2007-08-15 11:28:01 -03:00
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*****************
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Unicode HOWTO
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*****************
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2010-06-20 18:45:45 -03:00
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:Release: 1.03
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2007-08-15 11:28:01 -03:00
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2010-06-20 18:45:45 -03:00
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This HOWTO discusses Python 2.x's support for Unicode, and explains
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various problems that people commonly encounter when trying to work
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with Unicode. For the Python 3 version, see
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<https://docs.python.org/3/howto/unicode.html>.
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Introduction to Unicode
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=======================
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History of Character Codes
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--------------------------
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In 1968, the American Standard Code for Information Interchange, better known by
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its acronym ASCII, was standardized. ASCII defined numeric codes for various
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characters, with the numeric values running from 0 to
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127. For example, the lowercase letter 'a' is assigned 97 as its code
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value.
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ASCII was an American-developed standard, so it only defined unaccented
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characters. There was an 'e', but no 'é' or 'Í'. This meant that languages
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which required accented characters couldn't be faithfully represented in ASCII.
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(Actually the missing accents matter for English, too, which contains words such
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as 'naïve' and 'café', and some publications have house styles which require
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spellings such as 'coöperate'.)
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For a while people just wrote programs that didn't display accents. I remember
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looking at Apple ][ BASIC programs, published in French-language publications in
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the mid-1980s, that had lines like these::
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2009-01-03 17:04:55 -04:00
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PRINT "FICHIER EST COMPLETE."
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PRINT "CARACTERE NON ACCEPTE."
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Those messages should contain accents, and they just look wrong to someone who
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can read French.
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In the 1980s, almost all personal computers were 8-bit, meaning that bytes could
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hold values ranging from 0 to 255. ASCII codes only went up to 127, so some
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machines assigned values between 128 and 255 to accented characters. Different
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machines had different codes, however, which led to problems exchanging files.
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Eventually various commonly used sets of values for the 128-255 range emerged.
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Some were true standards, defined by the International Standards Organization,
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and some were **de facto** conventions that were invented by one company or
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another and managed to catch on.
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255 characters aren't very many. For example, you can't fit both the accented
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characters used in Western Europe and the Cyrillic alphabet used for Russian
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into the 128-255 range because there are more than 128 such characters.
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You could write files using different codes (all your Russian files in a coding
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system called KOI8, all your French files in a different coding system called
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Latin1), but what if you wanted to write a French document that quotes some
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Russian text? In the 1980s people began to want to solve this problem, and the
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Unicode standardization effort began.
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Unicode started out using 16-bit characters instead of 8-bit characters. 16
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bits means you have 2^16 = 65,536 distinct values available, making it possible
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to represent many different characters from many different alphabets; an initial
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goal was to have Unicode contain the alphabets for every single human language.
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It turns out that even 16 bits isn't enough to meet that goal, and the modern
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Unicode specification uses a wider range of codes, 0-1,114,111 (0x10ffff in
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base-16).
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There's a related ISO standard, ISO 10646. Unicode and ISO 10646 were
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originally separate efforts, but the specifications were merged with the 1.1
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revision of Unicode.
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(This discussion of Unicode's history is highly simplified. I don't think the
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average Python programmer needs to worry about the historical details; consult
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the Unicode consortium site listed in the References for more information.)
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Definitions
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-----------
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A **character** is the smallest possible component of a text. 'A', 'B', 'C',
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etc., are all different characters. So are 'È' and 'Í'. Characters are
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abstractions, and vary depending on the language or context you're talking
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about. For example, the symbol for ohms (Ω) is usually drawn much like the
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capital letter omega (Ω) in the Greek alphabet (they may even be the same in
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some fonts), but these are two different characters that have different
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meanings.
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The Unicode standard describes how characters are represented by **code
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points**. A code point is an integer value, usually denoted in base 16. In the
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standard, a code point is written using the notation U+12ca to mean the
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character with value 0x12ca (4810 decimal). The Unicode standard contains a lot
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of tables listing characters and their corresponding code points::
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2009-01-03 17:04:55 -04:00
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0061 'a'; LATIN SMALL LETTER A
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0062 'b'; LATIN SMALL LETTER B
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0063 'c'; LATIN SMALL LETTER C
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...
