mirror of https://github.com/python/cpython
719 lines
31 KiB
ReStructuredText
719 lines
31 KiB
ReStructuredText
.. _unicode-howto:
|
|
|
|
*****************
|
|
Unicode HOWTO
|
|
*****************
|
|
|
|
:Release: 1.12
|
|
|
|
This HOWTO discusses Python support for Unicode, and explains
|
|
various problems that people commonly encounter when trying to work
|
|
with Unicode.
|
|
|
|
Introduction to Unicode
|
|
=======================
|
|
|
|
History of Character Codes
|
|
--------------------------
|
|
|
|
In 1968, the American Standard Code for Information Interchange, better known by
|
|
its acronym ASCII, was standardized. ASCII defined numeric codes for various
|
|
characters, with the numeric values running from 0 to 127. For example, the
|
|
lowercase letter 'a' is assigned 97 as its code value.
|
|
|
|
ASCII was an American-developed standard, so it only defined unaccented
|
|
characters. There was an 'e', but no 'é' or 'Í'. This meant that languages
|
|
which required accented characters couldn't be faithfully represented in ASCII.
|
|
(Actually the missing accents matter for English, too, which contains words such
|
|
as 'naïve' and 'café', and some publications have house styles which require
|
|
spellings such as 'coöperate'.)
|
|
|
|
For a while people just wrote programs that didn't display accents.
|
|
In the mid-1980s an Apple II BASIC program written by a French speaker
|
|
might have lines like these::
|
|
|
|
PRINT "FICHIER EST COMPLETE."
|
|
PRINT "CARACTERE NON ACCEPTE."
|
|
|
|
Those messages should contain accents (completé, caractère, accepté),
|
|
and they just look wrong to someone who can read French.
|
|
|
|
In the 1980s, almost all personal computers were 8-bit, meaning that bytes could
|
|
hold values ranging from 0 to 255. ASCII codes only went up to 127, so some
|
|
machines assigned values between 128 and 255 to accented characters. Different
|
|
machines had different codes, however, which led to problems exchanging files.
|
|
Eventually various commonly used sets of values for the 128--255 range emerged.
|
|
Some were true standards, defined by the International Standards Organization,
|
|
and some were *de facto* conventions that were invented by one company or
|
|
another and managed to catch on.
|
|
|
|
255 characters aren't very many. For example, you can't fit both the accented
|
|
characters used in Western Europe and the Cyrillic alphabet used for Russian
|
|
into the 128--255 range because there are more than 127 such characters.
|
|
|
|
You could write files using different codes (all your Russian files in a coding
|
|
system called KOI8, all your French files in a different coding system called
|
|
Latin1), but what if you wanted to write a French document that quotes some
|
|
Russian text? In the 1980s people began to want to solve this problem, and the
|
|
Unicode standardization effort began.
|
|
|
|
Unicode started out using 16-bit characters instead of 8-bit characters. 16
|
|
bits means you have 2^16 = 65,536 distinct values available, making it possible
|
|
to represent many different characters from many different alphabets; an initial
|
|
goal was to have Unicode contain the alphabets for every single human language.
|
|
It turns out that even 16 bits isn't enough to meet that goal, and the modern
|
|
Unicode specification uses a wider range of codes, 0 through 1,114,111 (
|
|
``0x10FFFF`` in base 16).
|
|
|
|
There's a related ISO standard, ISO 10646. Unicode and ISO 10646 were
|
|
originally separate efforts, but the specifications were merged with the 1.1
|
|
revision of Unicode.
|
|
|
|
(This discussion of Unicode's history is highly simplified. The
|
|
precise historical details aren't necessary for understanding how to
|
|
use Unicode effectively, but if you're curious, consult the Unicode
|
|
consortium site listed in the References or
|
|
the `Wikipedia entry for Unicode <http://en.wikipedia.org/wiki/Unicode#History>`_
|
|
for more information.)
|
|
|
|
|
|
Definitions
|
|
-----------
|
|
|
|
A **character** is the smallest possible component of a text. 'A', 'B', 'C',
|
|
etc., are all different characters. So are 'È' and 'Í'. Characters are
|
|
abstractions, and vary depending on the language or context you're talking
|
|
about. For example, the symbol for ohms (Ω) is usually drawn much like the
|
|
capital letter omega (Ω) in the Greek alphabet (they may even be the same in
|
|
some fonts), but these are two different characters that have different
|
|
meanings.
