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

Release:
   1.03

This HOWTO discusses Python 2.x’s support for Unicode, and explains
various problems that people commonly encounter when trying to work
with Unicode.  For the Python 3 version, see
<https://docs.python.org/3/howto/unicode.html>.


Unicode 入門
============


文字コードの歴史
----------------

1968年に American Standard Code for Information Interchange が標準化さ
れました。これは頭文字の ASCII でよく知られています。ASCII は0から127
までの、異なる文字の数値コードを定義していて、例えば、小文字の 『a』
にはコード値 97 が割り当てられています。

ASCII はアメリカの開発標準だったのでアクセント無しの文字のみを定義して
いて、 『e』 はありましたが、 『é』 や 『Í』 はありませんでした。つま
り、アクセント付きの文字を必要とする言語は ASCII できちんと表現すると
ができません。 (実際には英語でもアクセントが無いために起きる問題があり
ました、 『naïve』 や 『café』 のようなアクセントを含む単語や、いくつ
かの出版社は 『coöperate』 のような独自のスタイルのつづりを必要とする
など)

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 "MISE A JOUR TERMINEE"
   PRINT "PARAMETRES ENREGISTRES"

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 Organization for
Standardization, and some were *de facto* conventions that were
invented by one company or another and managed to catch on.

255文字というのは十分多い数ではありません。例えば、西ヨーロッパで使わ
れるアクセント付き文字とロシアで使われるキリルアルファベットの両方は
128文字以上あるので、128–255の間におさめることはできません。

異なる文字コードを使ってファイルを作成することは可能です (持っているロ
シア語のファイル全てを KOI8 と呼ばれるコーディングシステムで、持ってい
るフランス語のファイル全てを別の Latin1 と呼ばれるコーディングシステム
にすることで)、しかし、ロシア語の文章を引用するフランス語の文章を書き
たい場合にはどうでしょう? 1989年代にこの問題を解決したいという要望が上
って、Unicode 標準化の努力が始まりました。

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).

関連する ISO 標準も ISO 10646 があります。Unicode と ISO 10646 は元々
独立した成果でしたが、 Unicode の 1.1 リビジョンで仕様を併合しました。

(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.)


定義
----

**文字** は文章の構成要素の中の最小のものです。』A』, 『B』, 『C』 な
どは全て異なる文字です。 『È』 や 『Í』 も同様に異なる文字です。文字は
抽象的な概念で、言語や文脈に依存してさまざまに変化します。例えば、オー
ム(Ω) はふつう大文字ギリシャ文字のオメガ (Ω) で書かれますが (これらは
いくつかのフォントで全く同じ書体かもしれません) しかし、これらは異なる
意味を持つ異なる文字とみなされます。

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.

文字は画面や紙面上では **グリフ (glyph)** と呼ばれるグラフィック要素の
組で表示されます。大文字の A のグリフは例えば、厳密な形は使っているフ
ォントによって異なりますが、斜めの線と水平の線です。たいていの Python
コードではグリフの心配をする必要はありません; 一般的には表示する正しい
グリフを見付けることは GUI toolkit や端末のフォントレンダラーの仕事で
す。


エンコーディング
----------------

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

この表現は直接的でわかりやすい方法ですが、この表現を使うにはいくつかの
問題があります。

1. 可搬性がない; プロセッサが異なるとバイトの順序づけも変わってしま
   い ます。

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. "strlen()" のような現存する C 関数と互換性がありません、そのため
   ワ イド文字列関数一式が新たに必要となります。

4. 多くのインターネット標準がテキストデータとして定義されていて、そ
   れ らはゼロバイトの埋め込まれた内容を扱うことができません。

一般的にこのエンコーディングは使わず、変わりにより効率的で便利な他のエ
ンコーディングが選ばれています。 UTF-8 はたぶん最も一般的にサポートさ
れているエンコーディングです。このエンコーディングについては後で説明し
ます。

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. コードポイントは128より小さい場合、コードポイントと同じ値です。

2. コードポイントが128以上の場合、Unicode 文字列はエンコーディング
   で表 示することができません。 (この場合 Python は
   "UnicodeEncodeError" 例外を送出します。)

