[Python-Dev] PEP 393: Flexible String Representation (original) (raw)

Josiah Carlson josiah.carlson at gmail.com
Sat Jan 29 01:54:08 CET 2011

Pardon me for this drive-by posting, but this thread smells a lot like this old thread (don't be afraid to read it all, there are some good points in there; not directed at you Martin, but at all readers/posters in this thread)...

http://mail.python.org/pipermail/python-3000/2006-September/003795.html

<http://mail.python.org/pipermail/python-3000/2006-September/003795.html>I'm not averse to faster and/or more memory efficient unicode representations (I would be quite happy with them, actually). I do see the usefulness of having non-utf-8 representations, and caching them is a good idea, though I wonder if that is a "good for Python itself to cache", or "good for the application to cache".

The evil side of me says that we should just provide an API available in Python/C for "give me the representation of unicode string X using the 2byte/4byte code points", and have it just return the appropriate array.array() value (useful for passing to other APIs, or for those who need to do manual manipulation of code-points), or whatever structure is deemed to be appropriate.

The less evil side of me says that going with what the PEP offers isn't a bad idea, and might just be a good idea.

I'll defer my vote to Martin.

Regards,

On Mon, Jan 24, 2011 at 12:17 PM, "Martin v. Löwis" <martin at v.loewis.de>wrote:

I have been thinking about Unicode representation for some time now. This was triggered, on the one hand, by discussions with Glyph Lefkowitz (who complained that his server app consumes too much memory), and Carl Friedrich Bolz (who profiled Python applications to determine that Unicode strings are among the top consumers of memory in Python). On the other hand, this was triggered by the discussion on supporting surrogates in the library better.

I'd like to propose PEP 393, which takes a different approach, addressing both problems simultaneously: by getting a flexible representation (one that can be either 1, 2, or 4 bytes), we can support the full range of Unicode on all systems, but still use only one byte per character for strings that are pure ASCII (which will be the majority of strings for the majority of users). You'll find the PEP at http://www.python.org/dev/peps/pep-0393/ For convenience, I include it below. Regards, Martin PEP: 393 Title: Flexible String Representation Version: Revision:88168Revision: 88168 Revision:88168 Last-Modified: Date:2011−01−2421:14:21+0100(Mo,24.Jan2011)Date: 2011-01-24 21:14:21 +0100 (Mo, 24. Jan 2011) Date:2011−01−2421:14:21+0100(Mo,24.Jan2011) Author: Martin v. Löwis <martin at v.loewis.de> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 24-Jan-2010 Python-Version: 3.3 Post-History: Abstract ======== The Unicode string type is changed to support multiple internal representations, depending on the character with the largest Unicode ordinal (1, 2, or 4 bytes). This will allow a space-efficient representation in common cases, but give access to full UCS-4 on all systems. For compatibility with existing APIs, several representations may exist in parallel; over time, this compatibility should be phased out. Rationale ========= There are two classes of complaints about the current implementation of the unicode type: on systems only supporting UTF-16, users complain that non-BMP characters are not properly supported. On systems using UCS-4 internally (and also sometimes on systems using UCS-2), there is a complaint that Unicode strings take up too much memory - especially compared to Python 2.x, where the same code would often use ASCII strings (i.e. ASCII-encoded byte strings). With the proposed approach, ASCII-only Unicode strings will again use only one byte per character; while still allowing efficient indexing of strings containing non-BMP characters (as strings containing them will use 4 bytes per character). One problem with the approach is support for existing applications (e.g. extension modules). For compatibility, redundant representations may be computed. Applications are encouraged to phase out reliance on a specific internal representation if possible. As interaction with other libraries will often require some sort of internal representation, the specification choses UTF-8 as the recommended way of exposing strings to C code. For many strings (e.g. ASCII), multiple representations may actually share memory (e.g. the shortest form may be shared with the UTF-8 form if all characters are ASCII). With such sharing, the overhead of compatibility representations is reduced. Specification ============= The Unicode object structure is changed to this definition:: typedef struct { PyObjectHEAD Pyssizet length; void *str; Pyhasht hash; int state; Pyssizet utf8length; void *utf8; Pyssizet wstrlength; void *wstr; } PyUnicodeObject; These fields have the following interpretations: - length: number of code points in the string (result of sqlength) - str: shortest-form representation of the unicode string; the lower two bits of the pointer indicate the specific form: 01 => 1 byte (Latin-1); 11 => 2 byte (UCS-2); 11 => 4 byte (UCS-4); 00 => null pointer The string is null-terminated (in its respective representation). - hash, state: same as in Python 3.2 - utf8length, utf8: UTF-8 representation (null-terminated) - wstrlength, wstr: representation in platform's wchart (null-terminated). If wchart is 16-bit, this form may use surrogate pairs (in which cast wstrlength differs form length). All three representations are optional, although the str form is considered the canonical representation which can be absent only while the string is being created. The PyUNICODE type is still supported but deprecated. It is always defined as a typedef for wchart, so the wstr representation can double as PyUNICODE representation. The str and utf8 pointers point to the same memory if the string uses only ASCII characters (using only Latin-1 is not sufficient). The str and wstr pointers point to the same memory if the string happens to fit exactly to the wchart type of the platform (i.e. uses some BMP-not-Latin-1 characters if sizeof(wchart) is 2, and uses some non-BMP characters if sizeof(wchart) is 4). If the string is created directly with the canonical representation (see below), this representation doesn't take a separate memory block, but is allocated right after the PyUnicodeObject struct. String Creation --------------- The recommended way to create a Unicode object is to use the function PyUnicodeNew:: PyObject* PyUnicodeNew(Pyssizet size, PyUCS4 maxchar); Both parameters must denote the eventual size/range of the strings. In particular, codecs using this API must compute both the number of characters and the maximum character in advance. An string is allocated according to the specified size and character range and is null-terminated; the actual characters in it may be unitialized. PyUnicodeFromString and PyUnicodeFromStringAndSize remain supported for processing UTF-8 input; the input is decoded, and the UTF-8 representation is not yet set for the string. PyUnicodeFromUnicode remains supported but is deprecated. If the PyUNICODE pointer is non-null, the str representation is set. If the pointer is NULL, a properly-sized wstr representation is allocated, which can be modified until PyUnicodeFinalize() is called (explicitly or implicitly). Resizing a Unicode string remains possible until it is finalized. PyUnicodeFinalize() converts a string containing only a wstr representation into the canonical representation. Unless wstr and str can share the memory, the wstr representation is discarded after the conversion. String Access ------------- The canonical representation can be accessed using two macros PyUnicodeKind and PyUnicodeData. PyUnicodeKind gives one of the value PyUnicode1BYTE (1), PyUnicode2BYTE (2), or PyUnicode4BYTE (3). PyUnicodeData gives the void pointer to the data, masking out the pointer kind. All these functions call PyUnicodeFinalize in case the canonical representation hasn't been computed yet. A new function PyUnicodeAsUTF8 is provided to access the UTF-8 representation. It is thus identical to the existing PyUnicodeAsString, which is removed. The function will compute the utf8 representation when first called. Since this representation will consume memory until the string object is released, applications should use the existing PyUnicodeAsUTF8String where possible (which generates a new string object every time). API that implicitly converts a string to a char* (such as the ParseTuple functions) will use this function to compute a conversion. PyUnicodeAsUnicode is deprecated; it computes the wstr representation on first use. String Operations ----------------- Various convenience functions will be provided to deal with the canonical representation, in particular with respect to concatenation and slicing. Stable ABI ---------- None of the functions in this PEP become part of the stable ABI. Copyright ========= This document has been placed in the public domain.

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