|
|
|
|
The Unicode standard describes how characters are represented by **code
|
|
points**. A code point is an integer value, usually denoted in base 16. In the
|
|
standard, a code point is written using the notation ``U+12CA`` to mean the
|
|
character with value ``0x12ca`` (4,810 decimal). The Unicode standard contains
|
|
a lot of tables listing characters and their corresponding code points:
|
|
|
|
.. code-block:: none
|
|
|
|
0061 'a'; LATIN SMALL LETTER A
|
|
0062 'b'; LATIN SMALL LETTER B
|
|
0063 'c'; LATIN SMALL LETTER C
|
|
...
|
|
007B '{'; LEFT CURLY BRACKET
|
|
|
|
Strictly, these definitions imply that it's meaningless to say 'this is
|
|
character ``U+12CA``'. ``U+12CA`` is a code point, which represents some particular
|
|
character; in this case, it represents the character 'ETHIOPIC SYLLABLE WI'. In
|
|
informal contexts, this distinction between code points and characters will
|
|
sometimes be forgotten.
|
|
|
|
A character is represented on a screen or on paper by a set of graphical
|
|
elements that's called a **glyph**. The glyph for an uppercase A, for example,
|
|
is two diagonal strokes and a horizontal stroke, though the exact details will
|
|
depend on the font being used. Most Python code doesn't need to worry about
|
|
glyphs; figuring out the correct glyph to display is generally the job of a GUI
|
|
toolkit or a terminal's font renderer.
|
|
|
|
|
|
Encodings
|
|
---------
|
|
|
|
To summarize the previous section: a Unicode string is a sequence of code
|
|
points, which are numbers from 0 through ``0x10FFFF`` (1,114,111 decimal). This
|
|
sequence needs to be represented as a set of bytes (meaning, values
|
|
from 0 through 255) in memory. The rules for translating a Unicode string
|
|
into a sequence of bytes are called an **encoding**.
|
|
|
|
The first encoding you might think of is an array of 32-bit integers. In this
|
|
representation, the string "Python" would look like this:
|
|
|
|
.. code-block:: none
|
|
|
|
P y t h o n
|
|
0x50 00 00 00 79 00 00 00 74 00 00 00 68 00 00 00 6f 00 00 00 6e 00 00 00
|
|
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
|
|
|
|
This representation is straightforward but using it presents a number of
|
|
problems.
|
|
|
|
1. It's not portable; different processors order the bytes differently.
|
|
|
|
2. It's very wasteful of space. In most texts, the majority of the code points
|
|
are less than 127, or less than 255, so a lot of space is occupied by ``0x00``
|
|
bytes. The above string takes 24 bytes compared to the 6 bytes needed for an
|
|
ASCII representation. Increased RAM usage doesn't matter too much (desktop
|
|
computers have gigabytes of RAM, and strings aren't usually that large), but
|
|
expanding our usage of disk and network bandwidth by a factor of 4 is
|
|
intolerable.
|
|
|
|
3. It's not compatible with existing C functions such as ``strlen()``, so a new
|
|
family of wide string functions would need to be used.
|
|
|
|
4. Many Internet standards are defined in terms of textual data, and can't
|
|
handle content with embedded zero bytes.
|
|
|
|
Generally people don't use this encoding, instead choosing other
|
|
encodings that are more efficient and convenient. UTF-8 is probably
|
|
the most commonly supported encoding; it will be discussed below.
|
|
|
|
Encodings don't have to handle every possible Unicode character, and most
|
|
encodings don't. The rules for converting a Unicode string into the ASCII
|
|
encoding, for example, are simple; for each code point:
|
|
|
|
1. If the code point is < 128, each byte is the same as the value of the code
|
|
point.
|
|
|
|
2. If the code point is 128 or greater, the Unicode string can't be represented
|
|
in this encoding. (Python raises a :exc:`UnicodeEncodeError` exception in this
|
|
case.)
|
|
|
|
Latin-1, also known as ISO-8859-1, is a similar encoding. Unicode code points
|
|
0--255 are identical to the Latin-1 values, so converting to this encoding simply
|
|
requires converting code points to byte values; if a code point larger than 255
|
|
is encountered, the string can't be encoded into Latin-1.
