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Marc-Andre Lemburg: Python Unicode integration proposal, version 1.2.

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=============================================================================
Python Unicode Integration Proposal Version: 1.2
-----------------------------------------------------------------------------
Introduction:
-------------
The idea of this proposal is to add native Unicode 3.0 support to
Python in a way that makes use of Unicode strings as simple as
possible without introducing too many pitfalls along the way.
Since this goal is not easy to achieve -- strings being one of the
most fundamental objects in Python --, we expect this proposal to
undergo some significant refinements.
Note that the current version of this proposal is still a bit unsorted
due to the many different aspects of the Unicode-Python integration.
The latest version of this document is always available at:
http://starship.skyport.net/~lemburg/unicode-proposal.txt
Older versions are available as:
http://starship.skyport.net/~lemburg/unicode-proposal-X.X.txt
Conventions:
------------
· In examples we use u = Unicode object and s = Python string
· 'XXX' markings indicate points of discussion (PODs)
General Remarks:
----------------
· Unicode encoding names should be lower case on output and
case-insensitive on input (they will be converted to lower case
by all APIs taking an encoding name as input).
Encoding names should follow the name conventions as used by the
Unicode Consortium: spaces are converted to hyphens, e.g. 'utf 16' is
written as 'utf-16'.
Codec modules should use the same names, but with hyphens converted
to underscores, e.g. utf_8, utf_16, iso_8859_1.
· The <default encoding> should be the widely used 'utf-8' format. This
is very close to the standard 7-bit ASCII format and thus resembles the
standard used programming nowadays in most aspects.
Unicode Constructors:
---------------------
Python should provide a built-in constructor for Unicode strings which
is available through __builtins__:
u = unicode(encoded_string[,encoding=<default encoding>][,errors="strict"])
u = u'<unicode-escape encoded Python string>'
u = ur'<raw-unicode-escape encoded Python string>'
With the 'unicode-escape' encoding being defined as:
· all non-escape characters represent themselves as Unicode ordinal
(e.g. 'a' -> U+0061).
· all existing defined Python escape sequences are interpreted as
Unicode ordinals; note that \xXXXX can represent all Unicode
ordinals, and \OOO (octal) can represent Unicode ordinals up to U+01FF.
· a new escape sequence, \uXXXX, represents U+XXXX; it is a syntax
error to have fewer than 4 digits after \u.
For an explanation of possible values for errors see the Codec section
below.
Examples:
u'abc' -> U+0061 U+0062 U+0063
u'\u1234' -> U+1234
u'abc\u1234\n' -> U+0061 U+0062 U+0063 U+1234 U+005c
The 'raw-unicode-escape' encoding is defined as follows:
· \uXXXX sequence represent the U+XXXX Unicode character if and
only if the number of leading backslashes is odd
· all other characters represent themselves as Unicode ordinal
(e.g. 'b' -> U+0062)
Note that you should provide some hint to the encoding you used to
write your programs as pragma line in one the first few comment lines
of the source file (e.g. '# source file encoding: latin-1'). If you
only use 7-bit ASCII then everything is fine and no such notice is
needed, but if you include Latin-1 characters not defined in ASCII, it
may well be worthwhile including a hint since people in other
countries will want to be able to read you source strings too.
Unicode Type Object:
--------------------
Unicode objects should have the type UnicodeType with type name
'unicode', made available through the standard types module.
Unicode Output:
---------------
Unicode objects have a method .encode([encoding=<default encoding>])
which returns a Python string encoding the Unicode string using the
given scheme (see Codecs).
print u := print u.encode() # using the <default encoding>
str(u) := u.encode() # using the <default encoding>
repr(u) := "u%s" % repr(u.encode('unicode-escape'))
Also see Internal Argument Parsing and Buffer Interface for details on
how other APIs written in C will treat Unicode objects.
Unicode Ordinals:
-----------------
Since Unicode 3.0 has a 32-bit ordinal character set, the implementation
should provide 32-bit aware ordinal conversion APIs:
ord(u[:1]) (this is the standard ord() extended to work with Unicode
objects)
--> Unicode ordinal number (32-bit)
unichr(i)
--> Unicode object for character i (provided it is 32-bit);
ValueError otherwise
Both APIs should go into __builtins__ just like their string
counterparts ord() and chr().
