iSharkFly-Open
/
python-peps
mirror of https://github.com/python/peps.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Updated terminology and format.

This commit is contained in:
Paul Prescod 2001-07-01 19:52:25 +00:00
parent ec6436a790
commit bdff046670
1 changed files with 221 additions and 75 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

296
pep-0261.txt
View File

@ -11,122 +11,268 @@ Post-History: 27-Jun-2001
Abstract
Python 2.1 unicode characters can have ordinals only up to 65536.
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
2**20 + 2**16 - 1. For readability, we will call this TOPCHAR.
Python 2.1 unicode characters can have ordinals only up to 2**16 -1.
This range corresponds to a range in Unicode known as the Basic
Multilingual Plane. 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
A code point is an integer between 0 and TOPCHAR.
If you imagine Unicode as a mapping from integers to
characters, each integer is a code point. But the
integers between 0 and TOPCHAR that do not map to
characters are also code points. Some will someday
be used for characters. Some are guaranteed never
to be used for characters.
Codec
A set of functions for translating between physical
encodings (e.g. on disk or coming in from a network)
into logical Python objects.
Encoding
Mechanism for representing abstract characters in terms of
physical bits and bytes. Encodings allow us to store
Unicode characters on disk and transmit them over networks
in a manner that is compatible with other Unicode software.
Surrogate pair
Two physical characters that represent a single logical
character. Part of a convention for representing 32-bit
code points in terms of two 16-bit code points.
Unicode string
A Python type representing a sequence of code points with
"string semantics" (e.g. case conversions, regular
expression compatibility, etc.) Constructed with the
unicode() function.
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.
One solution would be to merely increase the maximum ordinal
to a larger value. Unfortunately the only straightforward
implementation of this idea is to use 4 bytes per character.
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 the 4-byte implementation 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
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.
* the \u and \U literal syntaxes will always generate the same
data that the unichr function would. They are just different
syntaxes for the same thing.
* unichr(i) for 0 <= i < 2**16 (0x10000) always returns a
length-one string.
* unichr(i) for 0 <= i <= 2**16 always returns a size-one string.
* unichr(i) for 2**16 <= i <= TOPCHAR will return a
length-one string on wide Python builds. On narrow builds it will
raise ValueError.
* unichr(i) for 2**16+1 <= i <= TOPCHAR will always return a
string representing the character.
ISSUE
* BUT on narrow builds of Python, the string will actually be
composed of two characters called a "surrogate pair".
Python currently allows \U literals that cannot be
represented as a single Python character. It generates two
Python characters known as a "surrogate pair". Should this
be disallowed on future narrow Python builds?
* ord() will now accept surrogate pairs and return the ordinal of
the "wide" character. Open question: should it accept surrogate
pairs on wide Python builds?
Pro:
Python already 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. It is also
somewhat logical considering that the Unicode-literal syntax
is basically a short-form way of invoking the unicode-escape
codec.
Con:
Surrogates could be easily created this way but the user
still needs to be careful about slicing, indexing, printing
etc. Therefore some have suggested that Unicode
literals should not support surrogates.
ISSUE
Should Python allow the construction of characters that do
not correspond to Unicode code points? Unassigned Unicode
code points 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?
Pro:
If a Python user thinks they know what they're doing why
should we try to prevent them from violating the Unicode
spec? After all, we don't stop 8-bit strings from
containing non-ASCII characters.
Con:
Codecs and other Unicode-consuming code will have to be
careful of these characters which are disallowed by the
Unicode specification.
* 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 on narrow builds of
Python. On wide builds it could be either TOPCHAR or 2**32-1.
That's an open question.
largest ordinal for a character in a Unicode string on the current
interpreter. sys.maxunicode is 2**16-1 (0xffff) on narrow builds
of Python and TOPCHAR on wide builds.
* Note that ord() can in some cases return ordinals higher than
sys.maxunicode because it accepts surrogate pairs on narrow
Python 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.unicodewidth which can take the
values 'wide' and 'narrow'.