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007B '{'; LEFT CURLY BRACKET
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Strictly, these definitions imply that it's meaningless to say 'this is
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character U+12ca'. U+12ca is a code point, which represents some particular
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character; in this case, it represents the character 'ETHIOPIC SYLLABLE WI'. In
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informal contexts, this distinction between code points and characters will
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sometimes be forgotten.
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A character is represented on a screen or on paper by a set of graphical
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elements that's called a **glyph**. The glyph for an uppercase A, for example,
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is two diagonal strokes and a horizontal stroke, though the exact details will
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depend on the font being used. Most Python code doesn't need to worry about
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glyphs; figuring out the correct glyph to display is generally the job of a GUI
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toolkit or a terminal's font renderer.
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Encodings
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---------
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To summarize the previous section: a Unicode string is a sequence of code
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points, which are numbers from 0 to 0x10ffff. This sequence needs to be
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represented as a set of bytes (meaning, values from 0-255) in memory. The rules
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for translating a Unicode string into a sequence of bytes are called an
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**encoding**.
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The first encoding you might think of is an array of 32-bit integers. In this
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representation, the string "Python" would look like this::
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P y t h o n
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0x50 00 00 00 79 00 00 00 74 00 00 00 68 00 00 00 6f 00 00 00 6e 00 00 00
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
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This representation is straightforward but using it presents a number of
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problems.
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1. It's not portable; different processors order the bytes differently.
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2. It's very wasteful of space. In most texts, the majority of the code points
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are less than 127, or less than 255, so a lot of space is occupied by zero
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bytes. The above string takes 24 bytes compared to the 6 bytes needed for an
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ASCII representation. Increased RAM usage doesn't matter too much (desktop
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computers have megabytes of RAM, and strings aren't usually that large), but
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expanding our usage of disk and network bandwidth by a factor of 4 is
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intolerable.
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3. It's not compatible with existing C functions such as ``strlen()``, so a new
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family of wide string functions would need to be used.
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4. Many Internet standards are defined in terms of textual data, and can't
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handle content with embedded zero bytes.
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Generally people don't use this encoding, instead choosing other
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encodings that are more efficient and convenient. UTF-8 is probably
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the most commonly supported encoding; it will be discussed below.
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Encodings don't have to handle every possible Unicode character, and most
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encodings don't. For example, Python's default encoding is the 'ascii'
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encoding. The rules for converting a Unicode string into the ASCII encoding are
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simple; for each code point:
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1. If the code point is < 128, each byte is the same as the value of the code
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point.
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2. If the code point is 128 or greater, the Unicode string can't be represented
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in this encoding. (Python raises a :exc:`UnicodeEncodeError` exception in this
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case.)
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Latin-1, also known as ISO-8859-1, is a similar encoding. Unicode code points
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0-255 are identical to the Latin-1 values, so converting to this encoding simply
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requires converting code points to byte values; if a code point larger than 255
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is encountered, the string can't be encoded into Latin-1.
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Encodings don't have to be simple one-to-one mappings like Latin-1. Consider
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IBM's EBCDIC, which was used on IBM mainframes. Letter values weren't in one
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block: 'a' through 'i' had values from 129 to 137, but 'j' through 'r' were 145
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through 153. If you wanted to use EBCDIC as an encoding, you'd probably use
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some sort of lookup table to perform the conversion, but this is largely an
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internal detail.
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UTF-8 is one of the most commonly used encodings. UTF stands for "Unicode
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Transformation Format", and the '8' means that 8-bit numbers are used in the
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encoding. (There's also a UTF-16 encoding, but it's less frequently used than
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UTF-8.) UTF-8 uses the following rules:
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1. If the code point is <128, it's represented by the corresponding byte value.
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2. If the code point is between 128 and 0x7ff, it's turned into two byte values
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between 128 and 255.