Note that Unicode provides space for private encodings. Usage of these
can cause different output representations on different machines. This
problem is not a Python or Unicode problem, but a machine setup and
maintenance one.
Comparison & Hash Value:
------------------------
Unicode objects should compare equal to other objects after these
other objects have been coerced to Unicode. For strings this means
that they are interpreted as Unicode string using the <default
encoding>.
For the same reason, Unicode objects should return the same hash value
as their UTF-8 equivalent strings.
Coercion:
---------
Using Python strings and Unicode objects to form new objects should
always coerce to the more precise format, i.e. Unicode objects.
u + s := u + unicode(s)
s + u := unicode(s) + u
All string methods should delegate the call to an equivalent Unicode
object method call by converting all envolved strings to Unicode and
then applying the arguments to the Unicode method of the same name,
e.g.
string.join((s,u),sep) := (s + sep) + u
sep.join((s,u)) := (s + sep) + u
For a discussion of %-formatting w/r to Unicode objects, see
Formatting Markers.
Exceptions:
-----------
UnicodeError is defined in the exceptions module as subclass of
ValueError. It is available at the C level via PyExc_UnicodeError.
All exceptions related to Unicode encoding/decoding should be
subclasses of UnicodeError.
Codecs (Coder/Decoders) Lookup:
-------------------------------
A Codec (see Codec Interface Definition) search registry should be
implemented by a module "codecs":
codecs.register(search_function)
Search functions are expected to take one argument, the encoding name
in all lower case letters, and return a tuple of functions (encoder,
decoder, stream_reader, stream_writer) taking the following arguments:
encoder and decoder:
These must be functions or methods which have the same
interface as the .encode/.decode methods of Codec instances
(see Codec Interface). The functions/methods are expected to
work in a stateless mode.
stream_reader and stream_writer:
These need to be factory functions with the following
interface:
factory(stream,errors='strict')
The factory functions must return objects providing
the interfaces defined by StreamWriter/StreamReader resp.
(see Codec Interface). Stream codecs can maintain state.
Possible values for errors are defined in the Codec
section below.
In case a search function cannot find a given encoding, it should
return None.
Aliasing support for encodings is left to the search functions
to implement.
The codecs module will maintain an encoding cache for performance
reasons. Encodings are first looked up in the cache. If not found, the
list of registered search functions is scanned. If no codecs tuple is
found, a LookupError is raised. Otherwise, the codecs tuple is stored
in the cache and returned to the caller.
To query the Codec instance the following API should be used:
codecs.lookup(encoding)
This will either return the found codecs tuple or raise a LookupError.
Standard Codecs:
----------------
Standard codecs should live inside an encodings/ package directory in the
Standard Python Code Library. The __init__.py file of that directory should
include a Codec Lookup compatible search function implementing a lazy module
based codec lookup.
Python should provide a few standard codecs for the most relevant
encodings, e.g.
'utf-8': 8-bit variable length encoding
'utf-16': 16-bit variable length encoding (litte/big endian)
'utf-16-le': utf-16 but explicitly little endian
'utf-16-be': utf-16 but explicitly big endian
'ascii': 7-bit ASCII codepage
'iso-8859-1': ISO 8859-1 (Latin 1) codepage
'unicode-escape': See Unicode Constructors for a definition
'raw-unicode-escape': See Unicode Constructors for a definition
'native': Dump of the Internal Format used by Python
Common aliases should also be provided per default, e.g. 'latin-1'
for 'iso-8859-1'.
Note: 'utf-16' should be implemented by using and requiring byte order
marks (BOM) for file input/output.
All other encodings such as the CJK ones to support Asian scripts
should be implemented in seperate packages which do not get included
in the core Python distribution and are not a part of this proposal.
Codecs Interface Definition:
----------------------------
The following base class should be defined in the module
"codecs". They provide not only templates for use by encoding module
implementors, but also define the interface which is expected by the
Unicode implementation.
Note that the Codec Interface defined here is well suitable for a
larger range of applications. The Unicode implementation expects
Unicode objects on input for .encode() and .write() and character
buffer compatible objects on input for .decode(). Output of .encode()
and .read() should be a Python string and .decode() must return an
Unicode object.
