How is unicode implemented behind the scenes?

Ned Batchelder ned at nedbatchelder.com
Sun Mar 9 04:48:51 CET 2014


On 3/8/14 9:08 PM, Dan Stromberg 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?
>
> Thanks.
>

In abstract terms, a Unicode string is a sequence of integers (code 
points).  There are lots of ways to store a sequence of integers.

In Python 2.x, it's either a vector of 16-bit ints, or 32-bit ints. 
These are the Unicode representations known as UTF-16 and UTF-32, 
respectively, and which you have depends on whether you have a "narrow" 
or "wide" build of Python.  You can tell the difference by examining 
sys.maxunicode, which is 65535 (narrow) or 1114111 (wide).

In Python 3.3, the representation was changed from narrow/wide to the 
so-called Flexible String Representation which others here have 
described.  It uses either 1-, 2-, or 4-bytes per code point, depending 
on the set of code points in the string.  It's specified in PEP 393: 
http://legacy.python.org/dev/peps/pep-0393/

-- 
Ned Batchelder, http://nedbatchelder.com




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