How is unicode implemented behind the scenes?

Chris Angelico rosuav at gmail.com
Sun Mar 9 03:56:39 CET 2014


On Sun, Mar 9, 2014 at 1:08 PM, Dan Stromberg <drsalists at gmail.com> wrote:
> OK, I know that Unicode data is stored in an encoding on disk.
>
> But how is it stored in RAM?
>
> I realize I shouldn't write code that depends on any relevant
> implementation details, but knowing some of the more common
> implementation options would probably help build an intuition for
> what's going on internally.
>
> I've heard that characters are no longer all c bytes wide internally,
> so is it sometimes utf-8?
>

As of Python 3.3, it's as MRAB described. If you like, Python chooses
between one of three (or four) encodings, based on what can handle the
string:

1) ASCII (there are some minor differences with 7-bit strings, eg it
knows the conversion to UTF-8 is the identity function)
2) Latin-1
3) UCS-2
4) UCS-4

This means that finding the Nth codepoint in a string is simply a
matter of shifting N by either 0, 0, 1, or 2, and picking the right
number of bytes from that position. You can read the gory details in
PEP 393:

http://www.python.org/dev/peps/pep-0393/

but the important bit here is the "kind", which is 01 for Latin-1, 10
for UCS-2, 11 for UCS-4. (The "ascii-only" flag is stored elsewhere.)
There's a functionally-identical field in Pike's strings, called
size_shift - 0 for ASCII or Latin-1, 1 for UCS-2, 2 for UCS-4.
Whichever it is, it's really efficient - and as an added bonus, all
those ASCII-only strings that scripts are full of (you know, words
like "print" and "len" and "int") are stored compactly, so it's much
tighter than the 3.2 builds, even narrow ones. It's pretty awesome!

ChrisA



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