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3. Code points >0x7ff are turned into three- or four-byte sequences, where each
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byte of the sequence is between 128 and 255.
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UTF-8 has several convenient properties:
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1. It can handle any Unicode code point.
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2. A Unicode string is turned into a string of bytes containing no embedded zero
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bytes. This avoids byte-ordering issues, and means UTF-8 strings can be
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processed by C functions such as ``strcpy()`` and sent through protocols that
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can't handle zero bytes.
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3. A string of ASCII text is also valid UTF-8 text.
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4. UTF-8 is fairly compact; the majority of code points are turned into two
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bytes, and values less than 128 occupy only a single byte.
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5. If bytes are corrupted or lost, it's possible to determine the start of the
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next UTF-8-encoded code point and resynchronize. It's also unlikely that
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random 8-bit data will look like valid UTF-8.
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References
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----------
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The Unicode Consortium site at <http://www.unicode.org> has character charts, a
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glossary, and PDF versions of the Unicode specification. Be prepared for some
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difficult reading. <http://www.unicode.org/history/> is a chronology of the
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origin and development of Unicode.
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To help understand the standard, Jukka Korpela has written an introductory guide
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to reading the Unicode character tables, available at
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<http://www.cs.tut.fi/~jkorpela/unicode/guide.html>.
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2009-09-16 06:30:48 -03:00
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Another good introductory article was written by Joel Spolsky
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<http://www.joelonsoftware.com/articles/Unicode.html>.
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If this introduction didn't make things clear to you, you should try reading this
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alternate article before continuing.
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.. Jason Orendorff XXX http://www.jorendorff.com/articles/unicode/ is broken
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Wikipedia entries are often helpful; see the entries for "character encoding"
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<http://en.wikipedia.org/wiki/Character_encoding> and UTF-8
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<http://en.wikipedia.org/wiki/UTF-8>, for example.
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2010-06-20 18:45:45 -03:00
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Python 2.x's Unicode Support
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============================
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Now that you've learned the rudiments of Unicode, we can look at Python's
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Unicode features.
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The Unicode Type
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----------------
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Unicode strings are expressed as instances of the :class:`unicode` type, one of
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Python's repertoire of built-in types. It derives from an abstract type called
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:class:`basestring`, which is also an ancestor of the :class:`str` type; you can
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therefore check if a value is a string type with ``isinstance(value,
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basestring)``. Under the hood, Python represents Unicode strings as either 16-
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or 32-bit integers, depending on how the Python interpreter was compiled.
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The :func:`unicode` constructor has the signature ``unicode(string[, encoding,
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errors])``. All of its arguments should be 8-bit strings. The first argument
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is converted to Unicode using the specified encoding; if you leave off the
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``encoding`` argument, the ASCII encoding is used for the conversion, so
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characters greater than 127 will be treated as errors::
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>>> unicode('abcdef')
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u'abcdef'
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>>> s = unicode('abcdef')
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>>> type(s)
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<type 'unicode'>
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>>> unicode('abcdef' + chr(255)) #doctest: +NORMALIZE_WHITESPACE
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Traceback (most recent call last):
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...
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UnicodeDecodeError: 'ascii' codec can't decode byte 0xff in position 6:
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ordinal not in range(128)
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The ``errors`` argument specifies the response when the input string can't be
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converted according to the encoding's rules. Legal values for this argument are
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'strict' (raise a ``UnicodeDecodeError`` exception), 'replace' (add U+FFFD,
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'REPLACEMENT CHARACTER'), or 'ignore' (just leave the character out of the
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Unicode result). The following examples show the differences::
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>>> unicode('\x80abc', errors='strict') #doctest: +NORMALIZE_WHITESPACE
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Traceback (most recent call last):
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...
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UnicodeDecodeError: 'ascii' codec can't decode byte 0x80 in position 0:
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ordinal not in range(128)
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>>> unicode('\x80abc', errors='replace')
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u'\ufffdabc'
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>>> unicode('\x80abc', errors='ignore')
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u'abc'
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2010-06-20 18:45:45 -03:00
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Encodings are specified as strings containing the encoding's name. Python 2.7
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comes with roughly 100 different encodings; see the Python Library Reference at
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:ref:`standard-encodings` for a list. Some encodings
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have multiple names; for example, 'latin-1', 'iso_8859_1' and '8859' are all
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synonyms for the same encoding.
