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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Encoding of surrogate code points to UTF-8
Date: Tue, 08 Oct 2013 18:17:01 -0400
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Xref: csiph.com comp.lang.python:56447

On 10/8/2013 5:47 PM, Terry Reedy wrote:
> On 10/8/2013 9:52 AM, Steven D'Aprano wrote:

>> But reading the previous entry in the FAQs:
>>
>> http://www.unicode.org/faq/utf_bom.html#utf8-4
>>
>> I interpret this as meaning that I should be able to encode valid pairs
>> of surrogates.
>
> It says you should be able to 'convert' them, and that the result for
> utf-8 encoding must be a single 4-bytes code for the corresponding
> supplementary codepoint.

To expand on this: The FAQ question is "How do I convert a UTF-16 
surrogate pair such as <D800 DC00> to UTF-8?" utf-16 and utf-8 are both 
byte (or double byte) encodings of codepoints. Direct conversion would 
be 'transcoding', not encoding. Python has a few bytes transcoders and 
one string transcoder (rot_13), listed at the end of
http://docs.python.org/3/library/codecs.html#python-specific-encodings
But in general, one must decode bytes to string and encode back to bytes.

>> So if I find a code point that encodes to a surrogate pair
>> in UTF-16:
>>
>> py> c = '\N{LINEAR B SYLLABLE B038 E}'
>> py> surr_pair = c.encode('utf-16be')
>> py> print(surr_pair)
>> b'\xd8\x00\xdc\x01'
>>
>> and then use those same values as the code points, I ought to be able to
>> encode to UTF-8, as if it were the same \N{LINEAR B SYLLABLE B038 E} code
>> point.

I believe the utf encodings are defined as 1 to 1. If the above worked, 
utf-8 would not be.

-- 
Terry Jan Reedy