*****************

  Unicode HOWTO

*****************

:Release: 1.02

This HOWTO discusses Python's support for Unicode, and explains various problems

that people commonly encounter when trying to work with Unicode.

Introduction to Unicode

=======================

History of Character Codes

--------------------------

In 1968, the American Standard Code for Information Interchange, better known by

its acronym ASCII, was standardized.  ASCII defined numeric codes for various

characters, with the numeric values running from 0 to

127.  For example, the lowercase letter 'a' is assigned 97 as its code

value.

ASCII was an American-developed standard, so it only defined unaccented

characters.  There was an 'e', but no 'é' or 'Í'.  This meant that languages

which required accented characters couldn't be faithfully represented in ASCII.

(Actually the missing accents matter for English, too, which contains words such

as 'naïve' and 'café', and some publications have house styles which require

spellings such as 'coöperate'.)

For a while people just wrote programs that didn't display accents.  I remember

looking at Apple ][ BASIC programs, published in French-language publications in

the mid-1980s, that had lines like these::

	PRINT "FICHER EST COMPLETE."

	PRINT "CARACTERE NON ACCEPTE."

Those messages should contain accents, and they just look wrong to someone who

can read French.

In the 1980s, almost all personal computers were 8-bit, meaning that bytes could

hold values ranging from 0 to 255.  ASCII codes only went up to 127, so some

machines assigned values between 128 and 255 to accented characters.  Different

machines had different codes, however, which led to problems exchanging files.

Eventually various commonly used sets of values for the 128-255 range emerged.

Some were true standards, defined by the International Standards Organization,

and some were **de facto** conventions that were invented by one company or

another and managed to catch on.

255 characters aren't very many.  For example, you can't fit both the accented

characters used in Western Europe and the Cyrillic alphabet used for Russian

into the 128-255 range because there are more than 127 such characters.

You could write files using different codes (all your Russian files in a coding

system called KOI8, all your French files in a different coding system called

Latin1), but what if you wanted to write a French document that quotes some

Russian text?  In the 1980s people began to want to solve this problem, and the

Unicode standardization effort began.

Unicode started out using 16-bit characters instead of 8-bit characters.  16

bits means you have 2^16 = 65,536 distinct values available, making it possible

to represent many different characters from many different alphabets; an initial

goal was to have Unicode contain the alphabets for every single human language.

It turns out that even 16 bits isn't enough to meet that goal, and the modern

Unicode specification uses a wider range of codes, 0-1,114,111 (0x10ffff in

base-16).

There's a related ISO standard, ISO 10646.  Unicode and ISO 10646 were

originally separate efforts, but the specifications were merged with the 1.1

revision of Unicode.

(This discussion of Unicode's history is highly simplified.  I don't think the

average Python programmer needs to worry about the historical details; consult

the Unicode consortium site listed in the References for more information.)

Definitions

-----------

A **character** is the smallest possible component of a text.  'A', 'B', 'C',

etc., are all different characters.  So are 'È' and 'Í'.  Characters are

abstractions, and vary depending on the language or context you're talking

about.  For example, the symbol for ohms (Ω) is usually drawn much like the

capital letter omega (Ω) in the Greek alphabet (they may even be the same in

some fonts), but these are two different characters that have different

meanings.

The Unicode standard describes how characters are represented by **code

points**.  A code point is an integer value, usually denoted in base 16.  In the

standard, a code point is written using the notation U+12ca to mean the

character with value 0x12ca (4810 decimal).  The Unicode standard contains a lot

of tables listing characters and their corresponding code points::

	0061    'a'; LATIN SMALL LETTER A

	0062    'b'; LATIN SMALL LETTER B

	0063    'c'; LATIN SMALL LETTER C

        ...

	007B	'{'; LEFT CURLY BRACKET

Strictly, these definitions imply that it's meaningless to say 'this is

character U+12ca'.  U+12ca is a code point, which represents some particular

character; in this case, it represents the character 'ETHIOPIC SYLLABLE WI'.  In

informal contexts, this distinction between code points and characters will

sometimes be forgotten.

A character is represented on a screen or on paper by a set of graphical

elements that's called a **glyph**.  The glyph for an uppercase A, for example,

is two diagonal strokes and a horizontal stroke, though the exact details will

depend on the font being used.  Most Python code doesn't need to worry about

glyphs; figuring out the correct glyph to display is generally the job of a GUI

toolkit or a terminal's font renderer.