Latin-1, ISO-8859-1 として知られるエンコーディングも同様のエンコーディ
ングです。Unicode コードポイントの 0–255 は Latin-1 の値と等価なので、
このエンコーディングの変換するには、単純にコードポイントをバイト値に変
換する必要があります; もしコードポイントが255より大きい場合に遭遇した
場合、文字列は Latin-1 にエンコードできません。

エンコーディングは Latin-1 のように単純な一対一対応を持っていません。
IBM メインフレームで使われていた IBM の EBCDIC で考えてみます。文字は
一つのブロックに収められていませんでした: 『a』 から 『i』 は 129 から
137 まででしたが、 『j』 から 『r』 までは 145 から 153 までした。
EBICIC を使いたいと思ったら、おそらく変換を実行するルックアップテーブ
ルの類を使う必要があるでしょう、これは内部の詳細のことになりますが。

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 はいくつかの便利な性質を持っています:

1. 任意の Unicode コードポイントを扱うことができる。

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. ASCII テキストの文字列は UTF-8 テキストとしても有効です。

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. バイトが欠落したり、失われた場合、次の UTF-8 でエンコードされた
   コー ドポイントの開始を決定し、再同期することができる可能性がありま
   す。 同様の理由でランダムな 8-bit データは正当な UTF-8 とみなされに
   くく なっています。


参考資料
--------

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

Another good introductory article was written by Joel Spolsky
<http://www.joelonsoftware.com/articles/Unicode.html>. If this
introduction didn’t make things clear to you, you should try reading
this alternate article 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 2.x’s Unicode Support
============================

ここまでで Unicode の基礎を学びました、ここから Python の Unicode 機能
に触れます。


The Unicode Type
----------------

Unicode strings are expressed as instances of the "unicode" type, one
of Python’s repertoire of built-in types.  It derives from an abstract
type called "basestring", which is also an ancestor of the "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 "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))    #doctest: +NORMALIZE_WHITESPACE
   Traceback (most recent call last):
   ...
   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')     #doctest: +NORMALIZE_WHITESPACE
   Traceback (most recent call last):
       ...
   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 2.7 comes with roughly 100 different encodings; see the Python
Library Reference at 標準エンコーディング 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 "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 "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 "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')                   #doctest: +NORMALIZE_WHITESPACE
   Traceback (most recent call last):
       ...
   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')                       #doctest: +NORMALIZE_WHITESPACE
   Traceback (most recent call last):
       ...
   UnicodeEncodeError: 'ascii' codec can't encode character u'\ua000' in
   position 0: ordinal not in range(128)
   >>> u.encode('ascii', 'ignore')
   'abcd'
   >>> u.encode('ascii', 'replace')
   '?abcd?'
   >>> u.encode('ascii', 'xmlcharrefreplace')
   '&#40960;abcd&#1972;'

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 "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.

The most commonly used part of the "codecs" module is the
"codecs.open()" function which will be discussed in the section on
input and output.


Python ソースコード内の Unicode リテラル
----------------------------------------

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
"unichr()" built-in function, but this is even more tedious.

理想的にはあなたの言語の自然なエンコーディングでリテラルを書くことでし
ょう。そうなれば、Python のソースコードをアクセント付きの文字を自然に
表示するお気に入りのエディタで編集し、実行時に正しい文字が得られます。

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])

この構文は Emacs のファイル固有の変数を指定する表記から影響を受けてい
ます。Emacs は様々な変数をサポートしていますが、Python がサポートして
いるのは 『coding』 のみです。 "-*-" の記法は Emacs に対してコメントが
特別であることを示します。これは Python にとって意味はありませんが慣習
で使われています。 Python はコメント中に "coding: name" または
"coding=name" を探します。

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:~$ python2.4 p263.py
   sys:1: DeprecationWarning: Non-ASCII character '\xe9'
        in file p263.py on line 2, but no encoding declared;
        see https://www.python.org/peps/pep-0263.html for details

Python 2.5 and higher are stricter and will produce a syntax error:

   amk:~$ python2.5 p263.py
   File "/tmp/p263.py", line 2
   SyntaxError: Non-ASCII character '\xc3' in file /tmp/p263.py
     on line 2, but no encoding declared; see
     https://www.python.org/peps/pep-0263.html for details


Unicode プロパティ
------------------

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.

