This patch by Martin v. Loewis changes the UTF-16 codec to only

malemburg · malemburg · commit 92b550cdd854 · 2001-06-19T20:07:51.000Z
write a BOM at the start of the stream and also to only read it as
BOM at the start of a stream.

Subsequent reading/writing of BOMs will read/write the BOM as ZWNBSP
character. This is in sync with the Unicode specifications.

Note that UTF-16 files will now *have* to start with a BOM mark
in order to be readable by the codec.
diff --git a/Lib/encodings/utf_16.py b/Lib/encodings/utf_16.py
@@ -6,7 +6,7 @@
 (c) Copyright CNRI, All Rights Reserved. NO WARRANTY.
 
 """
-import codecs
+import codecs, sys
 
 ### Codec APIs
 
@@ -18,10 +18,40 @@ class Codec(codecs.Codec):
     decode = codecs.utf_16_decode
 
 class StreamWriter(Codec,codecs.StreamWriter):
-    pass
+    def __init__(self, stream, errors='strict'):
+        self.bom_written = 0
+        codecs.StreamWriter.__init__(self, stream, errors)
+
+    def write(self, data):
+        result = codecs.StreamWriter.write(self, data)
+        if not self.bom_written:
+            self.bom_written = 1
+            if sys.byteorder == 'little':
+                self.encode = codecs.utf_16_le_encode
+            else:
+                self.encode = codecs.utf_16_be_encode
+        return result
         
 class StreamReader(Codec,codecs.StreamReader):
-    pass
+    def __init__(self, stream, errors='strict'):
+        self.bom_read = 0
+        codecs.StreamReader.__init__(self, stream, errors)
+
+    def read(self, size=-1):
+        if not self.bom_read:
+            signature = self.stream.read(2)
+            if signature == codecs.BOM_BE:
+                self.decode = codecs.utf_16_be_decode
+            elif signature == codecs.BOM_LE:
+                self.decode = codecs.utf_16_le_decode
+            else:
+                raise UnicodeError,"UTF-16 stream does not start with BOM"
+            if size > 2:
+                size -= 2
+            elif size >= 0:
+                size = 0
+            self.bom_read = 1
+        return codecs.StreamReader.read(self, size)
 
 ### encodings module API
 
