[Python-Dev] Internal representation of strings and Micropython

[Glenn Linderman]

On 6/4/2014 5:03 PM, Greg Ewing wrote:
> Serhiy Storchaka wrote:
>> html.HTMLParser, json.JSONDecoder, re.compile, tokenize.tokenize 
>> don't use iterators. They use indices, str.find and/or regular 
>> expressions. Common use case is quickly find substring starting from 
>> current position using str.find or re.search, process found token, 
>> advance position and repeat.
>
> For that kind of thing, you don't need an actual character
> index, just some way of referring to a place in a string.

I think you meant codepoint index, rather than character index.

>
> Instead of an integer, str.find() etc. could return a
> StringPosition, which would be an opaque reference to a
> particular point in a particular string. You would be
> able to pass StringPositions to indexing and slicing
> operations to get fast indexing into the string that
> they were derived from.
>
> StringPositions could support the following operations:
>
>    StringPosition + int --> StringPosition
>    StringPosition - int --> StringPosition
>    StringPosition - StringPosition --> int
>
> These would be computed by counting characters forwards
> or backwards in the string, which would be slower than
> int arithmetic but still faster than counting from the
> beginning of the string every time.
>
> In other contexts, StringPositions would coerce to ints
> (maybe being an int subclass?) allowing them to be used
> in any existing algorithm that slices strings using ints.
>
This starts to diverge from Python codepoint indexing via integers. 
Calculating or caching the codepoint index to byte offset as part of the 
str implementation stays compatible with Python. Introducing 
StringPosition makes a Python-like language. Or so it seems to me.
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