以下のプログラムはいくつかの文字に対する情報を表示し、特定の文字の数値
を印字します:

   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/reports/tr44/#General_Category_Values> for a
list of category codes.


参考資料
--------

The Unicode and 8-bit string types are described in the Python library
reference at Sequence Types — str, unicode, list, tuple, bytearray,
buffer, xrange.

"unicodedata" モジュールについてのドキュメント。

"codecs" モジュールについてのドキュメント。

Marc-André Lemburg gave a presentation at EuroPython 2002 titled 「
Python and Unicode」.  A PDF version of his slides is available at
<https://downloads.egenix.com/python/Unicode-EPC2002-Talk.pdf>, and is
an excellent overview of the design of Python’s Unicode features.


Unicode データを読み書きする
============================

一旦 Unicode データに対してコードが動作するように書き終えたら、次の問
題は入出力です。プログラムは Unicode 文字列をどう受けとり、どう
Unicode を外部記憶装置や送受信装置に適した形式に変換するのでしょう?

入力ソースと出力先に依存しないような方法は可能です; アプリケーションに
利用されているライブラリが Unicode をそのままサポートしているかを調べ
なければいけません。例えば XML パーサーは大抵 Unicode データを返します
。多くのリレーショナルデータベースも Unicode 値の入ったコラムをサポー
トしていますし、 SQL の問い合わせで Unicode 値を返すことができます。

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 "codecs" module includes a version
of the "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, errors='strict', buffering=1)".  "mode" can be "'r'",
"'w'", or "'a'", just like the corresponding parameter to the regular
built-in "open()" function; add a "'+'" to update the file.
"buffering" is similarly parallel to the standard function’s
parameter.  "encoding" is a string giving the encoding to use; if it’s
left as "None", a regular Python file object that accepts 8-bit
strings is returned.  Otherwise, a wrapper object is returned, and
data written to or read from the wrapper object will be converted as
needed. "errors" specifies the action for encoding errors and can be
one of the usual values of 『strict』, 『ignore』, and 『replace』.

そのためファイルから Unicode を読むのは単純です:

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

読み書きの両方ができる update モードでファイルを開くことも可能です:

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

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.


Unicode ファイル名
------------------

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.

"sys.getfilesystemencoding()" 関数は現在のシステムで利用するエンコーデ
ィングを返し、エンコーディングを手動で設定したい場合利用します、ただし
わざわざそうする積極的な理由はありません。読み書きのためにファイルを開
く時には、ファイル名を Unicode 文字列として渡すだけで正しいエンコーデ
ィングに自動的に変更されます:

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

"os.stat()" のような "os" モジュールの関数も Unicode のファイル名を受
け付けます。

"os.listdir()", which returns filenames, raises an issue: should it
return the Unicode version of filenames, or should it return 8-bit
strings containing the encoded versions?  "os.listdir()" will do both,
depending on whether you provided the directory path as an 8-bit
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 an 8-bit path will
return the 8-bit versions of the filenames.  For example, assuming the
default filesystem encoding is UTF-8, running the following program:

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

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

以下の出力結果が生成されます:

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

最初のリストは UTF-8 でエンコーディングされたファイル名を含み、第二の
リストは Unicode 版を含んでいます。


Unicode 対応のプログラムを書くための Tips
-----------------------------------------

この章では Unicode を扱うプログラムを書くためのいくつかの提案を紹介し
ます。

最も重要な助言としては:

   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 8-bit strings, you will find your program vulnerable to bugs
wherever you combine the two different kinds of strings.  Python’s
default encoding is ASCII, so whenever a character with an ASCII value
> 127 is in the input data, you’ll get a "UnicodeDecodeError" because
that character can’t be handled by the ASCII encoding.

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.


参考資料
--------

The PDF slides for Marc-André Lemburg’s presentation 「Writing
Unicode-aware Applications in Python」 are available at
<https://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 1.03: posted June 20 2010.  Notes that Python 3.x is not
covered, and that the HOWTO only covers 2.x.