First, we have the stateless encoders/decoders. These do not work in
chunks as the stream codecs (see below) do, because all components are
expected to be available in memory.
class Codec:
""" Defines the interface for stateless encoders/decoders.
The .encode()/.decode() methods may implement different error
handling schemes by providing the errors argument. These
string values are defined:
'strict' - raise an error (or a subclass)
'ignore' - ignore the character and continue with the next
'replace' - replace with a suitable replacement character;
Python will use the official U+FFFD REPLACEMENT
CHARACTER for the builtin Unicode codecs.
"""
def encode(self,input,errors='strict'):
""" Encodes the object intput and returns a tuple (output
object, length consumed).
errors defines the error handling to apply. It defaults to
'strict' handling.
The method may not store state in the Codec instance. Use
SteamCodec for codecs which have to keep state in order to
make encoding/decoding efficient.
"""
...
def decode(self,input,errors='strict'):
""" Decodes the object input and returns a tuple (output
object, length consumed).
input must be an object which provides the bf_getreadbuf
buffer slot. Python strings, buffer objects and memory
mapped files are examples of objects providing this slot.
errors defines the error handling to apply. It defaults to
'strict' handling.
The method may not store state in the Codec instance. Use
SteamCodec for codecs which have to keep state in order to
make encoding/decoding efficient.
"""
...
StreamWriter and StreamReader define the interface for stateful
encoders/decoders which work on streams. These allow processing of the
data in chunks to efficiently use memory. If you have large strings in
memory, you may want to wrap them with cStringIO objects and then use
these codecs on them to be able to do chunk processing as well,
e.g. to provide progress information to the user.
class StreamWriter(Codec):
def __init__(self,stream,errors='strict'):
""" Creates a StreamWriter instance.
stream must be a file-like object open for writing
(binary) data.
The StreamWriter may implement different error handling
schemes by providing the errors keyword argument. These
parameters are defined:
'strict' - raise a ValueError (or a subclass)
'ignore' - ignore the character and continue with the next
'replace'- replace with a suitable replacement character
"""
self.stream = stream
self.errors = errors
def write(self,object):
""" Writes the object's contents encoded to self.stream.
"""
data, consumed = self.encode(object,self.errors)
self.stream.write(data)
def reset(self):
""" Flushes and resets the codec buffers used for keeping state.
Calling this method should ensure that the data on the
output is put into a clean state, that allows appending
of new fresh data without having to rescan the whole
stream to recover state.
"""
pass
def __getattr__(self,name,
getattr=getattr):
""" Inherit all other methods from the underlying stream.
"""
return getattr(self.stream,name)
class StreamReader(Codec):
def __init__(self,stream,errors='strict'):
""" Creates a StreamReader instance.
stream must be a file-like object open for reading
(binary) data.
The StreamReader may implement different error handling
schemes by providing the errors keyword argument. These
parameters are defined:
'strict' - raise a ValueError (or a subclass)
'ignore' - ignore the character and continue with the next
'replace'- replace with a suitable replacement character;
"""
self.stream = stream
self.errors = errors
def read(self,size=-1):
""" Decodes data from the stream self.stream and returns the
resulting object.
size indicates the approximate maximum number of bytes to
read from the stream for decoding purposes. The decoder
can modify this setting as appropriate. The default value
-1 indicates to read and decode as much as possible. size
is intended to prevent having to decode huge files in one
step.
The method should use a greedy read strategy meaning that
it should read as much data as is allowed within the
definition of the encoding and the given size, e.g. if
optional encoding endings or state markers are available
on the stream, these should be read too.
"""
# Unsliced reading:
if size < 0:
return self.decode(self.stream.read())[0]
# Sliced reading:
read = self.stream.read
decode = self.decode
data = read(size)
i = 0
while 1:
try:
object, decodedbytes = decode(data)
except ValueError,why:
# This method is slow but should work under pretty much
# all conditions; at most 10 tries are made
i = i + 1
newdata = read(1)
if not newdata or i > 10:
raise
data = data + newdata
else:
return object
def reset(self):
""" Resets the codec buffers used for keeping state.
Note that no stream repositioning should take place.
This method is primarely intended to be able to recover
from decoding errors.