* codecs will be upgraded to support "wide characters". On narrow
Python builds, the codecs will generate surrogate pairs, on wide
Python builds they will generate a single character.
* every Python Unicode character represents exactly one Unicode code
point (i.e. Python Unicode Character = Abstract Unicode character).
* new codecs will be written for 4-byte Unicode and older codecs
will be updated to recognize surrogates and map them to wide
characters on wide Pythons.
* codecs will be upgraded to support "wide characters"
(represented directly in UCS-4, and as variable-length sequences
in UTF-8 and UTF-16). 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
"lone surrogates". The codecs should disallow reading these but
you could construct them using string literals or unichr().
* There is a convention in the Unicode world for encoding a 32-bit
code point in terms of two 16-bit code points. These are known
as "surrogate pairs". Python's codecs will adopt this convention
and encode 32-bit code points as surrogate pairs on narrow Python
builds.
ISSUE
Should there be a way to tell codecs not to generate
surrogates and instead treat wide characters as
errors?
Pro:
I might want to write code that works only with
fixed-width characters and does not have to worry about
surrogates.
Con:
No clear proposal of how to communicate this to codecs.
* 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 from files, but you could construct them using string
literals or unichr().
Implementation
There is a new (experimental) define in Include/unicodeobject.h:
There is a new (experimental) define:
#undef USE_UCS4_STORAGE
#define PY_UNICODE_SIZE 2
if defined, Py_UNICODE is set to the same thing as Py_UCS4.
USE_UCS4_STORAGE
There is a new configure options:
There is a new configure option:
--enable-unicode=ucs2 configures a narrow Py_UNICODE, and uses
wchar_t if it fits
--enable-unicode=ucs4 configures a wide Py_UNICODE likewise
--enable-unicode configures Py_UNICODE to wchar_t if available,
and to UCS-4 if not; this is the default
--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.
It is also proposed that one day --enable-unicode will just
default to the width of your platforms wchar_t.
Windows builds will be narrow for a while based on the fact that
there have been few requests for wide characters, those requests
are mostly from hard-core programmers with the ability to buy
their own Python and Windows itself is strongly biased towards
16-bit characters.
Notes
Note that len(unichr(i))==2 for i>=0x10000 on narrow machines.
This means (for example) that the following code is not portable:
x = 0x10000
if unichr(x) in somestring:
...
In general, you should be careful using "in" if the character that
is searched for could have been generated from unichr applied to a
number greater than 0x10000 or from a string literal greater than
0x10000.
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.
4-byte encoding for their files on disk or sent over the network.
It only allows them to do so. For example, ASCII is still a
legitimate (7-bit) Unicode-encoding.
It has been proposed that there should be a module that handles
surrogates in narrow Python builds for programmers. If someone
wants to implement that, it will be another PEP. It might also be
combined with features that allow other kinds of character-,
word- and line- based indexing.
Open Questions
Rejected Suggestions
"Code points" above TOPCHAR cannot be expressed in two 16-bit
characters. These are not assigned to Unicode characters and
supposedly will never be. Should we allow them to be passed as
arguments to unichr() anyhow? We could allow knowledgable
programmers to use these "unused" characters for whatever they
want, though Unicode does not address them.
More or less the status-quo
"Lone surrogates" "should not" occur on wide platforms. Should
ord() still accept them?
We could officially say that Python characters are 16-bit and
require programmers to implement wide characters in their
application logic by combining surrogate pairs. 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.
"Space-efficient Unicode" type
Another 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. Guido is not willing to undertake the
implementation right now.
Two types
We could introduce a 32-bit Unicode type alongside the 16-bit
type. There is a lot of code that expects there to be only a
single Unicode type.
This PEP represents the least-effort solution. Over the next
several years, 32-bit Unicode characters will become more common
and that may either convince us that we need a more sophisticated
solution or (on the other hand) convince us that simply
mandating wide Unicode characters is an appropriate solution.
Right now the two options on the table are do nothing or do
this.
References
Unicode Glossary: http://www.unicode.org/glossary/
Copyright
This document has been placed in the public domain.
Local Variables: