Issue #9548: Add a minimal "_bootlocale" module that is imported by the _io module instead of the full locale module.

Lib/_bootlocale.py

@@ -0,0 +1,34 @@
+"""A minimal subset of the locale module used at interpreter startup
+(imported by the _io module), in order to reduce startup time.
+
+Don't import directly from third-party code; use the `locale` module instead!
+"""
+
+import sys
+import _locale
+
+if sys.platform.startswith("win"):
+    def getpreferredencoding(do_setlocale=True):
+        return _locale._getdefaultlocale()[1]
+else:
+    try:
+        _locale.CODESET
+    except ImportError:
+        def getpreferredencoding(do_setlocale=True):
+            # This path for legacy systems needs the more complex
+            # getdefaultlocale() function, import the full locale module.
+            import locale
+            return locale.getpreferredencoding(do_setlocale)
+    else:
+        def getpreferredencoding(do_setlocale=True):
+            assert not do_setlocale
+            result = _locale.nl_langinfo(_locale.CODESET)
+            if not result and sys.platform == 'darwin':
+                # nl_langinfo can return an empty string
+                # when the setting has an invalid value.
+                # Default to UTF-8 in that case because
+                # UTF-8 is the default charset on OSX and
+                # returning nothing will crash the
+                # interpreter.
+                result = 'UTF-8'
+            return result

Lib/locale.py

@@ -554,8 +554,8 @@ if sys.platform.startswith("win"):
     # On Win32, this will return the ANSI code page
     def getpreferredencoding(do_setlocale = True):
         """Return the charset that the user is likely using."""
-        import _locale
+        import _bootlocale
-        return _locale._getdefaultlocale()[1]
+        return _bootlocale.getpreferredencoding(False)
 else:
     # On Unix, if CODESET is available, use that.
     try:
@@ -574,27 +574,16 @@ else:
         def getpreferredencoding(do_setlocale = True):
             """Return the charset that the user is likely using,
             according to the system configuration."""
+            import _bootlocale
             if do_setlocale:
                 oldloc = setlocale(LC_CTYPE)
                 try:
                     setlocale(LC_CTYPE, "")
                 except Error:
                     pass
-                result = nl_langinfo(CODESET)
+            result = _bootlocale.getpreferredencoding(False)
-                if not result and sys.platform == 'darwin':
+            if do_setlocale:
-                    # nl_langinfo can return an empty string
-                    # when the setting has an invalid value.
-                    # Default to UTF-8 in that case because
-                    # UTF-8 is the default charset on OSX and
-                    # returning nothing will crash the
-                    # interpreter.
-                    result = 'UTF-8'
                 setlocale(LC_CTYPE, oldloc)
-            else:
-                result = nl_langinfo(CODESET)
-                if not result and sys.platform == 'darwin':
-                    # See above for explanation
-                    result = 'UTF-8'
             return result
 
 

Lib/site.py

@@ -426,8 +426,8 @@ def aliasmbcs():
     while they are always available as "mbcs" in each locale. Make
     them usable by aliasing to "mbcs" in such a case."""
     if sys.platform == 'win32':
-        import locale, codecs
+        import _bootlocale, codecs
-        enc = locale.getdefaultlocale()[1]
+        enc = _bootlocale.getpreferredencoding(False)
         if enc.startswith('cp'):            # "cp***" ?
             try:
                 codecs.lookup(enc)

Lib/test/test_subprocess.py

@@ -883,8 +883,9 @@ class ProcessTestCase(BaseTestCase):
         #
         # UTF-16 and UTF-32-BE are sufficient to check both with BOM and
         # without, and UTF-16 and UTF-32.
+        import _bootlocale
         for encoding in ['utf-16', 'utf-32-be']:
-            old_getpreferredencoding = locale.getpreferredencoding
+            old_getpreferredencoding = _bootlocale.getpreferredencoding
             # Indirectly via io.TextIOWrapper, Popen() defaults to
             # locale.getpreferredencoding(False) and earlier in Python 3.2 to
             # locale.getpreferredencoding().
@@ -895,7 +896,7 @@ class ProcessTestCase(BaseTestCase):
                     encoding)
             args = [sys.executable, '-c', code]
             try:
-                locale.getpreferredencoding = getpreferredencoding
+                _bootlocale.getpreferredencoding = getpreferredencoding
                 # We set stdin to be non-None because, as of this writing,
                 # a different code path is used when the number of pipes is
                 # zero or one.
@@ -904,7 +905,7 @@ class ProcessTestCase(BaseTestCase):
                                          stdout=subprocess.PIPE)
                 stdout, stderr = popen.communicate(input='')
             finally:
-                locale.getpreferredencoding = old_getpreferredencoding
+                _bootlocale.getpreferredencoding = old_getpreferredencoding
             self.assertEqual(stdout, '1\n2\n3\n4')
 
     def test_no_leaking(self):

Misc/NEWS

@@ -36,6 +36,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #9548: Add a minimal "_bootlocale" module that is imported by the
+  _io module instead of the full locale module.
+
 - Issue #18764: remove the 'print' alias for the PDB 'p' command so that it no
   longer shadows the print function.
 

Modules/_io/_iomodule.c

@@ -546,7 +546,7 @@ _PyIO_get_locale_module(_PyIO_State *state)
         }
         Py_CLEAR(state->locale_module);
     }
-    mod = PyImport_ImportModule("locale");
+    mod = PyImport_ImportModule("_bootlocale");
     if (mod == NULL)
         return NULL;
     state->locale_module = PyWeakref_NewRef(mod, NULL);