"""
pass
def __getattr__(self,name,
getattr=getattr):
""" Inherit all other methods from the underlying stream.
"""
return getattr(self.stream,name)
XXX What about .readline(), .readlines() ? These could be implemented
using .read() as generic functions instead of requiring their
implementation by all codecs. Also see Line Breaks.
Stream codec implementors are free to combine the StreamWriter and
StreamReader interfaces into one class. Even combining all these with
the Codec class should be possible.
Implementors are free to add additional methods to enhance the codec
functionality or provide extra state information needed for them to
work. The internal codec implementation will only use the above
interfaces, though.
It is not required by the Unicode implementation to use these base
classes, only the interfaces must match; this allows writing Codecs as
extensions types.
As guideline, large mapping tables should be implemented using static
C data in separate (shared) extension modules. That way multiple
processes can share the same data.
A tool to auto-convert Unicode mapping files to mapping modules should be
provided to simplify support for additional mappings (see References).
Whitespace:
-----------
The .split() method will have to know about what is considered
whitespace in Unicode.
Case Conversion:
----------------
Case conversion is rather complicated with Unicode data, since there
are many different conditions to respect. See
http://www.unicode.org/unicode/reports/tr13/
for some guidelines on implementing case conversion.
For Python, we should only implement the 1-1 conversions included in
Unicode. Locale dependent and other special case conversions (see the
Unicode standard file SpecialCasing.txt) should be left to user land
routines and not go into the core interpreter.
The methods .capitalize() and .iscapitalized() should follow the case
mapping algorithm defined in the above technical report as closely as
possible.
Line Breaks:
------------
Line breaking should be done for all Unicode characters having the B
property as well as the combinations CRLF, CR, LF (interpreted in that
order) and other special line separators defined by the standard.
The Unicode type should provide a .splitlines() method which returns a
list of lines according to the above specification. See Unicode
Methods.
Unicode Character Properties:
-----------------------------
A separate module "unicodedata" should provide a compact interface to
all Unicode character properties defined in the standard's
UnicodeData.txt file.
Among other things, these properties provide ways to recognize
numbers, digits, spaces, whitespace, etc.
Since this module will have to provide access to all Unicode
characters, it will eventually have to contain the data from
UnicodeData.txt which takes up around 600kB. For this reason, the data
should be stored in static C data. This enables compilation as shared
module which the underlying OS can shared between processes (unlike
normal Python code modules).
There should be a standard Python interface for accessing this information
so that other implementors can plug in their own possibly enhanced versions,
e.g. ones that do decompressing of the data on-the-fly.
Private Code Point Areas:
-------------------------
Support for these is left to user land Codecs and not explicitly
intergrated into the core. Note that due to the Internal Format being
implemented, only the area between \uE000 and \uF8FF is useable for
private encodings.
Internal Format:
----------------
The internal format for Unicode objects should use a Python specific
fixed format <PythonUnicode> implemented as 'unsigned short' (or
another unsigned numeric type having 16 bits). Byte order is platform
dependent.
This format will hold UTF-16 encodings of the corresponding Unicode
ordinals. The Python Unicode implementation will address these values
as if they were UCS-2 values. UCS-2 and UTF-16 are the same for all
currently defined Unicode character points. UTF-16 without surrogates
provides access to about 64k characters and covers all characters in
the Basic Multilingual Plane (BMP) of Unicode.
It is the Codec's responsibility to ensure that the data they pass to
the Unicode object constructor repects this assumption. The
constructor does not check the data for Unicode compliance or use of
surrogates.
Future implementations can extend the 32 bit restriction to the full
set of all UTF-16 addressable characters (around 1M characters).
The Unicode API should provide inteface routines from <PythonUnicode>
to the compiler's wchar_t which can be 16 or 32 bit depending on the
compiler/libc/platform being used.
Unicode objects should have a pointer to a cached Python string object
<defencstr> holding the object's value using the current <default
encoding>. This is needed for performance and internal parsing (see
Internal Argument Parsing) reasons. The buffer is filled when the
first conversion request to the <default encoding> is issued on the
object.