Encodings

---------

To summarize the previous section: a Unicode string is a sequence of code

points, which are numbers from 0 to 0x10ffff.  This sequence needs to be

represented as a set of bytes (meaning, values from 0-255) in memory.  The rules

for translating a Unicode string into a sequence of bytes are called an

**encoding**.

The first encoding you might think of is an array of 32-bit integers.  In this

representation, the string "Python" would look like this::

       P           y           t           h           o           n

    0x50 00 00 00 79 00 00 00 74 00 00 00 68 00 00 00 6f 00 00 00 6e 00 00 00 

       0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 

This representation is straightforward but using it presents a number of

problems.

1. It's not portable; different processors order the bytes differently.

2. It's very wasteful of space.  In most texts, the majority of the code points

   are less than 127, or less than 255, so a lot of space is occupied by zero

   bytes.  The above string takes 24 bytes compared to the 6 bytes needed for an

   ASCII representation.  Increased RAM usage doesn't matter too much (desktop

   computers have megabytes of RAM, and strings aren't usually that large), but

   expanding our usage of disk and network bandwidth by a factor of 4 is

   intolerable.

3. It's not compatible with existing C functions such as ``strlen()``, so a new

   family of wide string functions would need to be used.

4. Many Internet standards are defined in terms of textual data, and can't

   handle content with embedded zero bytes.

Generally people don't use this encoding, instead choosing other encodings that

are more efficient and convenient.

Encodings don't have to handle every possible Unicode character, and most

encodings don't.  For example, Python's default encoding is the 'ascii'

encoding.  The rules for converting a Unicode string into the ASCII encoding are

simple; for each code point:

1. If the code point is < 128, each byte is the same as the value of the code

   point.

2. If the code point is 128 or greater, the Unicode string can't be represented

   in this encoding.  (Python raises a :exc:`UnicodeEncodeError` exception in this

   case.)

Latin-1, also known as ISO-8859-1, is a similar encoding.  Unicode code points

0-255 are identical to the Latin-1 values, so converting to this encoding simply

requires converting code points to byte values; if a code point larger than 255

is encountered, the string can't be encoded into Latin-1.

Encodings don't have to be simple one-to-one mappings like Latin-1.  Consider

IBM's EBCDIC, which was used on IBM mainframes.  Letter values weren't in one

block: 'a' through 'i' had values from 129 to 137, but 'j' through 'r' were 145

through 153.  If you wanted to use EBCDIC as an encoding, you'd probably use

some sort of lookup table to perform the conversion, but this is largely an

internal detail.

UTF-8 is one of the most commonly used encodings.  UTF stands for "Unicode

Transformation Format", and the '8' means that 8-bit numbers are used in the

encoding.  (There's also a UTF-16 encoding, but it's less frequently used than

UTF-8.)  UTF-8 uses the following rules:

1. If the code point is <128, it's represented by the corresponding byte value.

2. If the code point is between 128 and 0x7ff, it's turned into two byte values

   between 128 and 255.

3. Code points >0x7ff are turned into three- or four-byte sequences, where each

   byte of the sequence is between 128 and 255.

UTF-8 has several convenient properties:

1. It can handle any Unicode code point.

2. A Unicode string is turned into a string of bytes containing no embedded zero

   bytes.  This avoids byte-ordering issues, and means UTF-8 strings can be

   processed by C functions such as ``strcpy()`` and sent through protocols that

   can't handle zero bytes.

3. A string of ASCII text is also valid UTF-8 text.

4. UTF-8 is fairly compact; the majority of code points are turned into two

   bytes, and values less than 128 occupy only a single byte.

5. If bytes are corrupted or lost, it's possible to determine the start of the

   next UTF-8-encoded code point and resynchronize.  It's also unlikely that

   random 8-bit data will look like valid UTF-8.

References

----------

The Unicode Consortium site at <http://www.unicode.org> has character charts, a

glossary, and PDF versions of the Unicode specification.  Be prepared for some

difficult reading.  <http://www.unicode.org/history/> is a chronology of the

origin and development of Unicode.

