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For object serialization and some other purposes, Java encodes unicode
strings with a modified version of utf-8:
http://en.wikipedia.org/wiki/UTF-8#Java
http://java.sun.com/javase/6/docs/api/java/io/DataInput.html#modified-utf-8
It is used in Lucene index files among other places.
It would be useful if Python had a codec for this, maybe called "UTF-8J"
or something like that.

What would you use such a codec for ?
From the references you gave, it is only used internally for Java object
serialization, so wouldn't really be of much use in Python.

Some java applications use it externally. The purpose seems to be to
prevent NUL bytes from appearing inside encoded strings which can
confuse C libraries that expect NUL's to terminate strings. My
immediate application is parsing lucene indexes:
http://lucene.apache.org/java/docs/fileformats.html#Chars 

Also, according to wikipedia, tcl also uses that encoding.

TCL only uses the codec for internal represenation. You might want to
interface to TCL at the C level and use the codec there, but is that
really a good reason to include the codec in the Python stdlib ?
Dito for parsing Lucene indexes.
I think you're better off writing your own codec and registering it with
the Python codec registry at application start-up time.

I'm not sure what you mean by "ditto for Lucene indexes". I wasn't
planning to use C code. I was hoping to write Python code to parse
those indexes, then found they use this weird encoding, and Python's
codec set is fairly inclusive already, so this codec sounded like a
reasonably useful addition. It probably shows up other places as well.
 It might even be a reasonable internal representation for Python, which
as I understand it currently can't represent codepoints outside the BMP.
 Also, it is used in Java serialization, which I think of as a somewhat
weird and whacky thing, but it's conceivable that somebody someday might
want to write a Python program that speaks the Java serialization
protocol (I don't have a good sense of whether that's feasible).
Writing an application specific codec with the C API is doable in
principle, but it seems like an awful lot of effort for just one quickie
program. These indexes are very large and so writing the codec in
Python would probably be painfully slow.

Since we also support oddball codecs like UTF-8-SIG, why not this one too?
Given the importance of UTF-8, it seems a good idea to support common
variations.

Ok, if you can write a patch implementing the codec, we'll add it.
Please use the name "utf-8-java" and codec name utf_8_java.py.

> TCL only uses the codec for internal represenation. You might want to
> interface to TCL at the C level and use the codec there, but is that
> really a good reason to include the codec in the Python stdlib ?
I wonder if tkinter should use this encoding.

> I wonder if tkinter should use this encoding.
Tkinter is used to build graphical interfaces. I don't think that users write nul bytes with their keyboard. But there is maybe a use case?

I use the hachoir Python package to parse Java .class files and extract the strings from them and having support for Java modified UTF-8 would have been nice.

utf_8_java.patch: Implement "utf-8-java" encoding.
 * It has no alias
 * 'a0円b'.encode('utf-8-java') returns b'a\xc0\x80b'
 * b'a\xc0\x80b'.decode('utf-8-java') returns 'a\x00b'
 * I added some tests to utf-8 codec (test_invalid, test_null_byte)
 * I added many tests for utf-8-java codec
 * I choosed to copy utf8_code_length as utf8java_code_length instead of adding some if to not slow down UTF-8 codec
 * Decoder: 2 byte sequences may be *a little bit* slower for UTF-8:
"if ((s[1] & 0xc0) != 0x80)"
 is replaced by 
"if ((ch <= 0x007F && (ch != 0x0000 || !java)) || ch > 0x07FF)"
 * Encoder: encode chars in U+0000-U+007F may be *a little bit* slower for UTF-8: I added (ch == 0x00 && java) test
For the doc, I just added a line "utf-8-java" in the codec list, but I did not add a paragraph to explain how this codec is different to utf-8. Does anyone have a suggestion?

Thanks for the patch, Victor.
Some comments on the patch:
 * the codec will have to be able to work with lone surrogates
 (see the wikipedia page explaining this detail), which the
 UTF-8 codec in Python 3.x no longer does, so another special
 case is due for this difference
 * we should not make the standard UTF-8 codec slower just to
 support a variant of UTF-8 which will only get marginal use;
 for the decoder, the changes are minimal, so that's fine,
 but for the decoder you are changing the most often used
 code branch to check for NUL bytes - we need a better solution
 for this, even if it means having to use a separte encode_utf8java
 function
Since the ticket was opened in 2008, the common name of the
codec appears to have changed from "UTF-8 Java" to "Modified UTF-8"
or "MUTF-8" as short alias:
 * http://en.wikipedia.org/wiki/UTF-8#Modified_UTF-8
 (change in http://en.wikipedia.org/w/index.php?title=UTF-8&diff=next&oldid=291829304)
 * http://java.sun.com/developer/technicalArticles/Intl/Supplementary/
 (scroll down to "Modified UTF-8")
 * http://developer.android.com/reference/java/io/DataInput.html
 (this is for Android)
So I guess we should adapt to the name to the now common name
and call it "ModifiedUTF8" in the C API and add these aliases:
"utf-8-modified", "mutf-8" and "modified-utf-8".