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One-character Unicode strings can also be created with the :func:`unichr`
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built-in function, which takes integers and returns a Unicode string of length 1
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that contains the corresponding code point. The reverse operation is the
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built-in :func:`ord` function that takes a one-character Unicode string and
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returns the code point value::
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>>> unichr(40960)
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u'\ua000'
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>>> ord(u'\ua000')
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40960
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Instances of the :class:`unicode` type have many of the same methods as the
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8-bit string type for operations such as searching and formatting::
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>>> s = u'Was ever feather so lightly blown to and fro as this multitude?'
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>>> s.count('e')
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5
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>>> s.find('feather')
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9
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>>> s.find('bird')
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-1
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>>> s.replace('feather', 'sand')
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u'Was ever sand so lightly blown to and fro as this multitude?'
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>>> s.upper()
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u'WAS EVER FEATHER SO LIGHTLY BLOWN TO AND FRO AS THIS MULTITUDE?'
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Note that the arguments to these methods can be Unicode strings or 8-bit
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strings. 8-bit strings will be converted to Unicode before carrying out the
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operation; Python's default ASCII encoding will be used, so characters greater
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than 127 will cause an exception::
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2012-09-12 02:14:23 -03:00
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>>> s.find('Was\x9f') #doctest: +NORMALIZE_WHITESPACE
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Traceback (most recent call last):
|
2012-09-12 02:14:23 -03:00
|
|
|
...
|
|
|
|
UnicodeDecodeError: 'ascii' codec can't decode byte 0x9f in position 3:
|
|
|
|
ordinal not in range(128)
|
2007-08-15 11:28:01 -03:00
|
|
|
>>> 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'
|
2012-09-12 02:14:23 -03:00
|
|
|
>>> u.encode('ascii') #doctest: +NORMALIZE_WHITESPACE
|
2007-08-15 11:28:01 -03:00
|
|
|
Traceback (most recent call last):
|
2012-09-12 02:14:23 -03:00
|
|
|
...
|
|
|
|
UnicodeEncodeError: 'ascii' codec can't encode character u'\ua000' in
|
|
|
|
position 0: ordinal not in range(128)
|
2007-08-15 11:28:01 -03:00
|
|
|
>>> 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
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
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.
|
2009-01-03 16:55:06 -04:00
|
|
|
|
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
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"
|
2012-09-12 02:14:23 -03:00
|
|
|
... # ^^^^ two-digit hex escape
|
|
|
|
... # ^^^^^^ four-digit Unicode escape
|
|
|
|
... # ^^^^^^^^^^ eight-digit Unicode escape
|
2007-08-15 11:28:01 -03:00
|
|
|
>>> for c in s: print ord(c),
|
2009-01-03 16:55:06 -04:00
|
|
|
...
|
2007-08-15 11:28:01 -03:00
|
|
|
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 -*-
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
u = u'abcdé'
|
|
|
|
print ord(u[-1])
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
The syntax is inspired by Emacs's notation for specifying variables local to a
|
|
|
|
file. Emacs supports many different variables, but Python only supports
|
2008-11-22 06:08:50 -04:00
|
|
|
'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.
