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unicode(s, enc).encode(enc) == s ?

I have checks in code, to ensure a decode/encode cycle returns the
original string.

Given no UnicodeErrors, are there any cases for the following not to
be True?

unicode(s, enc).encode(enc) == s

mario

Dec 27 '07 #1
8 2134
Given no UnicodeErrors, are there any cases for the following not to
be True?

unicode(s, enc).encode(enc) == s
Certainly. ISO-2022 is famous for having ambiguous encodings. Try
these:

unicode("Hallo","iso-2022-jp")
unicode("\x1b(BHallo","iso-2022-jp")
unicode("\x1b(JHallo","iso-2022-jp")
unicode("\x1b(BHal\x1b(Jlo","iso-2022-jp")

or likewise

unicode("\x1b$@BB","iso-2022-jp")
unicode("\x1b$BBB","iso-2022-jp")

In iso-2022-jp-3, there are even more ways to encode the same string.

Regards,
Martin
Dec 27 '07 #2
On Dec 27, 7:37 pm, "Martin v. Lwis" <mar...@v.loewis.dewrote:
Certainly. ISO-2022 is famous for having ambiguous encodings. Try
these:

unicode("Hallo","iso-2022-jp")
unicode("\x1b(BHallo","iso-2022-jp")
unicode("\x1b(JHallo","iso-2022-jp")
unicode("\x1b(BHal\x1b(Jlo","iso-2022-jp")

or likewise

unicode("\x1b$@BB","iso-2022-jp")
unicode("\x1b$BBB","iso-2022-jp")

In iso-2022-jp-3, there are even more ways to encode the same string.
Wow, that's not easy to see why would anyone ever want that? Is there
any logic behind this?

In your samples both of unicode("\x1b(BHallo","iso-2022-jp") and
unicode("\x1b(JHallo","iso-2022-jp") give u"Hallo" -- does this mean
that the ignored/lost bytes in the original strings are not illegal
but *represent nothing* in this encoding?

I.e. in practice (in a context limited to the encoding in question)
should this be considered as a data loss, or should these strings be
considered "equivalent"?

Thanks!

mario
Dec 28 '07 #3
On Fri, 28 Dec 2007 03:00:59 -0800, mario wrote:
On Dec 27, 7:37 pm, "Martin v. Löwis" <mar...@v.loewis.dewrote:
>Certainly. ISO-2022 is famous for having ambiguous encodings. Try
these:

unicode("Hallo","iso-2022-jp")
unicode("\x1b(BHallo","iso-2022-jp")
unicode("\x1b(JHallo","iso-2022-jp")
unicode("\x1b(BHal\x1b(Jlo","iso-2022-jp")

or likewise

unicode("\x1b$@BB","iso-2022-jp")
unicode("\x1b$BBB","iso-2022-jp")

In iso-2022-jp-3, there are even more ways to encode the same string.

Wow, that's not easy to see why would anyone ever want that? Is there
any logic behind this?

In your samples both of unicode("\x1b(BHallo","iso-2022-jp") and
unicode("\x1b(JHallo","iso-2022-jp") give u"Hallo" -- does this mean
that the ignored/lost bytes in the original strings are not illegal
but *represent nothing* in this encoding?
They are not lost or ignored but escape sequences that tell how the
following bytes should be interpreted '\x1b(B' switches to ASCII and
'\x1b(J' to some "roman" encoding which is a superset of ASCII, so it
doesn't matter which one you choose unless the following bytes are all
ASCII. And of course you can use that escape prefix as often as you want
within a string of ASCII byte values.

http://en.wikipedia.org/wiki/ISO-202...Character_Sets
I.e. in practice (in a context limited to the encoding in question)
should this be considered as a data loss, or should these strings be
considered "equivalent"?
Equivalent I would say. As Unicode they contain the same characters.
Just differently encoded as bytes.

Ciao,
Marc 'BlackJack' Rintsch
Dec 28 '07 #4
Wow, that's not easy to see why would anyone ever want that? Is there
any logic behind this?
It's the pre-Unicode solution to the "we want to have many characters
encoded in a single file" problem.

Suppose you have pre-defined characters sets A, B, C, and you want text
to contain characters from all three sets, one possible encoding is

<switch-to-A>CharactersInA<switch-to-B>CharactersFromB<and-so-on>

Now also suppose that A, B, and C are not completely different, but
have slight overlap - and you get ambiguous encodings.

ISO-2022 works that way. IPSJ maintains a registry of character
sets for ISO, and assigns escape codes to them. There are currently
about 200 character sets registered.

Somebody decoding this would have to know all the character sets
(remember it's a growing registry), hence iso-2022-jp restricts
the character sets that you can use for that particular encoding.
(Likewise, iso-2022-kr also restricts it, but to a different set
of sets).