|
|
|
|
Encodings don't have to be simple one-to-one mappings like Latin-1. Consider
|
|
IBM's EBCDIC, which was used on IBM mainframes. Letter values weren't in one
|
|
block: 'a' through 'i' had values from 129 to 137, but 'j' through 'r' were 145
|
|
through 153. If you wanted to use EBCDIC as an encoding, you'd probably use
|
|
some sort of lookup table to perform the conversion, but this is largely an
|
|
internal detail.
|
|
|
|
UTF-8 is one of the most commonly used encodings. UTF stands for "Unicode
|
|
Transformation Format", and the '8' means that 8-bit numbers are used in the
|
|
encoding. (There are also a UTF-16 and UTF-32 encodings, but they are less
|
|
frequently used than UTF-8.) UTF-8 uses the following rules:
|
|
|
|
1. If the code point is < 128, it's represented by the corresponding byte value.
|
|
2. If the code point is >= 128, it's turned into a sequence of two, three, or
|
|
four bytes, where each byte of the sequence is between 128 and 255.
|
|
|
|
UTF-8 has several convenient properties:
|
|
|
|
1. It can handle any Unicode code point.
|
|
2. A Unicode string is turned into a string of bytes containing no embedded zero
|
|
bytes. This avoids byte-ordering issues, and means UTF-8 strings can be
|
|
processed by C functions such as ``strcpy()`` and sent through protocols that
|
|
can't handle zero bytes.
|
|
3. A string of ASCII text is also valid UTF-8 text.
|
|
4. UTF-8 is fairly compact; the majority of commonly used characters can be
|
|
represented with one or two bytes.
|
|
5. If bytes are corrupted or lost, it's possible to determine the start of the
|
|
next UTF-8-encoded code point and resynchronize. It's also unlikely that
|
|
random 8-bit data will look like valid UTF-8.
|
|
|
|
|
|
|
|
References
|
|
----------
|
|
|
|
The `Unicode Consortium site <http://www.unicode.org>`_ has character charts, a
|
|
glossary, and PDF versions of the Unicode specification. Be prepared for some
|
|
difficult reading. `A chronology <http://www.unicode.org/history/>`_ of the
|
|
origin and development of Unicode is also available on the site.
|
|
|
|
To help understand the standard, Jukka Korpela has written `an introductory
|
|
guide <http://www.cs.tut.fi/~jkorpela/unicode/guide.html>`_ to reading the
|
|
Unicode character tables.
|
|
|
|
Another `good introductory article <http://www.joelonsoftware.com/articles/Unicode.html>`_
|
|
was written by Joel Spolsky.
|
|
If this introduction didn't make things clear to you, you should try
|
|
reading this alternate article before continuing.
|
|
|
|
Wikipedia entries are often helpful; see the entries for "`character encoding
|
|
<http://en.wikipedia.org/wiki/Character_encoding>`_" and `UTF-8
|
|
<http://en.wikipedia.org/wiki/UTF-8>`_, for example.
|
|
|
|
|
|
Python's Unicode Support
|
|
========================
|
|
|
|
Now that you've learned the rudiments of Unicode, we can look at Python's
|
|
Unicode features.
|
|
|
|
The String Type
|
|
---------------
|
|
|
|
Since Python 3.0, the language features a :class:`str` type that contain Unicode
|
|
characters, meaning any string created using ``"unicode rocks!"``, ``'unicode
|
|
rocks!'``, or the triple-quoted string syntax is stored as Unicode.
|
|
|
|
The default encoding for Python source code is UTF-8, so you can simply
|
|
include a Unicode character in a string literal::
|
|
|
|
try:
|
|
with open('/tmp/input.txt', 'r') as f:
|
|
...
|
|
except IOError:
|
|
# 'File not found' error message.
|
|
print("Fichier non trouvé")
|
|
|
|
You can use a different encoding from UTF-8 by putting a specially-formatted
|
|
comment as the first or second line of the source code::
|
|
|
|
# -*- coding: <encoding name> -*-
|
|
|
|
Side note: Python 3 also supports using Unicode characters in identifiers::
|
|
|
|
répertoire = "/tmp/records.log"
|
|
with open(répertoire, "w") as f:
|
|
f.write("test\n")
|
|
|
|
If you can't enter a particular character in your editor or want to
|
|
keep the source code ASCII-only for some reason, you can also use
|
|
escape sequences in string literals. (Depending on your system,
|
|
you may see the actual capital-delta glyph instead of a \u escape.) ::
|
|
|
|
>>> "\N{GREEK CAPITAL LETTER DELTA}" # Using the character name
|
|
'\u0394'
|
|
>>> "\u0394" # Using a 16-bit hex value
|
|
'\u0394'
|
|
>>> "\U00000394" # Using a 32-bit hex value
|
|
'\u0394'
|
|
|
|
In addition, one can create a string using the :func:`~bytes.decode` method of
|
|
:class:`bytes`. This method takes an *encoding* argument, such as ``UTF-8``,
|
|
and optionally an *errors* argument.
