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.. _unicode-howto:
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*****************
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Unicode HOWTO
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*****************
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:Release: 1.12
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This HOWTO discusses Python 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.
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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 127. For example, the
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lowercase letter 'a' is assigned 97 as its code 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.
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In the mid-1980s an Apple II BASIC program written by a French speaker
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might have lines like these::
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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 (completé, caractère, accepté),
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and they just look wrong to someone who 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 127 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 through 1,114,111 (
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``0x10FFFF`` in 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. The
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precise historical details aren't necessary for understanding how to
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use Unicode effectively, but if you're curious, consult the Unicode
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consortium site listed in the References or
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the `Wikipedia entry for Unicode <http://en.wikipedia.org/wiki/Unicode#History>`_
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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`` (4,810 decimal). The Unicode standard contains
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a lot of tables listing characters and their corresponding code points:
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.. code-block:: none
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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 through ``0x10FFFF`` (1,114,111 decimal). This
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sequence needs to be represented as a set of bytes (meaning, values
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from 0 through 255) in memory. The rules for translating a Unicode string
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into a sequence of bytes are called an **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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.. code-block:: none
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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 ``0x00``
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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 gigabytes 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. The rules for converting a Unicode string into the ASCII
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encoding, for example, are 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 are also a UTF-16 and UTF-32 encodings, but they are less
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frequently used than 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 >= 128, it's turned into a sequence of two, three, or
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four bytes, where each 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 commonly used characters can be
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represented with one or two bytes.
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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 <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. `A chronology <http://www.unicode.org/history/>`_ of the
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origin and development of Unicode is also available on the site.
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To help understand the standard, Jukka Korpela has written `an introductory
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guide <http://www.cs.tut.fi/~jkorpela/unicode/guide.html>`_ to reading the
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Unicode character tables.
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Another `good introductory article <http://www.joelonsoftware.com/articles/Unicode.html>`_
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was written by Joel Spolsky.
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If this introduction didn't make things clear to you, you should try
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reading this alternate article before continuing.
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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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Python'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 String Type
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---------------
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Since Python 3.0, the language features a :class:`str` type that contain Unicode
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characters, meaning any string created using ``"unicode rocks!"``, ``'unicode
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rocks!'``, or the triple-quoted string syntax is stored as Unicode.
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The default encoding for Python source code is UTF-8, so you can simply
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include a Unicode character in a string literal::
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try:
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with open('/tmp/input.txt', 'r') as f:
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...
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except IOError:
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# 'File not found' error message.
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print("Fichier non trouvé")
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You can use a different encoding from UTF-8 by putting a specially-formatted
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comment as the first or second line of the source code::
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# -*- coding: <encoding name> -*-
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Side note: Python 3 also supports using Unicode characters in identifiers::
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répertoire = "/tmp/records.log"
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with open(répertoire, "w") as f:
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f.write("test\n")
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If you can't enter a particular character in your editor or want to
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keep the source code ASCII-only for some reason, you can also use
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escape sequences in string literals. (Depending on your system,
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you may see the actual capital-delta glyph instead of a \u escape.) ::
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>>> "\N{GREEK CAPITAL LETTER DELTA}" # Using the character name
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'\u0394'
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>>> "\u0394" # Using a 16-bit hex value
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'\u0394'
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>>> "\U00000394" # Using a 32-bit hex value
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'\u0394'
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In addition, one can create a string using the :func:`~bytes.decode` method of
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:class:`bytes`. This method takes an *encoding* argument, such as ``UTF-8``,
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and optionally an *errors* argument.
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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 :exc:`UnicodeDecodeError` exception), ``'replace'`` (use
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``U+FFFD``, ``REPLACEMENT CHARACTER``), or ``'ignore'`` (just leave the
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character out of the Unicode result).
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The following examples show the differences::
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>>> b'\x80abc'.decode("utf-8", "strict") #doctest: +NORMALIZE_WHITESPACE
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Traceback (most recent call last):
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...
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UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0:
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invalid start byte
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>>> b'\x80abc'.decode("utf-8", "replace")
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'\ufffdabc'
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>>> b'\x80abc'.decode("utf-8", "ignore")
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'abc'
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(In this code example, the Unicode replacement character has been replaced by
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a question mark because it may not be displayed on some systems.)
