Python UTF-16 WAVY DASH encoding question / issue

Question

Hi. I was doing some work today, and came across an issue where something "looked funny". I had been interpreting some string data as utf-8, and checking the encoded form. The data was coming from ldap (Specifically, Active Directory) via python-ldap. No surprises there.

So I came upon the byte sequence '\xe3\x80\xb0' a few times, which, when decoded as utf-8, is unicode codepoint 3030 (wavy dash). I need the string data in utf-16, so naturally I converted it via .encode('utf-16'). Unfortunately, it seems python doesn't like this character:

D:\> python
Python 2.6.4 (r264:75708, Oct 26 2009, 08:23:19) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> u"\u3030"
u'\u3030'
>>> u"\u3030".encode("utf-8")
'\xe3\x80\xb0'
>>> u"\u3030".encode("utf-16-le")
'00'
>>> u"\u3030".encode("utf-16-be")
'00'
>>> '\xe3\x80\xb0'.decode('utf-8')
u'\u3030'
>>> '\xe3\x80\xb0'.decode('utf-8').encode('utf-16')
'\xff\xfe00'
>>> '\xe3\x80\xb0'.decode('utf-8').encode('utf-16-le').decode('utf-8')
u'00'

It seems IronPython isn't a fan either:

D:\ipy
IronPython 2.6 Beta 2 (2.6.0.20) on .NET 2.0.50727.3053
Type "help", "copyright", "credits" or "license" for more information.
>>> u"\u3030"
u'\u3030'
>>> u"\u3030".encode('utf-8')
u'\xe3\x80\xb0'
>>> u"\u3030".encode('utf-16-le')
'00'

If somebody could tell me what, exactly, is going on here, it'd be much appreciated.

Answer 1

This seems to be the correct behaviour. The character u'\u3030' when encoded in UTF-16 is the same as the encoding of '00' in UTF-8. It looks strange, but it's correct.

The '\xff\xfe' you can see is just a Byte Order Mark.

Are you sure you want a wavy dash, and not some other character? If you were hoping for a different character then it might be because it had already been misencoded before entering your application.

Answer 2

But it decodes okay:

>>> u"\u3030".encode("utf-16-le")
'00'
>>> '00'.decode("utf-16-le")
u'\u3030'

It's that the UTF-16 encoding of that character happens to coincide with the ASCII code for '0'. You could also represent it with '\x30\x30':

>>> '00' == '\x30\x30'
True

Answer 3

You are being confused by two things here (threw me off too):

1. utf-16 and utf-32 encodings use a BOM unless you specify which byte order to use, via utf-16-be and such. This is the \xff\xfe in the second last line.

2. '00' is two of the characters digit zero. It is not a null character. That'd print differently anyway:

   >>> '\0\0'
   '\x00\x00'
   

Answer 4

There is a basic error in your sample code above. Remember, you encode Unicode to an encoded string, and you decode from an encoded string back to Unicode. So, you do:

'\xe3\x80\xb0'.decode('utf-8').encode('utf-16-le').decode('utf-8')

which translates to the following steps:

'\xe3\x80\xb0' # (some string)
.decode('utf-8') # decode above text as UTF-8 encoded text, giving u'\u3030'
.encode('utf-16-le') # encode u'\u3030' as UTF-16-LE, i.e. '00'
.decode('utf-8') # OOPS! decode using the wrong encoding here!

u'\u3030' is indeed encoded as '00' (ascii zero twice) in UTF-16LE but you somehow think that this is a null byte ('\0') or something.

Remember, you can't reach to the same character if you encode with one and decode with another encoding:

>>> import unicodedata as ud
>>> c= unichr(193)
>>> ud.name(c)
'LATIN CAPITAL LETTER A WITH ACUTE'
>>> ud.name(c.encode("cp1252").decode("cp1253"))
'GREEK CAPITAL LETTER ALPHA'

In this code, I encoded to Windows-1252 and decoded from Windows-1253. In your code, you encoded to UTF-16LE and decoded from UTF-8.