To help understand the standard, Jukka Korpela has written an introductory guide

to reading the Unicode character tables, available at

<http://www.cs.tut.fi/~jkorpela/unicode/guide.html>.

Two other good introductory articles were written by Joel Spolsky

<http://www.joelonsoftware.com/articles/Unicode.html> and Jason Orendorff

<http://www.jorendorff.com/articles/unicode/>.  If this introduction didn't make

things clear to you, you should try reading one of these alternate articles

before continuing.

Wikipedia entries are often helpful; see the entries for "character encoding"

<http://en.wikipedia.org/wiki/Character_encoding> and UTF-8

<http://en.wikipedia.org/wiki/UTF-8>, for example.

Python's Unicode Support

========================

Now that you've learned the rudiments of Unicode, we can look at Python's

Unicode features.

The Unicode Type

----------------

Unicode strings are expressed as instances of the :class:`unicode` type, one of

Python's repertoire of built-in types.  It derives from an abstract type called

:class:`basestring`, which is also an ancestor of the :class:`str` type; you can

therefore check if a value is a string type with ``isinstance(value,

basestring)``.  Under the hood, Python represents Unicode strings as either 16-

or 32-bit integers, depending on how the Python interpreter was compiled.

The :func:`unicode` constructor has the signature ``unicode(string[, encoding,

errors])``.  All of its arguments should be 8-bit strings.  The first argument

is converted to Unicode using the specified encoding; if you leave off the

``encoding`` argument, the ASCII encoding is used for the conversion, so

characters greater than 127 will be treated as errors::

    >>> unicode('abcdef')

    u'abcdef'

    >>> s = unicode('abcdef')

    >>> type(s)

    <type 'unicode'>

    >>> unicode('abcdef' + chr(255))

    Traceback (most recent call last):

      File "<stdin>", line 1, in ?

    UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 6: 

                        ordinal not in range(128)

The ``errors`` argument specifies the response when the input string can't be

converted according to the encoding's rules.  Legal values for this argument are

'strict' (raise a ``UnicodeDecodeError`` exception), 'replace' (add U+FFFD,

'REPLACEMENT CHARACTER'), or 'ignore' (just leave the character out of the

Unicode result).  The following examples show the differences::

    >>> unicode('\x80abc', errors='strict')

    Traceback (most recent call last):

      File "<stdin>", line 1, in ?

    UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 0: 

                        ordinal not in range(128)

    >>> unicode('\x80abc', errors='replace')

    u'\ufffdabc'

    >>> unicode('\x80abc', errors='ignore')

    u'abc'

Encodings are specified as strings containing the encoding's name.  Python 2.4

comes with roughly 100 different encodings; see the Python Library Reference at

:ref:`standard-encodings` for a list.  Some encodings

have multiple names; for example, 'latin-1', 'iso_8859_1' and '8859' are all

synonyms for the same encoding.

One-character Unicode strings can also be created with the :func:`unichr`

built-in function, which takes integers and returns a Unicode string of length 1

that contains the corresponding code point.  The reverse operation is the

built-in :func:`ord` function that takes a one-character Unicode string and

returns the code point value::

    >>> unichr(40960)

    u'\ua000'

    >>> ord(u'\ua000')

    40960

Instances of the :class:`unicode` type have many of the same methods as the

8-bit string type for operations such as searching and formatting::

    >>> s = u'Was ever feather so lightly blown to and fro as this multitude?'

    >>> s.count('e')

    5

    >>> s.find('feather')

    9

    >>> s.find('bird')

    -1

    >>> s.replace('feather', 'sand')

    u'Was ever sand so lightly blown to and fro as this multitude?'

    >>> s.upper()

    u'WAS EVER FEATHER SO LIGHTLY BLOWN TO AND FRO AS THIS MULTITUDE?'

Note that the arguments to these methods can be Unicode strings or 8-bit

strings.  8-bit strings will be converted to Unicode before carrying out the

operation; Python's default ASCII encoding will be used, so characters greater

than 127 will cause an exception::

    >>> s.find('Was\x9f')

    Traceback (most recent call last):

      File "<stdin>", line 1, in ?

    UnicodeDecodeError: 'ascii' codec can't decode byte 0x9f in position 3: ordinal not in range(128)

    >>> s.find(u'Was\x9f')

    -1

Much Python code that operates on strings will therefore work with Unicode

strings without requiring any changes to the code.  (Input and output code needs

more updating for Unicode; more on this later.)