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Encodings are specified as strings containing the encoding's name. Python 3.2
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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 have multiple names; for
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example, ``'latin-1'``, ``'iso_8859_1'`` and ``'8859``' are all synonyms for
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the same encoding.
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One-character Unicode strings can also be created with the :func:`chr`
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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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>>> chr(57344)
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'\ue000'
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>>> ord('\ue000')
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57344
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Converting to Bytes
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-------------------
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The opposite method of :meth:`bytes.decode` is :meth:`str.encode`,
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which returns a :class:`bytes` representation of the Unicode string, encoded in the
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requested *encoding*.
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The *errors* parameter is the same as the parameter of the
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:meth:`~bytes.decode` method but supports a few more possible handlers. As well as
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``'strict'``, ``'ignore'``, and ``'replace'`` (which in this case
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inserts a question mark instead of the unencodable character), there is
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also ``'xmlcharrefreplace'`` (inserts an XML character reference) and
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``backslashreplace`` (inserts a ``\uNNNN`` escape sequence).
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The following example shows the different results::
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>>> u = chr(40960) + 'abcd' + chr(1972)
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>>> u.encode('utf-8')
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b'\xea\x80\x80abcd\xde\xb4'
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>>> u.encode('ascii') #doctest: +NORMALIZE_WHITESPACE
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Traceback (most recent call last):
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...
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UnicodeEncodeError: 'ascii' codec can't encode character '\ua000' in
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position 0: ordinal not in range(128)
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>>> u.encode('ascii', 'ignore')
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b'abcd'
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>>> u.encode('ascii', 'replace')
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b'?abcd?'
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>>> u.encode('ascii', 'xmlcharrefreplace')
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b'ꀀabcd޴'
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>>> u.encode('ascii', 'backslashreplace')
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b'\\ua000abcd\\u07b4'
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The low-level routines for registering and accessing the available
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encodings are found in the :mod:`codecs` module. Implementing new
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encodings also requires understanding the :mod:`codecs` module.
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However, the encoding and decoding functions returned by this module
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are usually more low-level than is comfortable, and writing new encodings
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is a specialized task, so the module won't be covered in this HOWTO.
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Unicode Literals in Python Source Code
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--------------------------------------
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In Python source code, specific Unicode code points can be written using the
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``\u`` escape sequence, which is followed by four hex digits giving the code
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point. The ``\U`` escape sequence is similar, but expects eight hex digits,
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not four::
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>>> s = "a\xac\u1234\u20ac\U00008000"
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... # ^^^^ two-digit hex escape
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... # ^^^^^^ four-digit Unicode escape
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... # ^^^^^^^^^^ eight-digit Unicode escape
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>>> [ord(c) for c in s]
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[97, 172, 4660, 8364, 32768]
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Using escape sequences for code points greater than 127 is fine in small doses,
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but becomes an annoyance if you're using many accented characters, as you would
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in a program with messages in French or some other accent-using language. You
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can also assemble strings using the :func:`chr` built-in function, but this is
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even more tedious.
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Ideally, you'd want to be able to write literals in your language's natural
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encoding. You could then edit Python source code with your favorite editor
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which would display the accented characters naturally, and have the right
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characters used at runtime.
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Python supports writing source code in UTF-8 by default, but you can use almost
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any encoding if you declare the encoding being used. This is done by including
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a special comment as either the first or second line of the source file::
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#!/usr/bin/env python
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# -*- coding: latin-1 -*-
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u = 'abcdé'
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print(ord(u[-1]))
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The syntax is inspired by Emacs's notation for specifying variables local to a
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file. Emacs supports many different variables, but Python only supports
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'coding'. The ``-*-`` symbols indicate to Emacs that the comment is special;
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they have no significance to Python but are a convention. Python looks for
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``coding: name`` or ``coding=name`` in the comment.
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If you don't include such a comment, the default encoding used will be UTF-8 as
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already mentioned. See also :pep:`263` for more information.
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Unicode Properties
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------------------
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The Unicode specification includes a database of information about code points.
