
Unicode HOWTO
*************

Release:
   1.1

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 "FICHIER 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
   ``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 String Type
---------------

Since Python 3.0, the language features a ``str`` type that contain
Unicode characters, meaning any string created using ``"unicode
rocks!"``, ``'unicode rocks!'``, or the triple-quoted string syntax is
stored as Unicode.

To insert a Unicode character that is not part ASCII, e.g., any
letters with accents, one can use escape sequences in their string
literals as such:

   >>> "\N{GREEK CAPITAL LETTER DELTA}"  # Using the character name
   '\u0394'
   >>> "\u0394"                          # Using a 16-bit hex value
   '\u0394'
   >>> "\U00000394"                      # Using a 32-bit hex value
   '\u0394'

In addition, one can create a string using the ``decode()`` method of
``bytes``.  This method takes an encoding, such as UTF-8, and,
optionally, an *errors* argument.

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' (use U+FFFD, 'REPLACEMENT CHARACTER'), or
'ignore' (just leave the character out of the Unicode result).  The
following examples show the differences:

   >>> b'\x80abc'.decode("utf-8", "strict")
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   UnicodeDecodeError: 'utf8' codec can't decode byte 0x80 in position 0:
                       unexpected code byte
   >>> b'\x80abc'.decode("utf-8", "replace")
   '\ufffdabc'
   >>> b'\x80abc'.decode("utf-8", "ignore")
   'abc'

Encodings are specified as strings containing the encoding's name.
Python comes with roughly 100 different encodings; see the Python
Library Reference at *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 ``chr()``
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 ``ord()`` function that takes a one-
character Unicode string and returns the code point value:

   >>> chr(40960)
   '\ua000'
   >>> ord('\ua000')
   40960


Converting to Bytes
-------------------

Another important str method is ``.encode([encoding],
[errors='strict'])``, which returns a ``bytes`` representation of the
Unicode string, encoded in the requested encoding.  The ``errors``
parameter is the same as the parameter of the ``decode()`` method,
with one additional possibility; as well as 'strict', 'ignore', and
'replace' (which in this case inserts a question mark instead of the
unencodable character), you can also pass 'xmlcharrefreplace' which
uses XML's character references.  The following example shows the
different results:

   >>> u = chr(40960) + 'abcd' + chr(1972)
   >>> u.encode('utf-8')
   b'\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')
   b'abcd'
   >>> u.encode('ascii', 'replace')
   b'?abcd?'
   >>> u.encode('ascii', 'xmlcharrefreplace')
   b'&#40960;abcd&#1972;'

The low-level routines for registering and accessing the available
encodings are found in the ``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 ``codecs``
module here.  If you need to implement a completely new encoding,
you'll need to learn about the ``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.


Unicode Literals in Python Source Code
--------------------------------------

In Python source code, specific Unicode 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:

   >>> s = "a\xac\u1234\u20ac\U00008000"
             ^^^^ two-digit hex escape
                  ^^^^^ four-digit Unicode escape
                             ^^^^^^^^^^ eight-digit Unicode escape
   >>> for c in s:  print(ord(c), end=" ")
   ...
   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
``chr()`` 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 source code in UTF-8 by default, but you can
use almost any encoding if you 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 = '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 to Emacs that
the comment is special; they have no significance to Python but are a
convention.  Python looks for ``coding: name`` or ``coding=name`` in
the comment.

If you don't include such a comment, the default encoding used will be
UTF-8 as already mentioned.


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 = chr(233) + chr(0x0bf2) + chr(3972) + chr(6000) + chr(13231)

   for i, c in enumerate(u):
       print(i, '%04x' % ord(c), unicodedata.category(c), end=" ")
       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 ``str`` type is described in the Python library reference at
*Sequence Types --- str, bytes, bytearray, list, tuple, range*.

The documentation for the ``unicodedata`` module.

The documentation for the ``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
(based on Python 2, where the Unicode string type is called
``unicode`` and literals start with ``u``).


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 byte string from it, and
convert the string with ``str(bytes, 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 built-in ``open()`` function can
return 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()``.  This works through ``open()``'s
*encoding* and *errors* parameters which are interpreted just like
those in string objects' ``encode()`` and ``decode()`` methods.

Reading Unicode from a file is therefore simple:

   f = open('unicode.rst', encoding='utf-8')
   for line in f:
       print(repr(line))

It's also possible to open files in update mode, allowing both reading
and writing:

   f = open('test', encoding='utf-8', mode='w+')
   f.write('\u4500 blah blah blah\n')
   f.seek(0)
   print(repr(f.readline()[:1]))
   f.close()

The Unicode character U+FEFF is used as a byte-order mark (BOM), and
is often written as the first character of a file in order to assist
with autodetection of the file's byte ordering.  Some encodings, such
as UTF-16, expect a BOM to be present at the start of a file; when
such an encoding is used, the BOM will be automatically written as the
first character and will be silently dropped when the file is read.
There are variants of these encodings, such as 'utf-16-le' and
'utf-16-be' for little-endian and big-endian encodings, that specify
one particular byte ordering and don't skip the BOM.