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
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::
|
|
|
|
|
2010-06-20 18:45:45 -03:00
|
|
|
amk:~$ python2.4 p263.py
|
2009-01-03 16:55:06 -04:00
|
|
|
sys:1: DeprecationWarning: Non-ASCII character '\xe9'
|
|
|
|
in file p263.py on line 2, but no encoding declared;
|
2014-10-29 04:36:35 -03:00
|
|
|
see https://www.python.org/peps/pep-0263.html for details
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2010-06-20 18:45:45 -03:00
|
|
|
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
|
2014-10-29 04:36:35 -03:00
|
|
|
https://www.python.org/peps/pep-0263.html for details
|
2010-06-20 18:45:45 -03:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
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
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
u = unichr(233) + unichr(0x0bf2) + unichr(3972) + unichr(6000) + unichr(13231)
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
for i, c in enumerate(u):
|
|
|
|
print i, '%04x' % ord(c), unicodedata.category(c),
|
|
|
|
print unicodedata.name(c)
|
2009-01-03 16:55:06 -04:00
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
# 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
|
2010-03-22 20:07:32 -03:00
|
|
|
<http://www.unicode.org/reports/tr44/#General_Category_Values> for a
|
2007-08-15 11:28:01 -03:00
|
|
|
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
|
2008-03-14 21:20:19 -03:00
|
|
|
<http://downloads.egenix.com/python/Unicode-EPC2002-Talk.pdf>, and is an
|
2007-08-15 11:28:01 -03:00
|
|
|
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,
|
|
|
|
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'.
|
|
|
|
|
|
|
|
Reading Unicode from a file is therefore simple::
|
|
|
|
|
|
|
|
import codecs
|
|
|
|
f = codecs.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 = 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 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
|
2008-09-13 14:41:16 -03:00
|
|
|
example, Mac OS X uses UTF-8 while Windows uses a configurable encoding; on
|
2007-08-15 11:28:01 -03:00
|
|
|
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 :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
|
|
|
|
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 = u'filename\u4500abc'
|
|
|
|
f = open(filename, 'w')
|
|
|
|
f.write('blah\n')
|
|
|
|
f.close()
|
|
|
|
|
|
|
|
Functions in the :mod:`os` module such as :func:`os.stat` will also accept Unicode
|
|
|
|
filenames.
|
|
|
|
|
|
|
|
:func:`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? :func:`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::
|
|
|
|
|
2009-01-03 17:04:55 -04:00
|
|
|
fn = u'filename\u4500abc'
|
|
|
|
f = open(fn, 'w')
|
|
|
|
f.close()
|
2007-08-15 11:28:01 -03:00
|
|
|
|
2009-01-03 17:04:55 -04:00
|
|
|
import os
|
|
|
|
print os.listdir('.')
|
|
|
|
print os.listdir(u'.')
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
will produce the following output::
|
|
|
|
|
2009-01-03 17:04:55 -04:00
|
|
|
amk:~$ python t.py
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['.svn', 'filename\xe4\x94\x80abc', ...]
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[u'.svn', u'filename\u4500abc', ...]
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2007-08-15 11:28:01 -03:00
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The first list contains UTF-8-encoded filenames, and the second list contains
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the Unicode versions.
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2009-01-03 16:55:06 -04:00
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2007-08-15 11:28:01 -03:00
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Tips for Writing Unicode-aware Programs
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---------------------------------------
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This section provides some suggestions on writing software that deals with
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Unicode.
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The most important tip is:
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Software should only work with Unicode strings internally, converting to a
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particular encoding on output.
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If you attempt to write processing functions that accept both Unicode and 8-bit
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strings, you will find your program vulnerable to bugs wherever you combine the
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two different kinds of strings. Python's default encoding is ASCII, so whenever
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a character with an ASCII value > 127 is in the input data, you'll get a
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:exc:`UnicodeDecodeError` because that character can't be handled by the ASCII
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encoding.
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It's easy to miss such problems if you only test your software with data that
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doesn't contain any accents; everything will seem to work, but there's actually
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a bug in your program waiting for the first user who attempts to use characters
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> 127. A second tip, therefore, is:
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Include characters > 127 and, even better, characters > 255 in your test
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data.
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When using data coming from a web browser or some other untrusted source, a
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common technique is to check for illegal characters in a string before using the
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string in a generated command line or storing it in a database. If you're doing
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this, be careful to check the string once it's in the form that will be used or
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stored; it's possible for encodings to be used to disguise characters. This is
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especially true if the input data also specifies the encoding; many encodings
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leave the commonly checked-for characters alone, but Python includes some
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encodings such as ``'base64'`` that modify every single character.