It's a mess, sure, and one of the primary driving force of Unicode
(which even has the unification - ie. lack of ambiguity - in
its name).
In your samples both of unicode("\x1b(BHallo","iso-2022-jp") and
unicode("\x1b(JHallo","iso-2022-jp") give u"Hallo" -- does this mean
that the ignored/lost bytes in the original strings are not illegal
but *represent nothing* in this encoding?
See above, and Marc's explanation.
ESC ( B switches to "ISO 646, USA Version X3.4 - 1968";
ESC ( J to "ISO 646, Japanese Version for Roman Characters JIS C6220-1969"

These are identical, except for the following differences:
- The USA version has "reverse solidus" at 5/12; the Japanese
version "Yen sign"
- The USA version has "Tilde (overline; general accent)" at
7/14 (depicted as tilde); the Japanese version "Overline"
(depicted as straight overline)
- The Japanese version specifies that you can switch between
roman and katakana mode by sending shift out (SO, '\x0e')
and shift-in (SI, '\x0F') respectively; this switches to
the "JIS KATAKANA character set".
(source:
http://www.itscj.ipsj.or.jp/ISO-IR/006.pdf
http://www.itscj.ipsj.or.jp/ISO-IR/014.pdf
)
I.e. in practice (in a context limited to the encoding in question)
should this be considered as a data loss, or should these strings be
considered "equivalent"?
These particular differences should be considered as irrelevant. There
are some cases where Unicode had introduced particular compatibility
characters to accommodate such encodings (specifically, the "full-width"
latin (*) and "half-width" Japanese characters). Good codecs are
supposed to round-trip the relevant differences to Unicode, and generate
the appropriate compatibility characters.

Bad codecs might not, and in some cases, users might complain that
certain compatibility characters are lacking in Unicode so that correct
round-tripping is not possible. I believe the Unicode consortium has
resolved all these complaints by adding the missing characters; but
I'm not sure.

Regards,
Martin

(*) As an example for full-width characters, consider these
two strings:
Hello
Hello
Should they be equivalent, or not? They are under NFKD, but not
NFD.
Dec 29 '07 #5
Thanks a lot Martin and Marc for the really great explanations! I was
wondering if it would be reasonable to imagine a utility that will
determine whether, for a given encoding, two byte strings would be
equivalent. But I think such a utility will require *extensive*
knowledge about many bizarrities of many encodings -- and has little
chance of being pretty!

In any case, it goes well beyond the situation that triggered my
original question in the first place, that basically was to provide a
reasonable check on whether round-tripping a string is successful --
this is in the context of a small utility to guess an encoding and to
use it to decode a byte string. This utility module was triggered by
one that Skip Montanaro had written some time ago, but I wanted to add
and combine several ideas and techniques (and support for my usage
scenarios) for guessing a string's encoding in one convenient place. I
provide a write-up and the code for it here:

http://gizmojo.org/code/decodeh/

I will be very interested in any remarks any of you may have!

Best regards, mario
Jan 2 '08 #6
Thanks a lot Martin and Marc for the really great explanations! I was
wondering if it would be reasonable to imagine a utility that will
determine whether, for a given encoding, two byte strings would be
equivalent.
But that is much easier to answer:

s1.decode(enc) == s2.decode(enc)

Assuming Unicode's unification, for a single encoding, this should
produce correct results in all cases I'm aware of.

If the you also have different encodings, you should add

def normal_decode(s, enc):
return unicode.normalize("NFKD", s.decode(enc))

normal_decode(s1, enc) == normal_decode(s2, enc)

This would flatten out compatibility characters, and ambiguities
left in Unicode itself.
But I think such a utility will require *extensive*
knowledge about many bizarrities of many encodings -- and has little
chance of being pretty!
See above.
In any case, it goes well beyond the situation that triggered my
original question in the first place, that basically was to provide a
reasonable check on whether round-tripping a string is successful --
this is in the context of a small utility to guess an encoding and to
use it to decode a byte string. This utility module was triggered by
one that Skip Montanaro had written some time ago, but I wanted to add
and combine several ideas and techniques (and support for my usage
scenarios) for guessing a string's encoding in one convenient place.
Notice that this algorithm is not capable of detecting the ISO-2022
encodings - they look like ASCII to this algorithm. This is by design,
as the encoding was designed to only use 7-bit bytes, so that you can
safely transport them in Email and such (*)

If you want to add support for ISO-2022, you should look for escape
characters, and then check whether the escape sequences are among
the ISO-2022 ones:
- ESC ( - 94-character graphic character set, G0
- ESC ) - 94-character graphic character set, G1
- ESC * - 94-character graphic character set, G2
- ESC + - 94-character graphic character set, G3
- ESC - - 96-character graphic character set, G1
- ESC . - 96-character graphic character set, G2
- ESC / - 96-character graphic character set, G3
- ESC $ - Multibyte
( G0
) G1
* G2
+ G3
- ESC % - Non-ISO-2022 (e.g. UTF-8)

If you see any of these, it should be ISO-2022; see
the Wiki page as to what subset may be in use.

