How is unicode implemented behind the scenes?

[MRAB]

On 2014-03-09 02:40, MRAB wrote:
> On 2014-03-09 02:08, 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?
>>
> No.
>
>   From Python 3.3, it's an array of 1, 2 or 4 bytes per codepoint.
>
> In Python terms:
>
> if all(c <= '\xFF' for c in string):
>       use 1 byte per codepoint
> elif all(c <= '\xFFFF' for c in string):
>       use 2 bytes per codepoint
> else:
>       use 4 bytes per codepoint
>
Oops! That should, of course, be:

if all(c <= '\xFF' for c in string):
     use 1 byte per codepoint
elif all(c <= '\uFFFF' for c in string):
     use 2 bytes per codepoint
else:
     use 4 bytes per codepoint