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,
@@ -535,7 +536,14 @@ if supports_bytes_environ:
 
     __all__.extend(("environb", "getenvb"))
 
-def fsencode(filename):
+def _fscodec():
+    encoding = sys.getfilesystemencoding()
+    if encoding == 'mbcs':
+        errors = None   # strict
+    else:
+        errors = 'surrogateescape'
+
+    def fsencode(filename):
         """
         Encode filename to the filesystem encoding with 'surrogateescape' error
         handler, return bytes unchanged. On Windows, use 'strict' error handler if
@@ -544,15 +552,11 @@ def fsencode(filename):
         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')
+            return filename.encode(encoding, errors)
         else:
             raise TypeError("expect bytes or str, not %s" % type(filename).__name__)
 
-def fsdecode(filename):
+    def fsdecode(filename):
         """
         Decode filename from the filesystem encoding with 'surrogateescape' error
         handler, return str unchanged. On Windows, use 'strict' error handler if
@@ -561,14 +565,15 @@ def fsdecode(filename):
         if isinstance(filename, str):
             return filename
         elif isinstance(filename, bytes):
-        encoding = sys.getfilesystemencoding()
-        if encoding == 'mbcs':
-            return filename.decode(encoding)
-        else:
-            return filename.decode(encoding, 'surrogateescape')
+            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()