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bpo-20906: Various revisions to the Unicode howto (#8394) 

* bpo-20906: Corrections & revisions to Unicode HOWTO

* bpo-34484: don't describe range as a Private Use Area
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@ -6,95 +6,48 @@
:Release: 1.12
This HOWTO discusses Python support for Unicode, and explains
various problems that people commonly encounter when trying to work
with Unicode.
This HOWTO discusses Python's support for the Unicode specification
for representing textual data, 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.
In the mid-1980s an Apple II BASIC program written by a French speaker
might have lines like these:
.. code-block:: basic
PRINT "MISE A JOUR TERMINEE"
PRINT "PARAMETRES ENREGISTRES"
Those messages should contain accents (terminée, paramètre, enregistrés) 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 Organization for
Standardization, 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 128 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 through 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. The
precise historical details aren't necessary for understanding how to
use Unicode effectively, but if you're curious, consult the Unicode
consortium site listed in the References or
the `Wikipedia entry for Unicode <https://en.wikipedia.org/wiki/Unicode#History>`_
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.
Today's programs need to be able to handle a wide variety of
characters. Applications are often internationalized to display
messages and output in a variety of user-selectable languages; the
same program might need to output an error message in English, French,
Japanese, Hebrew, or Russian. Web content can be written in any of
these languages and can also include a variety of emoji symbols.
Python's string type uses the Unicode Standard for representing
characters, which lets Python programs work with all these different
possible characters.
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`` (4,810 decimal). The Unicode standard contains
a lot of tables listing characters and their corresponding code points:
Unicode (https://www.unicode.org/) is a specification that aims to
list every character used by human languages and give each character
its own unique code. The Unicode specifications are continually
revised and updated to add new languages and symbols.
A **character** is the smallest possible component of a text. 'A', 'B', 'C',
etc., are all different characters. So are 'È' and 'Í'. Characters vary
depending on the language or context you're talking
about. For example, there's a character for "Roman Numeral One", 'Ⅰ', that's
separate from the uppercase letter 'I'. They'll usually look the same,
but these are two different characters that have different meanings.
The Unicode standard describes how characters are represented by
**code points**. A code point value is an integer in the range 0 to
0x10FFFF (about 1.1 million values, with some 110 thousand assigned so
far). In the standard and in this document, a code point is written
using the notation ``U+265E`` to mean the character with value
``0x265e`` (9,822 in decimal).
The Unicode standard contains a lot of tables listing characters and
their corresponding code points:
.. code-block:: none
@ -103,10 +56,21 @@ a lot of tables listing characters and their corresponding code points:
0063 'c'; LATIN SMALL LETTER C
...
007B '{'; LEFT CURLY BRACKET
...
2167 'Ⅶ': ROMAN NUMERAL EIGHT
2168 'Ⅸ': ROMAN NUMERAL NINE
...
265E '♞': BLACK CHESS KNIGHT
265F '♟': BLACK CHESS PAWN
...
1F600 '😀': GRINNING FACE
1F609 '😉': WINKING FACE
...
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
character ``U+265E``'. ``U+265E`` is a code point, which represents some particular
character; in this case, it represents the character 'BLACK CHESS KNIGHT',
'♞'. In
informal contexts, this distinction between code points and characters will
sometimes be forgotten.
@ -121,14 +85,17 @@ 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 through ``0x10FFFF`` (1,114,111 decimal). This
sequence needs to be represented as a set of bytes (meaning, values
from 0 through 255) in memory. The rules for translating a Unicode string
into a sequence of bytes are called an **encoding**.
To summarize the previous section: a Unicode string is a sequence of
code points, which are numbers from 0 through ``0x10FFFF`` (1,114,111
decimal). This sequence of code points needs to be represented in
memory as a set of **code units**, and **code units** are then mapped
to 8-bit bytes. The rules for translating a Unicode string into a
sequence of bytes are called a **character encoding**, or just
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:
The first encoding you might think of is using 32-bit integers as the
code unit, and then using the CPU's representation of 32-bit integers.
In this representation, the string "Python" might look like this:
.. code-block:: none
@ -152,40 +119,14 @@ problems.
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.
Therefore this encoding isn't used very much, and people instead choose other
encodings that are more efficient and convenient, such as UTF-8.