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For example, let's say you have a content management system that takes a Unicode
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filename, and you want to disallow paths with a '/' character. You might write
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this code::
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def read_file (filename, encoding):
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if '/' in filename:
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raise ValueError("'/' not allowed in filenames")
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unicode_name = filename.decode(encoding)
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f = open(unicode_name, 'r')
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# ... return contents of file ...
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2009-01-03 16:55:06 -04:00
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2007-08-15 11:28:01 -03:00
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However, if an attacker could specify the ``'base64'`` encoding, they could pass
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``'L2V0Yy9wYXNzd2Q='``, which is the base-64 encoded form of the string
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``'/etc/passwd'``, to read a system file. The above code looks for ``'/'``
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characters in the encoded form and misses the dangerous character in the
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resulting decoded form.
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References
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----------
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The PDF slides for Marc-André Lemburg's presentation "Writing Unicode-aware
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Applications in Python" are available at
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2008-03-14 21:20:19 -03:00
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<http://downloads.egenix.com/python/LSM2005-Developing-Unicode-aware-applications-in-Python.pdf>
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2007-08-15 11:28:01 -03:00
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and discuss questions of character encodings as well as how to internationalize
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and localize an application.
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Revision History and Acknowledgements
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=====================================
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Thanks to the following people who have noted errors or offered suggestions on
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this article: Nicholas Bastin, Marius Gedminas, Kent Johnson, Ken Krugler,
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Marc-André Lemburg, Martin von Löwis, Chad Whitacre.
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Version 1.0: posted August 5 2005.
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Version 1.01: posted August 7 2005. Corrects factual and markup errors; adds
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several links.
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Version 1.02: posted August 16 2005. Corrects factual errors.
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2010-06-20 18:45:45 -03:00
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Version 1.03: posted June 20 2010. Notes that Python 3.x is not covered,
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and that the HOWTO only covers 2.x.
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|
2007-08-15 11:28:01 -03:00
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|
2010-06-20 18:45:45 -03:00
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.. comment Describe Python 3.x support (new section? new document?)
|
2007-08-15 11:28:01 -03:00
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.. comment Additional topic: building Python w/ UCS2 or UCS4 support
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.. comment Describe obscure -U switch somewhere?
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.. comment Describe use of codecs.StreamRecoder and StreamReaderWriter
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|
2009-01-03 16:55:06 -04:00
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.. comment
|
2007-08-15 11:28:01 -03:00
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Original outline:
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|
- [ ] Unicode introduction
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|
- [ ] ASCII
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- [ ] Terms
|
2009-01-03 17:04:55 -04:00
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- [ ] Character
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- [ ] Code point
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- [ ] Encodings
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- [ ] Common encodings: ASCII, Latin-1, UTF-8
|
2007-08-15 11:28:01 -03:00
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- [ ] Unicode Python type
|
2009-01-03 17:04:55 -04:00
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- [ ] Writing unicode literals
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|
- [ ] Obscurity: -U switch
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- [ ] Built-ins
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|
- [ ] unichr()
|
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|
- [ ] ord()
|
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|
- [ ] unicode() constructor
|
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|
- [ ] Unicode type
|
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|
- [ ] encode(), decode() methods
|
2007-08-15 11:28:01 -03:00
|
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|
- [ ] Unicodedata module for character properties
|
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|
- [ ] I/O
|
2009-01-03 17:04:55 -04:00
|
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|
- [ ] Reading/writing Unicode data into files
|
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|
- [ ] Byte-order marks
|
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|
- [ ] Unicode filenames
|
2007-08-15 11:28:01 -03:00
|
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- [ ] Writing Unicode programs
|
2009-01-03 17:04:55 -04:00
|
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|
- [ ] Do everything in Unicode
|
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|
|
- [ ] Declaring source code encodings (PEP 263)
|
2007-08-15 11:28:01 -03:00
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|
- [ ] Other issues
|
2009-01-03 17:04:55 -04:00
|
|
|
- [ ] Building Python (UCS2, UCS4)
|