
 
@@ -2807,7 +2807,7 @@ init_sys_streams(PyThreadState *tstate)
     }
 #endif
 
-    if (!(iomod = PyImport_ImportModule("io"))) {
+    if (!(iomod = PyImport_ImportModule("_io"))) {
         goto error;
     }