Generally people don't use this encoding, instead choosing other
encodings that are more efficient and convenient. UTF-8 is probably
the most commonly supported encoding; it will be discussed below.
Encodings don't have to handle every possible Unicode character, and most
encodings don't. The rules for converting a Unicode string into the ASCII
encoding, for example, 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 are also a UTF-16 and UTF-32 encodings, but they are less
frequently used than UTF-8.) UTF-8 uses the following rules:
UTF-8 is one of the most commonly used encodings, and Python often
defaults to using it. UTF stands for "Unicode Transformation Format",
and the '8' means that 8-bit values are used in the encoding. (There
are also UTF-16 and UTF-32 encodings, but they are 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 >= 128, it's turned into a sequence of two, three, or
@ -215,6 +156,10 @@ glossary, and PDF versions of the Unicode specification. Be prepared for some
difficult reading. `A chronology <http://www.unicode.org/history/>`_ of the
origin and development of Unicode is also available on the site.
On the Computerphile Youtube channel, Tom Scott briefly
`discusses the history of Unicode and UTF-8 <https://www.youtube.com/watch?v=MijmeoH9LT4>`
(9 minutes 36 seconds).
To help understand the standard, Jukka Korpela has written `an introductory
guide <http://jkorpela.fi/unicode/guide.html>`_ to reading the
Unicode character tables.
@ -238,7 +183,7 @@ Unicode features.
The String Type
---------------
Since Python 3.0, the language features a :class:`str` type that contain Unicode
Since Python 3.0, the language's :class:`str` type contains Unicode
characters, meaning any string created using ``"unicode rocks!"``, ``'unicode
rocks!'``, or the triple-quoted string syntax is stored as Unicode.
@ -252,11 +197,6 @@ include a Unicode character in a string literal::
# 'File not found' error message.
print("Fichier non trouvé")
You can use a different encoding from UTF-8 by putting a specially-formatted
comment as the first or second line of the source code::
# -*- coding: <encoding name> -*-
Side note: Python 3 also supports using Unicode characters in identifiers::
répertoire = "/tmp/records.log"
@ -299,7 +239,7 @@ The following examples show the differences::
>>> b'\x80abc'.decode("utf-8", "ignore")
'abc'
Encodings are specified as strings containing the encoding's name. Python 3.2
Encodings are specified as strings containing the encoding's name. Python
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
@ -409,12 +349,13 @@ already mentioned. See also :pep:`263` for more information.
Unicode Properties
------------------
The Unicode specification includes a database of information about code points.
For each defined code point, 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 Unicode specification includes a database of information about
code points. For each defined code point, the information includes
the character's name, its category, the numeric value if applicable
(for characters representing numeric concepts such as the Roman
numerals, fractions such as one-third and four-fifths, etc.). There
are also display-related properties, such as how to use the code point
in bidirectional text.
The following program displays some information about several characters, and
prints the numeric value of one particular character::
@ -451,6 +392,88 @@ other". See
list of category codes.
Comparing Strings
-----------------
Unicode adds some complication to comparing strings, because the same
set of characters can be represented by different sequences of code
points. For example, a letter like 'ê' can be represented as a single
code point U+00EA, or as U+0065 U+0302, which is the code point for
'e' followed by a code point for 'COMBINING CIRCUMFLEX ACCENT'. These
will produce the same output when printed, but one is a string of
length 1 and the other is of length 2.
One tool for a case-insensitive comparison is the
:meth:`~str.casefold` string method that converts a string to a
case-insensitive form following an algorithm described by the Unicode
Standard. This algorithm has special handling for characters such as
the German letter 'ß' (code point U+00DF), which becomes the pair of
lowercase letters 'ss'.