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For each defined code point, the information includes the character's
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name, its category, the numeric value if applicable (Unicode has characters
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representing the Roman numerals and fractions such as one-third and
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four-fifths). There are also properties related to the code point's use in
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bidirectional text and other display-related properties.
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The following program displays some information about several characters, and
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prints the numeric value of one particular character::
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import unicodedata
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u = chr(233) + chr(0x0bf2) + chr(3972) + chr(6000) + chr(13231)
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for i, c in enumerate(u):
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print(i, '%04x' % ord(c), unicodedata.category(c), end=" ")
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print(unicodedata.name(c))
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# Get numeric value of second character
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print(unicodedata.numeric(u[1]))
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When run, this prints:
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.. code-block:: none
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0 00e9 Ll LATIN SMALL LETTER E WITH ACUTE
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1 0bf2 No TAMIL NUMBER ONE THOUSAND
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2 0f84 Mn TIBETAN MARK HALANTA
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3 1770 Lo TAGBANWA LETTER SA
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4 33af So SQUARE RAD OVER S SQUARED
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1000.0
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The category codes are abbreviations describing the nature of the character.
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These are grouped into categories such as "Letter", "Number", "Punctuation", or
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"Symbol", which in turn are broken up into subcategories. To take the codes
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from the above output, ``'Ll'`` means 'Letter, lowercase', ``'No'`` means
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"Number, other", ``'Mn'`` is "Mark, nonspacing", and ``'So'`` is "Symbol,
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other". See
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`the General Category Values section of the Unicode Character Database documentation <http://www.unicode.org/reports/tr44/#General_Category_Values>`_ for a
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list of category codes.
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Unicode Regular Expressions
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---------------------------
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The regular expressions supported by the :mod:`re` module can be provided
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either as bytes or strings. Some of the special character sequences such as
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``\d`` and ``\w`` have different meanings depending on whether
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the pattern is supplied as bytes or a string. For example,
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``\d`` will match the characters ``[0-9]`` in bytes but
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in strings will match any character that's in the ``'Nd'`` category.
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The string in this example has the number 57 written in both Thai and
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Arabic numerals::
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import re
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p = re.compile('\d+')
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s = "Over \u0e55\u0e57 57 flavours"
|
|
m = p.search(s)
|
|
print(repr(m.group()))
|
|
|
|
When executed, ``\d+`` will match the Thai numerals and print them
|
|
out. If you supply the :const:`re.ASCII` flag to
|
|
:func:`~re.compile`, ``\d+`` will match the substring "57" instead.
|
|
|
|
Similarly, ``\w`` matches a wide variety of Unicode characters but
|
|
only ``[a-zA-Z0-9_]`` in bytes or if :const:`re.ASCII` is supplied,
|
|
and ``\s`` will match either Unicode whitespace characters or
|
|
``[ \t\n\r\f\v]``.
|
|
|
|
|
|
References
|
|
----------
|
|
|
|
.. comment should these be mentioned earlier, e.g. at the start of the "introduction to Unicode" first section?
|
|
|
|
Some good alternative discussions of Python's Unicode support are:
|
|
|
|
* `Processing Text Files in Python 3 <http://python-notes.curiousefficiency.org/en/latest/python3/text_file_processing.html>`_, by Nick Coghlan.
|
|
* `Pragmatic Unicode <http://nedbatchelder.com/text/unipain.html>`_, a PyCon 2012 presentation by Ned Batchelder.
|
|
|
|
The :class:`str` type is described in the Python library reference at
|
|
:ref:`textseq`.
|
|
|
|
The documentation for the :mod:`unicodedata` module.
|
|
|
|
The documentation for the :mod:`codecs` module.
|
|
|
|
Marc-André Lemburg gave `a presentation titled "Python and Unicode" (PDF slides) <http://downloads.egenix.com/python/Unicode-EPC2002-Talk.pdf>`_ at
|
|
EuroPython 2002. The slides are an excellent overview of the design
|
|
of Python 2's Unicode features (where the Unicode string type is
|
|
called ``unicode`` and literals start with ``u``).