Interning is not needed (for now), since Python identifiers are
defined as being ASCII only.
codecs.BOM should return the byte order mark (BOM) for the format
used internally. The codecs module should provide the following
additional constants for convenience and reference (codecs.BOM will
either be BOM_BE or BOM_LE depending on the platform):
BOM_BE: '\376\377'
(corresponds to Unicode U+0000FEFF in UTF-16 on big endian
platforms == ZERO WIDTH NO-BREAK SPACE)
BOM_LE: '\377\376'
(corresponds to Unicode U+0000FFFE in UTF-16 on little endian
platforms == defined as being an illegal Unicode character)
BOM4_BE: '\000\000\376\377'
(corresponds to Unicode U+0000FEFF in UCS-4)
BOM4_LE: '\377\376\000\000'
(corresponds to Unicode U+0000FFFE in UCS-4)
Note that Unicode sees big endian byte order as being "correct". The
swapped order is taken to be an indicator for a "wrong" format, hence
the illegal character definition.
The configure script should provide aid in deciding whether Python can
use the native wchar_t type or not (it has to be a 16-bit unsigned
type).
Buffer Interface:
-----------------
Implement the buffer interface using the <defencstr> Python string
object as basis for bf_getcharbuf (corresponds to the "t#" argument
parsing marker) and the internal buffer for bf_getreadbuf (corresponds
to the "s#" argument parsing marker). If bf_getcharbuf is requested
and the <defencstr> object does not yet exist, it is created first.
This has the advantage of being able to write to output streams (which
typically use this interface) without additional specification of the
encoding to use.
The internal format can also be accessed using the 'unicode-internal'
codec, e.g. via u.encode('unicode-internal').
Pickle/Marshalling:
-------------------
Should have native Unicode object support. The objects should be
encoded using platform independent encodings.
Marshal should use UTF-8 and Pickle should either choose
Raw-Unicode-Escape (in text mode) or UTF-8 (in binary mode) as
encoding. Using UTF-8 instead of UTF-16 has the advantage of
eliminating the need to store a BOM mark.
Regular Expressions:
--------------------
Secret Labs AB is working on a Unicode-aware regular expression
machinery. It works on plain 8-bit, UCS-2, and (optionally) UCS-4
internal character buffers.
Also see
http://www.unicode.org/unicode/reports/tr18/
for some remarks on how to treat Unicode REs.
Formatting Markers:
-------------------
Format markers are used in Python format strings. If Python strings
are used as format strings, the following interpretations should be in
effect:
'%s': '%s' does str(u) for Unicode objects embedded
in Python strings, so the output will be
u.encode(<default encoding>)
In case the format string is an Unicode object, all parameters are coerced
to Unicode first and then put together and formatted according to the format
string. Numbers are first converted to strings and then to Unicode.
'%s': Python strings are interpreted as Unicode
string using the <default encoding>. Unicode
objects are taken as is.
All other string formatters should work accordingly.
Example:
u"%s %s" % (u"abc", "abc") == u"abc abc"
Internal Argument Parsing:
--------------------------
These markers are used by the PyArg_ParseTuple() APIs:
'U': Check for Unicode object and return a pointer to it
's': For Unicode objects: auto convert them to the <default encoding>
and return a pointer to the object's <defencstr> buffer.
's#': Access to the Unicode object via the bf_getreadbuf buffer interface
(see Buffer Interface); note that the length relates to the buffer
length, not the Unicode string length (this may be different
depending on the Internal Format).
't#': Access to the Unicode object via the bf_getcharbuf buffer interface
(see Buffer Interface); note that the length relates to the buffer
length, not necessarily to the Unicode string length (this may
be different depending on the <default encoding>).
File/Stream Output:
-------------------
Since file.write(object) and most other stream writers use the "s#"
argument parsing marker for binary files and "t#" for text files, the
buffer interface implementation determines the encoding to use (see
Buffer Interface).
For explicit handling of files using Unicode, the standard
stream codecs as available through the codecs module should
be used.
XXX There should be a short-cut open(filename,mode,encoding) available which
also assures that mode contains the 'b' character when needed.
File/Stream Input:
------------------
Only the user knows what encoding the input data uses, so no special
magic is applied. The user will have to explicitly convert the string
data to Unicode objects as needed or use the file wrappers defined in
the codecs module (see File/Stream Output).