::
>>> street = 'Gürzenichstraße'
>>> street.casefold()
'gürzenichstrasse'
A second tool is the :mod:`unicodedata` module's
:func:`~unicodedata.normalize` function that converts strings to one
of several normal forms, where letters followed by a combining
character are replaced with single characters. :func:`normalize` can
be used to perform string comparisons that won't falsely report
inequality if two strings use combining characters differently:
::
import unicodedata
def compare_strs(s1, s2):
def NFD(s):
return unicodedata.normalize('NFD', s)
return NFD(s1) == NFD(s2)
single_char = 'ê'
multiple_chars = '\N{LATIN SMALL LETTER E}\N{COMBINING CIRCUMFLEX ACCENT}'
print('length of first string=', len(single_char))
print('length of second string=', len(multiple_chars))
print(compare_strs(single_char, multiple_chars))
When run, this outputs:
.. code-block:: shell-session
$ python3 compare-strs.py
length of first string= 1
length of second string= 2
True
The first argument to the :func:`~unicodedata.normalize` function is a
string giving the desired normalization form, which can be one of
'NFC', 'NFKC', 'NFD', and 'NFKD'.
The Unicode Standard also specifies how to do caseless comparisons::
import unicodedata
def compare_caseless(s1, s2):
def NFD(s):
return unicodedata.normalize('NFD', s)
return NFD(NFD(s1).casefold()) == NFD(NFD(s2).casefold())
# Example usage
single_char = 'ê'
multiple_chars = '\N{LATIN CAPITAL LETTER E}\N{COMBINING CIRCUMFLEX ACCENT}'
print(compare_caseless(single_char, multiple_chars))
This will print ``True``. (Why is :func:`NFD` invoked twice? Because
there are a few characters that make :meth:`casefold` return a
non-normalized string, so the result needs to be normalized again. See
section 3.13 of the Unicode Standard for a discussion and an example.)
Unicode Regular Expressions
---------------------------
@ -567,22 +590,22 @@ 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.
The mark simply announces that the file is encoded in UTF-8. For reading such
files, use the 'utf-8-sig' codec to automatically skip the mark if present.
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 UTF-8.
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. Today Python is converging on using
UTF-8: Python on MacOS has used UTF-8 for several versions, and Python
3.6 switched to using UTF-8 on Windows as well. 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 again UTF-8.
The :func:`sys.getfilesystemencoding` function returns the encoding to use on
your current system, in case you want to do the encoding manually, but there's
@ -597,9 +620,9 @@ automatically converted to the right encoding for you::
Functions in the :mod:`os` module such as :func:`os.stat` will also accept Unicode
filenames.
The :func:`os.listdir` function returns filenames and raises an issue: should it return
The :func:`os.listdir` function returns filenames, which raises an issue: should it return
the Unicode version of filenames, or should it return bytes containing
the encoded versions? :func:`os.listdir` will do both, depending on whether you
the encoded versions? :func:`os.listdir` can do both, depending on whether you
provided the directory path as bytes 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
@ -619,16 +642,17 @@ will produce the following output:
.. code-block:: shell-session
amk:~$ python t.py
$ python listdir-test.py
[b'filename\xe4\x94\x80abc', ...]
['filename\u4500abc', ...]
The first list contains UTF-8-encoded filenames, and the second list contains
the Unicode versions.
Note that on most occasions, the Unicode APIs should be used. The bytes APIs
should only be used on systems where undecodable file names can be present,
i.e. Unix systems.
Note that on most occasions, you should can just stick with using
Unicode with these APIs. The bytes APIs should only be used on
systems where undecodable file names can be present; that's
pretty much only Unix systems now.
Tips for Writing Unicode-aware Programs
@ -695,10 +719,10 @@ with the ``surrogateescape`` error handler::
f.write(data)
The ``surrogateescape`` error handler will decode any non-ASCII bytes
as code points in the Unicode Private Use Area ranging from U+DC80 to
U+DCFF. These private code points will then be turned back into the
same bytes when the ``surrogateescape`` error handler is used when
encoding the data and writing it back out.
as code points in a special range running from U+DC80 to
U+DCFF. These code points will then turn back into the
same bytes when the ``surrogateescape`` error handler is used to
encode the data and write it back out.
References
@ -730,4 +754,5 @@ Andrew Kuchling, and Ezio Melotti.
Thanks to the following people who have noted errors or offered
suggestions on this article: Éric Araujo, Nicholas Bastin, Nick
Coghlan, Marius Gedminas, Kent Johnson, Ken Krugler, Marc-André
Lemburg, Martin von Löwis, Terry J. Reedy, Chad Whitacre.
Lemburg, Martin von Löwis, Terry J. Reedy, Serhiy Storchaka,
Eryk Sun, Chad Whitacre, Graham Wideman.