|
|
|
|
|
|
Reading and Writing Unicode Data
|
|
================================
|
|
|
|
Once you've written some code that works with Unicode data, the next problem is
|
|
input/output. How do you get Unicode strings into your program, and how do you
|
|
convert Unicode into a form suitable for storage or transmission?
|
|
|
|
It's possible that you may not need to do anything depending on your input
|
|
sources and output destinations; you should check whether the libraries used in
|
|
your application support Unicode natively. XML parsers often return Unicode
|
|
data, for example. Many relational databases also support Unicode-valued
|
|
columns and can return Unicode values from an SQL query.
|
|
|
|
Unicode data is usually converted to a particular encoding before it gets
|
|
written to disk or sent over a socket. It's possible to do all the work
|
|
yourself: open a file, read an 8-bit bytes object from it, and convert the string
|
|
with ``bytes.decode(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, 1024 or 4096 bytes), 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 2 GiB file, you need 2 GiB of RAM.
|
|
(More, really, since for at least a moment you'd need to have both the encoded
|
|
string and its Unicode version in memory.)
|
|
|
|
The solution would be to use the low-level decoding interface to catch the case
|
|
of partial coding sequences. The work of implementing this has already been
|
|
done for you: the built-in :func:`open` function can return a file-like object
|
|
that assumes the file's contents are in a specified encoding and accepts Unicode
|
|
parameters for methods such as :meth:`read` and :meth:`write`. This works through
|
|
:func:`open`\'s *encoding* and *errors* parameters which are interpreted just
|
|
like those in :meth:`str.encode` and :meth:`bytes.decode`.
|
|
|
|
Reading Unicode from a file is therefore simple::
|
|
|
|
with open('unicode.txt', encoding='utf-8') as f:
|
|
for line in f:
|
|
print(repr(line))
|
|
|
|
It's also possible to open files in update mode, allowing both reading and
|
|
writing::
|
|
|
|
with open('test', encoding='utf-8', mode='w+') as f:
|
|
f.write('\u4500 blah blah blah\n')
|
|
f.seek(0)
|
|
print(repr(f.readline()[:1]))
|
|
|
|
The Unicode character ``U+FEFF`` is used as a byte-order mark (BOM), and is often
|
|
written as the first character of a file in order to assist with autodetection
|
|
of the file's byte ordering. Some encodings, such as UTF-16, expect a BOM to be
|
|
present at the start of a file; when such an encoding is used, the BOM will be
|
|
automatically written as the first character and will be silently dropped when
|
|
the file is read. There are variants of these encodings, such as 'utf-16-le'
|
|
and 'utf-16-be' for little-endian and big-endian encodings, that specify one
|
|
particular byte ordering and don't skip the BOM.
|
|
|
|
In some areas, it is also convention to use a "BOM" at the start of UTF-8
|
|
encoded files; the name is misleading since UTF-8 is not byte-order dependent.
|
|
The mark simply announces that the file is encoded in UTF-8. Use the
|
|
'utf-8-sig' codec to automatically skip the mark if present for reading such
|
|
files.
|
|
|
|
|
|
Unicode filenames
|
|
-----------------
|
|
|
|
Most of the operating systems in common use today support filenames that contain
|
|
arbitrary Unicode characters. Usually this is implemented by converting the
|
|
Unicode string into some encoding that varies depending on the system. For
|
|
example, Mac OS X uses UTF-8 while Windows uses a configurable encoding; on
|
|
Windows, Python uses the name "mbcs" to refer to whatever the currently
|
|
configured encoding is. On Unix systems, there will only be a filesystem
|
|
encoding if you've set the ``LANG`` or ``LC_CTYPE`` environment variables; if
|
|
you haven't, the default encoding is 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
|
|
not much reason to bother. When opening a file for reading or writing, you can
|
|
usually just provide the Unicode string as the filename, and it will be
|
|
automatically converted to the right encoding for you::
|
|
|
|
filename = 'filename\u4500abc'
|
|
with open(filename, 'w') as f:
|
|
f.write('blah\n')
|
|
|
|
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 Unicode version of filenames, or should it return bytes containing
|
|
the encoded versions? :func:`os.listdir` will 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
|
|
path will return the filenames as bytes. For example,
|
|
assuming the default filesystem encoding is UTF-8, running the following
|
|
program::
|
|
|
|
fn = 'filename\u4500abc'
|
|
f = open(fn, 'w')
|
|
f.close()
|
|
|
|
import os
|
|
print(os.listdir(b'.'))
