os: fsencode(), fsdecode() and os.environ(b) internal encode-decode methods

keep a local copy of the fileystem encoding, instead of calling
sys.getfilesystemencoding() each time.

The filesystem encoding is now constant.

Lib/os.py

@@ -487,12 +487,13 @@ def _createenviron():
             data[encodekey(key)] = value
     else:
         # Where Env Var Names Can Be Mixed Case
+        encoding = sys.getfilesystemencoding()
         def encode(value):
             if not isinstance(value, str):
                 raise TypeError("str expected, not %s" % type(value).__name__)
-            return value.encode(sys.getfilesystemencoding(), 'surrogateescape')
+            return value.encode(encoding, 'surrogateescape')
         def decode(value):
-            return value.decode(sys.getfilesystemencoding(), 'surrogateescape')
+            return value.decode(encoding, 'surrogateescape')
         encodekey = encode
         data = environ
     return _Environ(data,
@@ -535,39 +536,43 @@ if supports_bytes_environ:
 
     __all__.extend(("environb", "getenvb"))
 
-def fsencode(filename):
-    """
-    Encode filename to the filesystem encoding with 'surrogateescape' error
-    handler, return bytes unchanged. On Windows, use 'strict' error handler if
-    the file system encoding is 'mbcs' (which is the default encoding).
-    """
-    if isinstance(filename, bytes):
-        return filename
-    elif isinstance(filename, str):
-        encoding = sys.getfilesystemencoding()
-        if encoding == 'mbcs':
-            return filename.encode(encoding)
-        else:
-            return filename.encode(encoding, 'surrogateescape')
+def _fscodec():
+    encoding = sys.getfilesystemencoding()
+    if encoding == 'mbcs':
+        errors = None   # strict
     else:
-        raise TypeError("expect bytes or str, not %s" % type(filename).__name__)
+        errors = 'surrogateescape'
 
-def fsdecode(filename):
-    """
-    Decode filename from the filesystem encoding with 'surrogateescape' error
-    handler, return str unchanged. On Windows, use 'strict' error handler if
-    the file system encoding is 'mbcs' (which is the default encoding).
-    """
-    if isinstance(filename, str):
-        return filename
-    elif isinstance(filename, bytes):
-        encoding = sys.getfilesystemencoding()
-        if encoding == 'mbcs':
-            return filename.decode(encoding)
+    def fsencode(filename):
+        """
+        Encode filename to the filesystem encoding with 'surrogateescape' error
+        handler, return bytes unchanged. On Windows, use 'strict' error handler if
+        the file system encoding is 'mbcs' (which is the default encoding).
+        """
+        if isinstance(filename, bytes):
+            return filename
+        elif isinstance(filename, str):
+            return filename.encode(encoding, errors)
         else:
-            return filename.decode(encoding, 'surrogateescape')
-    else:
-        raise TypeError("expect bytes or str, not %s" % type(filename).__name__)
+            raise TypeError("expect bytes or str, not %s" % type(filename).__name__)
+
+    def fsdecode(filename):
+        """
+        Decode filename from the filesystem encoding with 'surrogateescape' error
+        handler, return str unchanged. On Windows, use 'strict' error handler if
+        the file system encoding is 'mbcs' (which is the default encoding).
+        """
+        if isinstance(filename, str):
+            return filename
+        elif isinstance(filename, bytes):
+            return filename.decode(encoding, errors)
+        else:
+            raise TypeError("expect bytes or str, not %s" % type(filename).__name__)
+
+    return fsencode, fsdecode
+
+fsencode, fsdecode = _fscodec()
+del _fscodec
 
 def _exists(name):
     return name in globals()