G0..G3 means what register the character set is loaded
into; when you have loaded a character set into a register,
you can switch between registers through ^N (to G1),
^O (to G0), ESC n (to G2), ESC o (to G3) (*)
http://gizmojo.org/code/decodeh/

I will be very interested in any remarks any of you may have!
From a shallow inspection, it looks right. I would have spelled
"losses" as "loses".

Regards,
Martin

(*) For completeness: ISO-2022 also supports 8-bit characters,
and there are more control codes to shift between the various
registers.
Jan 2 '08 #7
Thanks again. I will chunk my responses as your message has too much
in it for me to process all at once...

On Jan 2, 9:34 pm, "Martin v. Lwis" <mar...@v.loewis.dewrote:
Thanks a lot Martin and Marc for the really great explanations! I was
wondering if it would be reasonable to imagine a utility that will
determine whether, for a given encoding, two byte strings would be
equivalent.

But that is much easier to answer:

s1.decode(enc) == s2.decode(enc)

Assuming Unicode's unification, for a single encoding, this should
produce correct results in all cases I'm aware of.

If the you also have different encodings, you should add

def normal_decode(s, enc):
return unicode.normalize("NFKD", s.decode(enc))

normal_decode(s1, enc) == normal_decode(s2, enc)

This would flatten out compatibility characters, and ambiguities
left in Unicode itself.
Hmmn, true, it would be that easy.

I am now not sure why I needed that check, or how to use this version
of it... I am always starting from one string, and decoding it... that
may be lossy when that is re-encoded, and compared to original.
However it is clear that the test above should always pass in this
case, so doing it seems superfluos.

Thanks for the unicodedata.normalize() tip.

mario

Jan 3 '08 #8
On Jan 2, 9:34 pm, "Martin v. Lwis" <mar...@v.loewis.dewrote:
In any case, it goes well beyond the situation that triggered my
original question in the first place, that basically was to provide a
reasonable check on whether round-tripping a string is successful --
this is in the context of a small utility to guess an encoding and to
use it to decode a byte string. This utility module was triggered by
one that Skip Montanaro had written some time ago, but I wanted to add
and combine several ideas and techniques (and support for my usage
scenarios) for guessing a string's encoding in one convenient place.

Notice that this algorithm is not capable of detecting the ISO-2022
encodings - they look like ASCII to this algorithm. This is by design,
as the encoding was designed to only use 7-bit bytes, so that you can
safely transport them in Email and such (*)
Well, one could specify decode_heuristically(s, enc="iso-2022-jp") and
that
encoding will be checked before ascii or any other encoding in the
list.
If you want to add support for ISO-2022, you should look for escape
characters, and then check whether the escape sequences are among
the ISO-2022 ones:
- ESC ( - 94-character graphic character set, G0
- ESC ) - 94-character graphic character set, G1
- ESC * - 94-character graphic character set, G2
- ESC + - 94-character graphic character set, G3
- ESC - - 96-character graphic character set, G1
- ESC . - 96-character graphic character set, G2
- ESC / - 96-character graphic character set, G3
- ESC $ - Multibyte
( G0
) G1
* G2
+ G3
- ESC % - Non-ISO-2022 (e.g. UTF-8)

If you see any of these, it should be ISO-2022; see
the Wiki page as to what subset may be in use.

G0..G3 means what register the character set is loaded
into; when you have loaded a character set into a register,
you can switch between registers through ^N (to G1),
^O (to G0), ESC n (to G2), ESC o (to G3) (*)
OK, suppose we do not know the string is likely to be iso-2022, but we
still want to detect it if it is. I have added a "may_do_better"
mechanism to the algorithm, to add special checks on a *guessed*
algorithm. I am not sure this will not however introduce more or other
problems than the one it is addressing...

I have re-instated checks for iso-8859-1 control chars (likely to be
cp1252), for special symbols in iso-8859-15 when they occur in
iso-8859-1 and cp1252, and for the iso-2022-jp escape sequences. To
flesh out with other checks is mechanical work...

If you could take a look at the updated page:
http://gizmojo.org/code/decodeh/
I still have issues with what happens in situations when for example a
file contains iso-2022 esc sequences but is anyway actally in ascii
or utf-8? e.g. this mail message! I'll let this issue turn for a
little while...
I will be very interested in any remarks any of you may have!

From a shallow inspection, it looks right. I would have spelled
"losses" as "loses".
Yes, corrected.

Thanks, mario
Jan 3 '08 #9

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