|
|
print(os.listdir('.'))
|
|
|
|
will produce the following output::
|
|
|
|
amk:~$ python t.py
|
|
[b'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.
|
|
|
|
|
|
Tips for Writing Unicode-aware Programs
|
|
---------------------------------------
|
|
|
|
This section provides some suggestions on writing software that deals with
|
|
Unicode.
|
|
|
|
The most important tip is:
|
|
|
|
Software should only work with Unicode strings internally, decoding the input
|
|
data as soon as possible and encoding the output only at the end.
|
|
|
|
If you attempt to write processing functions that accept both Unicode and byte
|
|
strings, you will find your program vulnerable to bugs wherever you combine the
|
|
two different kinds of strings. There is no automatic encoding or decoding: if
|
|
you do e.g. ``str + bytes``, a :exc:`TypeError` will be raised.
|
|
|
|
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 decoded string, not the encoded bytes data;
|
|
some encodings may have interesting properties, such as not being bijective
|
|
or not being fully ASCII-compatible. This is especially true if the input
|
|
data also specifies the encoding, since the attacker can then choose a
|
|
clever way to hide malicious text in the encoded bytestream.
|
|
|
|
|
|
Converting Between File Encodings
|
|
'''''''''''''''''''''''''''''''''
|
|
|
|
The :class:`~codecs.StreamRecoder` class can transparently convert between
|
|
encodings, taking a stream that returns data in encoding #1
|
|
and behaving like a stream returning data in encoding #2.
|
|
|
|
For example, if you have an input file *f* that's in Latin-1, you
|
|
can wrap it with a :class:`StreamRecoder` to return bytes encoded in UTF-8::
|
|
|
|
new_f = codecs.StreamRecoder(f,
|
|
# en/decoder: used by read() to encode its results and
|
|
# by write() to decode its input.
|
|
codecs.getencoder('utf-8'), codecs.getdecoder('utf-8'),
|
|
|
|
# reader/writer: used to read and write to the stream.
|
|
codecs.getreader('latin-1'), codecs.getwriter('latin-1') )
|
|
|
|
|
|
Files in an Unknown Encoding
|
|
''''''''''''''''''''''''''''
|
|
|
|
What can you do if you need to make a change to a file, but don't know
|
|
the file's encoding? If you know the encoding is ASCII-compatible and
|
|
only want to examine or modify the ASCII parts, you can open the file
|
|
with the ``surrogateescape`` error handler::
|
|
|
|
with open(fname, 'r', encoding="ascii", errors="surrogateescape") as f:
|
|
data = f.read()
|
|
|
|
# make changes to the string 'data'
|
|
|
|
with open(fname + '.new', 'w',
|
|
encoding="ascii", errors="surrogateescape") as f:
|
|
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.
|
|
|
|
|
|
References
|
|
----------
|
|
|
|
One section of `Mastering Python 3 Input/Output <http://pyvideo.org/video/289/pycon-2010--mastering-python-3-i-o>`_, a PyCon 2010 talk by David Beazley, discusses text processing and binary data handling.
|
|
|
|
The `PDF slides for Marc-André Lemburg's presentation "Writing Unicode-aware Applications in Python" <http://downloads.egenix.com/python/LSM2005-Developing-Unicode-aware-applications-in-Python.pdf>`_
|
|
discuss questions of character encodings as well as how to internationalize
|
|
and localize an application. These slides cover Python 2.x only.
|
|
|
|
`The Guts of Unicode in Python <http://pyvideo.org/video/1768/the-guts-of-unicode-in-python>`_ is a PyCon 2013 talk by Benjamin Peterson that discusses the internal Unicode representation in Python 3.3.
|
|
|
|
|
|
Acknowledgements
|
|
================
|
|
|
|
The initial draft of this document was written by Andrew Kuchling.
|
|
It has since been revised further by Alexander Belopolsky, Georg Brandl,
|
|
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.
|