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Issue #4153: Updated Unicode HOWTO.
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Alexander Belopolsky
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Unicode HOWTO
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*****************
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:Release: 1.11
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:Release: 1.12
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This HOWTO discusses Python 2.x's support for Unicode, and explains
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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. (This HOWTO has not yet been updated to cover the 3.x
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versions of Python.)
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with Unicode.
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Introduction to Unicode
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=======================
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@ -44,14 +42,14 @@ 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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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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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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@ -64,8 +62,8 @@ 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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Unicode specification uses a wider range of codes, 0 through 1,114,111 (0x10ffff
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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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@ -90,7 +88,7 @@ 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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character with value 0x12ca (4,810 decimal). The Unicode standard contains a lot
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of tables listing characters and their corresponding code points::
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0061 'a'; LATIN SMALL LETTER A
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@ -117,10 +115,10 @@ 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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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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@ -164,7 +162,7 @@ encoding, for example, are simple; for each code point:
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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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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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@ -226,8 +224,8 @@ Wikipedia entries are often helpful; see the entries for "character encoding"
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<http://en.wikipedia.org/wiki/UTF-8>, for example.
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Python 2.x's Unicode Support
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============================
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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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@ -265,7 +263,7 @@ Unicode result). The following examples show the differences::
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UnicodeDecodeError: 'utf8' codec can't decode byte 0x80 in position 0:
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unexpected code byte
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>>> b'\x80abc'.decode("utf-8", "replace")
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'\ufffdabc'
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'<EFBFBD>abc'
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>>> b'\x80abc'.decode("utf-8", "ignore")
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'abc'
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@ -281,10 +279,10 @@ 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(40960)
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'\ua000'
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>>> ord('\ua000')
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40960
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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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@ -326,7 +324,8 @@ Unicode Literals in Python Source Code
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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 8 hex digits, not 4::
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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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@ -465,18 +464,17 @@ like those in string objects' :meth:`encode` and :meth:`decode` methods.
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Reading Unicode from a file is therefore simple::
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f = open('unicode.rst', encoding='utf-8')
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with open('unicode.rst', encoding='utf-8') as f:
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for line in f:
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print(repr(line))
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It's also possible to open files in update mode, allowing both reading and
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writing::
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f = open('test', encoding='utf-8', mode='w+')
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with open('test', encoding='utf-8', mode='w+') as f:
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f.write('\u4500 blah blah blah\n')
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f.seek(0)
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print(repr(f.readline()[:1]))
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f.close()
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The Unicode character U+FEFF is used as a byte-order mark (BOM), and is often
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written as the first character of a file in order to assist with autodetection
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@ -513,14 +511,13 @@ usually just provide the Unicode string as the filename, and it will be
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automatically converted to the right encoding for you::
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filename = 'filename\u4500abc'
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f = open(filename, 'w')
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with open(filename, 'w') as f:
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f.write('blah\n')
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f.close()
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Functions in the :mod:`os` module such as :func:`os.stat` will also accept Unicode
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filenames.
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:func:`os.listdir`, which returns filenames, raises an issue: should it return
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Function :func:`os.listdir`, which returns filenames, raises an issue: should it return
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the Unicode version of filenames, or should it return byte strings containing
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the encoded versions? :func:`os.listdir` will do both, depending on whether you
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provided the directory path as a byte string or a Unicode string. If you pass a
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@ -569,14 +566,6 @@ strings, you will find your program vulnerable to bugs wherever you combine the
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two different kinds of strings. There is no automatic encoding or decoding if
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you do e.g. ``str + bytes``, a :exc:`TypeError` is raised for this expression.
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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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@ -594,7 +583,7 @@ this code::
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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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with open(unicode_name, 'r') as f:
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# ... return contents of file ...
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However, if an attacker could specify the ``'base64'`` encoding, they could pass
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@ -610,16 +599,19 @@ 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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<http://downloads.egenix.com/python/LSM2005-Developing-Unicode-aware-applications-in-Python.pdf>
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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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and localize an application. These slides cover Python 2.x only.
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Revision History and Acknowledgements
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=====================================
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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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.. comment
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Revision History
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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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