Issue #6268: More bugfixes about BOM, UTF-16 and UTF-32

 * Fix seek() method of codecs.open(), don't write the BOM twice after seek(0)
 * Fix reset() method of codecs, UTF-16, UTF-32 and StreamWriter classes
 * test_codecs: use "w+" mode instead of "wt+". "t" mode is not supported by
   Solaris or Windows, but does it really exist? I found it the in the issue.

Lib/codecs.py

@@ -370,6 +370,11 @@ class StreamWriter(Codec):
         """
         pass
 
+    def seek(self, offset, whence=0):
+        self.stream.seek(offset, whence)
+        if whence == 0 and offset == 0:
+            self.reset()
+
     def __getattr__(self, name,
                     getattr=getattr):
 
@@ -601,8 +606,8 @@ class StreamReader(Codec):
 
             Resets the codec buffers used for keeping state.
         """
-        self.reset()
         self.stream.seek(offset, whence)
+        self.reset()
 
     def next(self):
 
@@ -695,8 +700,10 @@ class StreamReaderWriter:
         self.writer.reset()
 
     def seek(self, offset, whence=0):
-        self.reader.seek(offset, whence)
-        self.writer.seek(offset, whence)
+        self.stream.seek(offset, whence)
+        self.reader.reset()
+        if whence == 0 and offset == 0:
+            self.writer.reset()
 
     def __getattr__(self, name,
                     getattr=getattr):

Lib/encodings/utf_16.py

@@ -58,17 +58,23 @@ class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
 
 class StreamWriter(codecs.StreamWriter):
     def __init__(self, stream, errors='strict'):
-        self.bom_written = False
         codecs.StreamWriter.__init__(self, stream, errors)
+        self.encoder = None
+
+    def reset(self):
+        codecs.StreamWriter.reset(self)
+        self.encoder = None
 
     def encode(self, input, errors='strict'):
-        self.bom_written = True
-        result = codecs.utf_16_encode(input, errors)
-        if sys.byteorder == 'little':
-            self.encode = codecs.utf_16_le_encode
+        if self.encoder is None:
+            result = codecs.utf_16encoder(input, errors)
+            if sys.byteorder == 'little':
+                self.encoder = codecs.utf_16_leencoder
+            else:
+                self.encoder = codecs.utf_16_beencoder
+            return result
         else:
-            self.encode = codecs.utf_16_be_encode
-        return result
+            return self.encoder(input, errors)
 
 class StreamReader(codecs.StreamReader):
 

Lib/encodings/utf_32.py

@@ -98,17 +98,23 @@ class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
 
 class StreamWriter(codecs.StreamWriter):
     def __init__(self, stream, errors='strict'):
-        self.bom_written = False
+        self.encoder = None
         codecs.StreamWriter.__init__(self, stream, errors)
 
+    def reset(self):
+        codecs.StreamWriter.reset(self)
+        self.encoder = None
+
     def encode(self, input, errors='strict'):
-        self.bom_written = True
-        result = codecs.utf_32_encode(input, errors)
-        if sys.byteorder == 'little':
-            self.encode = codecs.utf_32_le_encode
+        if self.encoder is None:
+            result = codecs.utf_32encoder(input, errors)
+            if sys.byteorder == 'little':
+                self.encoder = codecs.utf_32_leencoder
+            else:
+                self.encoder = codecs.utf_32_beencoder
+            return result
         else:
-            self.encode = codecs.utf_32_be_encode
-        return result
+            return self.encoder(input, errors)
 
 class StreamReader(codecs.StreamReader):
 

Lib/test/test_codecs.py

@@ -1498,7 +1498,7 @@ class WithStmtTest(unittest.TestCase):
 
 class BomTest(unittest.TestCase):
     def test_seek0(self):
-        data = "1234567890"
+        data = u"1234567890"
         tests = ("utf-16",
                  "utf-16-le",
                  "utf-16-be",
@@ -1506,8 +1506,8 @@ class BomTest(unittest.TestCase):
                  "utf-32-le",
                  "utf-32-be")
         for encoding in tests:
-            with codecs.open('foo', 'wt+', encoding=encoding) as f:
-                # Check if the BOM is written only once
+            # Check if the BOM is written only once
+            with codecs.open(test_support.TESTFN, 'w+', encoding=encoding) as f:
                 f.write(data)
                 f.write(data)
                 f.seek(0)
@@ -1515,6 +1515,42 @@ class BomTest(unittest.TestCase):
                 f.seek(0)
                 self.assertEquals(f.read(), data * 2)
 
+            # Check that the BOM is written after a seek(0)
+            with codecs.open(test_support.TESTFN, 'w+', encoding=encoding) as f:
+                f.write(data[0])
+                self.assertNotEquals(f.tell(), 0)
+                f.seek(0)
+                f.write(data)
+                f.seek(0)
+                self.assertEquals(f.read(), data)
+
+            # (StreamWriter) Check that the BOM is written after a seek(0)
+            with codecs.open(test_support.TESTFN, 'w+', encoding=encoding) as f:
+                f.writer.write(data[0])
+                self.assertNotEquals(f.writer.tell(), 0)
+                f.writer.seek(0)
+                f.writer.write(data)
+                f.seek(0)
+                self.assertEquals(f.read(), data)
+
+            # Check that the BOM is not written after a seek() at a position
+            # different than the start
+            with codecs.open(test_support.TESTFN, 'w+', encoding=encoding) as f:
+                f.write(data)
+                f.seek(f.tell())
+                f.write(data)
+                f.seek(0)
+                self.assertEquals(f.read(), data * 2)
+
+            # (StreamWriter) Check that the BOM is not written after a seek()
+            # at a position different than the start
+            with codecs.open(test_support.TESTFN, 'w+', encoding=encoding) as f:
+                f.writer.write(data)
+                f.writer.seek(f.writer.tell())
+                f.writer.write(data)
+                f.seek(0)
+                self.assertEquals(f.read(), data * 2)
+
 
 def test_main():
     test_support.run_unittest(

Misc/NEWS

@@ -31,8 +31,9 @@ Library
 
 - Issue #3924: Ignore cookies with invalid "version" field in cookielib.
 
-- Issue #6268: Fix seek() method of codecs.open(), don't read the BOM twice
-  after seek(0)
+- Issue #6268: Fix seek() method of codecs.open(), don't read or write the BOM
+  twice after seek(0). Fix also reset() method of codecs, UTF-16, UTF-32 and
+  StreamWriter classes.
 
 - Issue #5640: Fix Shift-JIS incremental encoder for error handlers different
   than strict