Another important method is ``.encode([encoding], [errors='strict'])``, which

returns an 8-bit string version of the Unicode string, encoded in the requested

encoding.  The ``errors`` parameter is the same as the parameter of the

``unicode()`` constructor, with one additional possibility; as well as 'strict',

'ignore', and 'replace', you can also pass 'xmlcharrefreplace' which uses XML's

character references.  The following example shows the different results::

    >>> u = unichr(40960) + u'abcd' + unichr(1972)

    >>> u.encode('utf-8')

    '\xea\x80\x80abcd\xde\xb4'

    >>> u.encode('ascii')

    Traceback (most recent call last):

      File "<stdin>", line 1, in ?

    UnicodeEncodeError: 'ascii' codec can't encode character '\ua000' in position 0: ordinal not in range(128)

    >>> u.encode('ascii', 'ignore')

    'abcd'

    >>> u.encode('ascii', 'replace')

    '?abcd?'

    >>> u.encode('ascii', 'xmlcharrefreplace')

    'ꀀabcd޴'

Python's 8-bit strings have a ``.decode([encoding], [errors])`` method that

interprets the string using the given encoding::

    >>> u = unichr(40960) + u'abcd' + unichr(1972)   # Assemble a string

    >>> utf8_version = u.encode('utf-8')             # Encode as UTF-8

    >>> type(utf8_version), utf8_version

    (<type 'str'>, '\xea\x80\x80abcd\xde\xb4')

    >>> u2 = utf8_version.decode('utf-8')            # Decode using UTF-8

    >>> u == u2                                      # The two strings match

    True

The low-level routines for registering and accessing the available encodings are

found in the :mod:`codecs` module.  However, the encoding and decoding functions

returned by this module are usually more low-level than is comfortable, so I'm

not going to describe the :mod:`codecs` module here.  If you need to implement a

completely new encoding, you'll need to learn about the :mod:`codecs` module

interfaces, but implementing encodings is a specialized task that also won't be

covered here.  Consult the Python documentation to learn more about this module.

The most commonly used part of the :mod:`codecs` module is the

:func:`codecs.open` function which will be discussed in the section on input and

output.

Unicode Literals in Python Source Code

--------------------------------------

In Python source code, Unicode literals are written as strings prefixed with the

'u' or 'U' character: ``u'abcdefghijk'``.  Specific code points can be written

using the ``\u`` escape sequence, which is followed by four hex digits giving

the code point.  The ``\U`` escape sequence is similar, but expects 8 hex

digits, not 4.

Unicode literals can also use the same escape sequences as 8-bit strings,

including ``\x``, but ``\x`` only takes two hex digits so it can't express an

arbitrary code point.  Octal escapes can go up to U+01ff, which is octal 777.

::

    >>> s = u"a\xac\u1234\u20ac\U00008000"

               ^^^^ two-digit hex escape

                   ^^^^^^ four-digit Unicode escape 

                               ^^^^^^^^^^ eight-digit Unicode escape

    >>> for c in s:  print ord(c),

    ... 

    97 172 4660 8364 32768

Using escape sequences for code points greater than 127 is fine in small doses,

but becomes an annoyance if you're using many accented characters, as you would

in a program with messages in French or some other accent-using language.  You

can also assemble strings using the :func:`unichr` built-in function, but this is

even more tedious.

Ideally, you'd want to be able to write literals in your language's natural

encoding.  You could then edit Python source code with your favorite editor

which would display the accented characters naturally, and have the right

characters used at runtime.

Python supports writing Unicode literals in any encoding, but you have to

declare the encoding being used.  This is done by including a special comment as

either the first or second line of the source file::

    #!/usr/bin/env python

    # -*- coding: latin-1 -*-

    u = u'abcdé'

    print ord(u[-1])

The syntax is inspired by Emacs's notation for specifying variables local to a

file.  Emacs supports many different variables, but Python only supports

'coding'.  The ``-*-`` symbols indicate that the comment is special; within

them, you must supply the name ``coding`` and the name of your chosen encoding,

separated by ``':'``.

If you don't include such a comment, the default encoding used will be ASCII.