In some areas, it is also convention to use a "BOM" at the start of
UTF-8 encoded files; the name is misleading since UTF-8 is not byte-
order dependent. The mark simply announces that the file is encoded in
UTF-8.  Use the 'utf-8-sig' codec to automatically skip the mark if
present for reading such files.


Unicode filenames
-----------------

Most of the operating systems in common use today support filenames
that contain arbitrary Unicode characters.  Usually this is
implemented by converting the Unicode string into some encoding that
varies depending on the system.  For example, Mac OS X uses UTF-8
while Windows uses a configurable encoding; on Windows, Python uses
the name "mbcs" to refer to whatever the currently configured encoding
is.  On Unix systems, there will only be a filesystem encoding if
you've set the ``LANG`` or ``LC_CTYPE`` environment variables; if you
haven't, the default encoding is ASCII.

The ``sys.getfilesystemencoding()`` function returns the encoding to
use on your current system, in case you want to do the encoding
manually, but there's not much reason to bother.  When opening a file
for reading or writing, you can usually just provide the Unicode
string as the filename, and it will be automatically converted to the
right encoding for you:

   filename = 'filename\u4500abc'
   f = open(filename, 'w')
   f.write('blah\n')
   f.close()

Functions in the ``os`` module such as ``os.stat()`` will also accept
Unicode filenames.

``os.listdir()``, which returns filenames, raises an issue: should it
return the Unicode version of filenames, or should it return byte
strings containing the encoded versions?  ``os.listdir()`` will do
both, depending on whether you provided the directory path as a byte
string or a Unicode string.  If you pass a Unicode string as the path,
filenames will be decoded using the filesystem's encoding and a list
of Unicode strings will be returned, while passing a byte path will
return the byte string versions of the filenames.  For example,
assuming the default filesystem encoding is UTF-8, running the
following program:

   fn = 'filename\u4500abc'
   f = open(fn, 'w')
   f.close()

   import os
   print(os.listdir(b'.'))
   print(os.listdir('.'))

will produce the following output:

   amk:~$ python t.py
   [b'.svn', b'filename\xe4\x94\x80abc', ...]
   ['.svn', 'filename\u4500abc', ...]

The first list contains UTF-8-encoded filenames, and the second list
contains the Unicode versions.

Note that in most occasions, the Uniode APIs should be used.  The
bytes APIs should only be used on systems where undecodable file names
can be present, i.e. Unix systems.


Tips for Writing Unicode-aware Programs
---------------------------------------

This section provides some suggestions on writing software that deals
with Unicode.

The most important tip is:

   Software should only work with Unicode strings internally,
   converting to a particular encoding on output.

If you attempt to write processing functions that accept both Unicode
and byte strings, you will find your program vulnerable to bugs
wherever you combine the two different kinds of strings.  There is no
automatic encoding or decoding if you do e.g. ``str + bytes``, a
``TypeError`` is raised for this expression.

It's easy to miss such problems if you only test your software with
data that doesn't contain any accents; everything will seem to work,
but there's actually a bug in your program waiting for the first user
who attempts to use characters > 127.  A second tip, therefore, is:

   Include characters > 127 and, even better, characters > 255 in your
   test data.

When using data coming from a web browser or some other untrusted
source, a common technique is to check for illegal characters in a
string before using the string in a generated command line or storing
it in a database.  If you're doing this, be careful to check the
string once it's in the form that will be used or stored; it's
possible for encodings to be used to disguise characters.  This is
especially true if the input data also specifies the encoding; many
encodings leave the commonly checked-for characters alone, but Python
includes some encodings such as ``'base64'`` that modify every single
character.

For example, let's say you have a content management system that takes
a Unicode filename, and you want to disallow paths with a '/'
character.  You might write this code:

   def read_file(filename, encoding):
       if '/' in filename:
           raise ValueError("'/' not allowed in filenames")
       unicode_name = filename.decode(encoding)
       f = open(unicode_name, 'r')
       # ... return contents of file ...

However, if an attacker could specify the ``'base64'`` encoding, they
could pass ``'L2V0Yy9wYXNzd2Q='``, which is the base-64 encoded form
of the string ``'/etc/passwd'``, to read a system file.  The above
code looks for ``'/'`` characters in the encoded form and misses the
dangerous character in the resulting decoded form.


References
----------

The PDF slides for Marc-André Lemburg's presentation "Writing Unicode-
aware Applications in Python" are available at
<http://downloads.egenix.com/python/LSM2005-Developing-Unicode-aware-
applications-in-Python.pdf> and discuss questions of character
encodings as well as how to internationalize and localize an
application.


Revision History and Acknowledgements
=====================================

Thanks to the following people who have noted errors or offered
suggestions on this article: Nicholas Bastin, Marius Gedminas, Kent
Johnson, Ken Krugler, Marc-André Lemburg, Martin von Löwis, Chad
Whitacre.

Version 1.0: posted August 5 2005.

Version 1.01: posted August 7 2005.  Corrects factual and markup
errors; adds several links.

Version 1.02: posted August 16 2005.  Corrects factual errors.

Version 1.1: Feb-Nov 2008.  Updates the document with respect to
Python 3 changes.
