[Python-Dev] PEP 393 Summer of Code Project (original) (raw)

Glenn Linderman v+python at g.nevcal.com
Wed Aug 24 20:52:51 CEST 2011

On 8/24/2011 9:00 AM, Stefan Behnel wrote:

Nick Coghlan, 24.08.2011 15:06:

On Wed, Aug 24, 2011 at 10:46 AM, Terry Reedy wrote:

In utf16.py, attached to http://bugs.python.org/issue12729 I propose for consideration a prototype of different solution to the 'mostly BMP chars, few non-BMP chars' case. Rather than expand every character from 2 bytes to 4, attach an array cpdex of character (ie code point, not code unit) indexes. Then for indexing and slicing, the correction is simple, simpler than I first expected: code-unit-index = char-index + bisect.bisectleft(cpdex, charindex) where code-unit-index is the adjusted index into the full underlying double-byte array. This adds a time penalty of log2(len(cpdex)), but avoids most of the space penalty and the consequent time penalty of moving more bytes around and increasing cache misses.

Interesting idea, but putting on my C programmer hat, I say -1. Non-uniform cell size = not a C array = standard C array manipulation idioms don't work = pain (no matter how simple the index correction happens to be). The nice thing about PEP 383 is that it gives us the smallest storage array that is both an ordinary C array and has sufficiently large individual elements to handle every character in the string. +1

Yes, this sounds like a nice benefit, but the problem is it is false.
The correct statement would be:

The nice thing about PEP 383 is that it gives us the smallest storage array that is both an ordinary C array and has sufficiently large individual elements to handle every Unicode codepoint in the string.

As Tom eloquently describes in the referenced issue (is Tom ever non-eloquent?), not all characters can be represented in a single codepoint.

It seems there are three concepts in Unicode, code units, codepoints, and characters, none of which are equivalent (and the first of which varies according to the encoding). It also seems (to me) that Unicode has failed in its original premise, of being an easy way to handle "big char" for "all languages" with fixed size elements, but it is not clear that its original premise is achievable regardless of the size of "big char", when mixed directionality is desired, and it seems that support of some single languages require mixed directionality, not to mention mixed language support.

Given the required variability of character size in all presently Unicode defined encodings, I tend to agree with Tom that UTF-8, together with some technique of translating character index to code unit offset, may provide the best overall space utilization, and adequate CPU efficiency. On the other hand, there are large subsets of applications that simply do not require support for bidirectional text or composed characters, and for those that do not, it remains to be seen if the price to be paid for supporting those features is too high a price for such applications. So far, we don't have implementations to benchmark to figure that out!

What does this mean for Python? Well, if Python is willing to limit its support for applications to the subset for which the "big char" solution sufficient, then PEP 393 provides a way to do that, that looks to be pretty effective for reducing memory consumption for those applications that use short strings most of which can be classified by content into the 1 byte or 2 byte representations. Applications that support long strings are more likely to bitten by the occasional "outlier" character that is longer than the average character, doubling or quadrupling the space needed to represent such strings, and eliminating a significant portion of the space savings the PEP is providing for other applications. Benchmarks may or may not fully reflect the actual requirements of all applications, so conclusions based on benchmarking can easily be blind-sided the realities of other applications, unless the benchmarks are carefully constructed.

It is possible that the ideas in PEP 393, with its support for multiple underlying representations, could be the basis for some more complex representations that would better support characters rather than only supporting code points, but Martin has stated he is not open to additional representations, so the PEP itself cannot be that basis (although with care which may or may not be taken in the implementation of the PEP, the implementation may still provide that basis). -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20110824/742d60db/attachment.html>

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