Versions of Python before 2.4 were Euro-centric and assumed Latin-1 as a default

encoding for string literals; in Python 2.4, characters greater than 127 still

work but result in a warning.  For example, the following program has no

encoding declaration::

    #!/usr/bin/env python

    u = u'abcdé'

    print ord(u[-1])

When you run it with Python 2.4, it will output the following warning::

    amk:~$ python p263.py

    sys:1: DeprecationWarning: Non-ASCII character '\xe9' 

         in file p263.py on line 2, but no encoding declared; 

         see http://www.python.org/peps/pep-0263.html for details

Unicode Properties

------------------

The Unicode specification includes a database of information about code points.

For each code point that's defined, the information includes the character's

name, its category, the numeric value if applicable (Unicode has characters

representing the Roman numerals and fractions such as one-third and

four-fifths).  There are also properties related to the code point's use in

bidirectional text and other display-related properties.

The following program displays some information about several characters, and

prints the numeric value of one particular character::

    import unicodedata

    u = unichr(233) + unichr(0x0bf2) + unichr(3972) + unichr(6000) + unichr(13231)

    for i, c in enumerate(u):

        print i, '%04x' % ord(c), unicodedata.category(c),

        print unicodedata.name(c)

    # Get numeric value of second character

    print unicodedata.numeric(u[1])

When run, this prints::

    0 00e9 Ll LATIN SMALL LETTER E WITH ACUTE

    1 0bf2 No TAMIL NUMBER ONE THOUSAND

    2 0f84 Mn TIBETAN MARK HALANTA

    3 1770 Lo TAGBANWA LETTER SA

    4 33af So SQUARE RAD OVER S SQUARED

    1000.0

The category codes are abbreviations describing the nature of the character.

These are grouped into categories such as "Letter", "Number", "Punctuation", or

"Symbol", which in turn are broken up into subcategories.  To take the codes

from the above output, ``'Ll'`` means 'Letter, lowercase', ``'No'`` means

"Number, other", ``'Mn'`` is "Mark, nonspacing", and ``'So'`` is "Symbol,

other".  See

<http://www.unicode.org/Public/UNIDATA/UCD.html#General_Category_Values> for a

list of category codes.

References

----------

The Unicode and 8-bit string types are described in the Python library reference

at :ref:`typesseq`.

The documentation for the :mod:`unicodedata` module.

The documentation for the :mod:`codecs` module.

Marc-André Lemburg gave a presentation at EuroPython 2002 titled "Python and

Unicode".  A PDF version of his slides is available at

<http://downloads.egenix.com/python/Unicode-EPC2002-Talk.pdf>, and is an

excellent overview of the design of Python's Unicode features.

Reading and Writing Unicode Data

================================

Once you've written some code that works with Unicode data, the next problem is

input/output.  How do you get Unicode strings into your program, and how do you

convert Unicode into a form suitable for storage or transmission?

It's possible that you may not need to do anything depending on your input

sources and output destinations; you should check whether the libraries used in

your application support Unicode natively.  XML parsers often return Unicode

data, for example.  Many relational databases also support Unicode-valued

columns and can return Unicode values from an SQL query.

Unicode data is usually converted to a particular encoding before it gets

written to disk or sent over a socket.  It's possible to do all the work

yourself: open a file, read an 8-bit string from it, and convert the string with

``unicode(str, encoding)``.  However, the manual approach is not recommended.

One problem is the multi-byte nature of encodings; one Unicode character can be

represented by several bytes.  If you want to read the file in arbitrary-sized

chunks (say, 1K or 4K), you need to write error-handling code to catch the case

where only part of the bytes encoding a single Unicode character are read at the

end of a chunk.  One solution would be to read the entire file into memory and

then perform the decoding, but that prevents you from working with files that

are extremely large; if you need to read a 2Gb file, you need 2Gb of RAM.

(More, really, since for at least a moment you'd need to have both the encoded

string and its Unicode version in memory.)

The solution would be to use the low-level decoding interface to catch the case

of partial coding sequences.  The work of implementing this has already been

done for you: the :mod:`codecs` module includes a version of the :func:`open`

function that returns a file-like object that assumes the file's contents are in

a specified encoding and accepts Unicode parameters for methods such as

``.read()`` and ``.write()``.

The function's parameters are ``open(filename, mode='rb', encoding=None,