Unicode Methods & Attributes:
-----------------------------
All Python string methods, plus:
.encode([encoding=<default encoding>][,errors="strict"])
--> see Unicode Output
.splitlines([include_breaks=0])
--> breaks the Unicode string into a list of (Unicode) lines;
returns the lines with line breaks included, if include_breaks
is true. See Line Breaks for a specification of how line breaking
is done.
Code Base:
----------
We should use Fredrik Lundh's Unicode object implementation as basis.
It already implements most of the string methods needed and provides a
well written code base which we can build upon.
The object sharing implemented in Fredrik's implementation should
be dropped.
Test Cases:
-----------
Test cases should follow those in Lib/test/test_string.py and include
additional checks for the Codec Registry and the Standard Codecs.
References:
-----------
Unicode Consortium:
http://www.unicode.org/
Unicode FAQ:
http://www.unicode.org/unicode/faq/
Unicode 3.0:
http://www.unicode.org/unicode/standard/versions/Unicode3.0.html
Unicode-TechReports:
http://www.unicode.org/unicode/reports/techreports.html
Unicode-Mappings:
ftp://ftp.unicode.org/Public/MAPPINGS/
Introduction to Unicode (a little outdated by still nice to read):
http://www.nada.kth.se/i18n/ucs/unicode-iso10646-oview.html
Encodings:
Overview:
http://czyborra.com/utf/
UTC-2:
http://www.uazone.com/multiling/unicode/ucs2.html
UTF-7:
Defined in RFC2152, e.g.
http://www.uazone.com/multiling/ml-docs/rfc2152.txt
UTF-8:
Defined in RFC2279, e.g.
http://info.internet.isi.edu/in-notes/rfc/files/rfc2279.txt
UTF-16:
http://www.uazone.com/multiling/unicode/wg2n1035.html
History of this Proposal:
-------------------------
1.2:
1.1: Added note about comparisons and hash values. Added note about
case mapping algorithms. Changed stream codecs .read() and
.write() method to match the standard file-like object methods
(bytes consumed information is no longer returned by the methods)
1.0: changed encode Codec method to be symmetric to the decode method
(they both return (object, data consumed) now and thus become
interchangeable); removed __init__ method of Codec class (the
methods are stateless) and moved the errors argument down to the
methods; made the Codec design more generic w/r to type of input
and output objects; changed StreamWriter.flush to StreamWriter.reset
in order to avoid overriding the stream's .flush() method;
renamed .breaklines() to .splitlines(); renamed the module unicodec
to codecs; modified the File I/O section to refer to the stream codecs.
0.9: changed errors keyword argument definition; added 'replace' error
handling; changed the codec APIs to accept buffer like objects on
input; some minor typo fixes; added Whitespace section and
included references for Unicode characters that have the whitespace
and the line break characteristic; added note that search functions
can expect lower-case encoding names; dropped slicing and offsets
in the codec APIs
0.8: added encodings package and raw unicode escape encoding; untabified
the proposal; added notes on Unicode format strings; added
.breaklines() method
0.7: added a whole new set of codec APIs; added a different encoder
lookup scheme; fixed some names
0.6: changed "s#" to "t#"; changed <defencbuf> to <defencstr> holding
a real Python string object; changed Buffer Interface to delegate
requests to <defencstr>'s buffer interface; removed the explicit
reference to the unicodec.codecs dictionary (the module can implement
this in way fit for the purpose); removed the settable default
encoding; move UnicodeError from unicodec to exceptions; "s#"
not returns the internal data; passed the UCS-2/UTF-16 checking
from the Unicode constructor to the Codecs
0.5: moved sys.bom to unicodec.BOM; added sections on case mapping,
private use encodings and Unicode character properties
0.4: added Codec interface, notes on %-formatting, changed some encoding
details, added comments on stream wrappers, fixed some discussion
points (most important: Internal Format), clarified the
'unicode-escape' encoding, added encoding references
0.3: added references, comments on codec modules, the internal format,
bf_getcharbuffer and the RE engine; added 'unicode-escape' encoding
proposed by Tim Peters and fixed repr(u) accordingly
0.2: integrated Guido's suggestions, added stream codecs and file
wrapping
0.1: first version
-----------------------------------------------------------------------------
Written by Marc-Andre Lemburg, 1999-2000, mal@lemburg.com
-----------------------------------------------------------------------------