See also issue #1028.

Benchmark:
a) ./python -m timeit "(b'\xc3\xa9' * 10000).decode('utf-8')"
b)./python -m timeit "(''.join( map(chr, range(0, 128)) )*1000).encode('utf-8')"
c) ./python -m timeit "f=open('Misc/ACKS', encoding='utf-8'); acks=f.read(); f.close()" "acks.encode('utf-8')"
d) ./python -m timeit "f=open('Misc/ACKS', 'rb'); acks=f.read(); f.close()" "acks.decode('utf-8')"
Original -> patched (smallest value of 3 runs):
a) 85.8 usec -> 83.4 usec (-2.8%)
b) 548 usec -> 688 usec (+25.5%)
c) 132 usec -> 144 usec (+9%)
d) 65.9 usec -> 67.3 usec (+2.1%)
Oh, decode 2 bytes sequences are faster with my patch. Strange :-)
But 25% slower to encode a pure ASCII text is not a good news.

Please do not call this "utf-8-java". It is called "cesu-8" per UTS#18 at:
 http://unicode.org/reports/tr26/
CESU-8 is *not* a a valid Unicode Transform Format and should not be called UTF-8. It is a real pain in the butt, caused by people who misunderand Unicode mis-encoding UCS-2 into UTF-8, screwing it up. I understand the need to be able to read it, but call it what it is, please.
Despite the talk about Lucene, I note that the Perl port of Lucene uses real UTF-8, not CESU-8.

+1 for calling it by the correct name (the docs can of course state that this is equivalent to "Java Modified UTF-8" or however they like to call it).

Tom Christiansen wrote:
> 
> Tom Christiansen <tchrist@perl.com> added the comment:
> 
> Please do not call this "utf-8-java". It is called "cesu-8" per UTS#18 at:
> 
> http://unicode.org/reports/tr26/
> 
> CESU-8 is *not* a a valid Unicode Transform Format and should not be called UTF-8. It is a real pain in the butt, caused by people who misunderand Unicode mis-encoding UCS-2 into UTF-8, screwing it up. I understand the need to be able to read it, but call it what it is, please.
> 
> Despite the talk about Lucene, I note that the Perl port of Lucene uses real UTF-8, not CESU-8.
CESU-8 is a different encoding than the one we are talking about.
The only difference between UTF-8 and the modified one is the different
encoding for the U+0000 code point to have the output not contain
any NUL bytes.

Corrected the title again. See my comment.

Marc-Andre Lemburg wrote:
> 
> Corrected the title again. See my comment.
Please open a new ticket, if you want to add a CESU-8 codec.
Looking at the relevant use cases, I'm at most +0 on adding the
modified UTF-8 codec. I think such codecs can well live outside
the stdlib on PyPI.

Python does have other "weird" encodings like bz2 or rot13.
Beside, batteries included :)

> Python does have other "weird" encodings like bz2 or rot13.
No, it has no more such weird encodings.

As far as I understand, this codec can be implemented in Python. There is no need to modify the interpreter core.
def decode_cesu8(b):
 return re.sub('[\uD800-\uDBFF][\uDC00\DFFF]', lambda m: chr(0x10000 | ((ord(m.group()[0]) & 0x3FF) << 10) | (ord(m.group()[1]) & 0x3FF)), b.decode('utf-8', 'surrogatepass'))
def encode_cesu8(s):
 return re.sub('[\U00010000-\U0010FFFF]', lambda m: chr(0xD800 | ((ord(m.group()) >> 10) & 0x3FF)) + chr(0xDC00 | (ord(m.group() & 0x3FF)), s).encode('utf-8', 'surrogatepass')
def decode_mutf8(b):
 return decode_cesu8(b.replace(b'\xC0\x80', b'\x00'))
def encode_mutf8(s):
 return encode_cesu8(s).replace(b'\x00', b'\xC0\x80')

Serhiy: your functions to not constitute a Python codec. For example, there is no support for error handlers in them.

> Serhiy: your functions to not constitute a Python codec. For example, there is no support for error handlers in them.
Yes, it is not a codec in Python library terminology. It's just a pair
of functions, the COder and DECoder, which is enough for the task of
hacking Java serialized data. I don't think that such specific task
leads to the change of the interpreter core.
However, translators that convert the non-BMP characters to a surrogate
pair and back, would be useful in the standard library. They need to
work with a non-standard encodings (CESU-8, MUTF-8, cp65001, some
Tk/IDLE issues). This is a fairly common task.

Ok, I'm closing this entire issue as "won't fix", then. There apparently is a need for functionality like this, but there is apparently also a concern that this is too specialized for the standard library.
As it is possible to implement this as a stand-alone library, I encourage interested users to design a package for PyPI that has this functionality collected for reuse. If the library is then widely used after some time, this issue can be reconsidered.