Slow python-dev day...consider this exiting new proposal to allow deal with important new characters like the Japanese dentristy symbols and ecological symbols (but not Klingon) -------- Original Message -------- Subject: PEP: Support for "wide" Unicode characters Date: Thu, 28 Jun 2001 15:33:00 -0700 From: Paul Prescod <paulp@ActiveState.com> Organization: ActiveState To: "python-list@python.org" <python-list@python.org> PEP: 261 Title: Support for "wide" Unicode characters Version: $Revision: 1.3 $ Author: paulp@activestate.com (Paul Prescod) Status: Draft Type: Standards Track Created: 27-Jun-2001 Python-Version: 2.2 Post-History: 27-Jun-2001, 28-Jun-2001 Abstract Python 2.1 unicode characters can have ordinals only up to 2**16 -1. These characters are known as Basic Multilinual Plane characters. There are now characters in Unicode that live on other "planes". The largest addressable character in Unicode has the ordinal 17 * 2**16 - 1 (0x10ffff). For readability, we will call this TOPCHAR and call characters in this range "wide characters". Glossary Character Used by itself, means the addressable units of a Python Unicode string. Code point If you imagine Unicode as a mapping from integers to characters, each integer represents a code point. Some are really used for characters. Some will someday be used for characters. Some are guaranteed never to be used for characters. Unicode character A code point defined in the Unicode standard whether it is already assigned or not. Identified by an integer. Code unit An integer representing a character in some encoding. Surrogate pair Two code units that represnt a single Unicode character. Proposed Solution One solution would be to merely increase the maximum ordinal to a larger value. Unfortunately the only straightforward implementation of this idea is to increase the character code unit to 4 bytes. This has the effect of doubling the size of most Unicode strings. In order to avoid imposing this cost on every user, Python 2.2 will allow 4-byte Unicode characters as a build-time option. Users can choose whether they care about wide characters or prefer to preserve memory. The 4-byte option is called "wide Py_UNICODE". The 2-byte option is called "narrow Py_UNICODE". Most things will behave identically in the wide and narrow worlds. * unichr(i) for 0 <= i < 2**16 (0x10000) always returns a length-one string. * unichr(i) for 2**16 <= i <= TOPCHAR will return a length-one string representing the character on wide Python builds. On narrow builds it will return ValueError. ISSUE: Python currently allows \U literals that cannot be represented as a single character. It generates two characters known as a "surrogate pair". Should this be disallowed on future narrow Python builds? ISSUE: Should Python allow the construction of characters that do not correspond to Unicode characters? Unassigned Unicode characters should obviously be legal (because they could be assigned at any time). But code points above TOPCHAR are guaranteed never to be used by Unicode. Should we allow access to them anyhow? * ord() is always the inverse of unichr() * There is an integer value in the sys module that describes the largest ordinal for a Unicode character on the current interpreter. sys.maxunicode is 2**16-1 (0xffff) on narrow builds of Python and TOPCHAR on wide builds. ISSUE: Should there be distinct constants for accessing TOPCHAR and the real upper bound for the domain of unichr (if they differ)? There has also been a suggestion of sys.unicodewith which can take the values 'wide' and 'narrow'. * codecs will be upgraded to support "wide characters" (represented directly in UCS-4, as surrogate pairs in UTF-16 and as multi-byte sequences in UTF-8). On narrow Python builds, the codecs will generate surrogate pairs, on wide Python builds they will generate a single character. This is the main part of the implementation left to be done. * there are no restrictions on constructing strings that use code points "reserved for surrogates" improperly. These are called "isolated surrogates". The codecs should disallow reading these but you could construct them using string literals or unichr(). unichr() is not restricted to values less than either TOPCHAR nor sys.maxunicode. Implementation There is a new (experimental) define: #define PY_UNICODE_SIZE 2 There is a new configure options: --enable-unicode=ucs2 configures a narrow Py_UNICODE, and uses wchar_t if it fits --enable-unicode=ucs4 configures a wide Py_UNICODE, and uses whchar_t if it fits --enable-unicode same as "=ucs2" The intention is that --disable-unicode, or --enable-unicode=no removes the Unicode type altogether; this is not yet implemented. Notes This PEP does NOT imply that people using Unicode need to use a 4-byte encoding. It only allows them to do so. For example, ASCII is still a legitimate (7-bit) Unicode-encoding. Rationale for Surrogate Creation Behaviour Python currently supports the construction of a surrogate pair for a large unicode literal character escape sequence. This is basically designed as a simple way to construct "wide characters" even in a narrow Python build. ISSUE: surrogates can be created this way but the user still needs to be careful about slicing, indexing, printing etc. Another option is to remove knowledge of surrogates from everything other than the codecs. Rejected Suggestions There were two primary solutions that were rejected. The first was more or less the status-quo. We could officially say that Python characters represent UTF-16 code units and require programmers to implement wide characters in their application logic. This is a heavy burden because emulating 32-bit characters is likely to be very inefficient if it is coded entirely in Python. Plus these abstracted pseudo-strings would not be legal as input to the regular expression engine. The other class of solution is to use some efficient storage internally but present an abstraction of wide characters to the programmer. Any of these would require a much more complex implementation than the accepted solution. For instance consider the impact on the regular expression engine. In theory, we could move to this implementation in the future without breaking Python code. A future Python could "emulate" wide Python semantics on narrow Python. Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil End: -- http://mail.python.org/mailman/listinfo/python-list