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