|
|
|
|
The *errors* argument specifies the response when the input string can't be
|
|
converted according to the encoding's rules. Legal values for this argument are
|
|
``'strict'`` (raise a :exc:`UnicodeDecodeError` exception), ``'replace'`` (use
|
|
``U+FFFD``, ``REPLACEMENT CHARACTER``), or ``'ignore'`` (just leave the
|
|
character out of the Unicode result).
|
|
The following examples show the differences::
|
|
|
|
>>> b'\x80abc'.decode("utf-8", "strict") #doctest: +NORMALIZE_WHITESPACE
|
|
Traceback (most recent call last):
|
|
...
|
|
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0:
|
|
invalid start byte
|
|
>>> b'\x80abc'.decode("utf-8", "replace")
|
|
'\ufffdabc'
|
|
>>> b'\x80abc'.decode("utf-8", "ignore")
|
|
'abc'
|
|
|
|
(In this code example, the Unicode replacement character has been replaced by
|
|
a question mark because it may not be displayed on some systems.)
|
|
|
|
Encodings are specified as strings containing the encoding's name. Python 3.2
|
|
comes with roughly 100 different encodings; see the Python Library Reference at
|
|
:ref:`standard-encodings` for a list. Some encodings have multiple names; for
|
|
example, ``'latin-1'``, ``'iso_8859_1'`` and ``'8859``' are all synonyms for
|
|
the same encoding.
|
|
|
|
One-character Unicode strings can also be created with the :func:`chr`
|
|
built-in function, which takes integers and returns a Unicode string of length 1
|
|
that contains the corresponding code point. The reverse operation is the
|
|
built-in :func:`ord` function that takes a one-character Unicode string and
|
|
returns the code point value::
|
|
|
|
>>> chr(57344)
|
|
'\ue000'
|
|
>>> ord('\ue000')
|
|
57344
|
|
|
|
Converting to Bytes
|
|
-------------------
|
|
|
|
The opposite method of :meth:`bytes.decode` is :meth:`str.encode`,
|
|
which returns a :class:`bytes` representation of the Unicode string, encoded in the
|
|
requested *encoding*.
|
|
|
|
The *errors* parameter is the same as the parameter of the
|
|
:meth:`~bytes.decode` method but supports a few more possible handlers. As well as
|
|
``'strict'``, ``'ignore'``, and ``'replace'`` (which in this case
|
|
inserts a question mark instead of the unencodable character), there is
|
|
also ``'xmlcharrefreplace'`` (inserts an XML character reference) and
|
|
``backslashreplace`` (inserts a ``\uNNNN`` escape sequence).
|
|
|
|
The following example shows the different results::
|
|
|
|
>>> u = chr(40960) + 'abcd' + chr(1972)
|
|
>>> u.encode('utf-8')
|
|
b'\xea\x80\x80abcd\xde\xb4'
|
|
>>> u.encode('ascii') #doctest: +NORMALIZE_WHITESPACE
|
|
Traceback (most recent call last):
|
|
...
|
|
UnicodeEncodeError: 'ascii' codec can't encode character '\ua000' in
|
|
position 0: ordinal not in range(128)
|
|
>>> u.encode('ascii', 'ignore')
|
|
b'abcd'
|
|
>>> u.encode('ascii', 'replace')
|
|
b'?abcd?'
|
|
>>> u.encode('ascii', 'xmlcharrefreplace')
|
|
b'ꀀabcd޴'
|
|
>>> u.encode('ascii', 'backslashreplace')
|
|
b'\\ua000abcd\\u07b4'
|
|
|
|
The low-level routines for registering and accessing the available
|
|
encodings are found in the :mod:`codecs` module. Implementing new
|
|
encodings also requires understanding the :mod:`codecs` module.
|
|
However, the encoding and decoding functions returned by this module
|
|
are usually more low-level than is comfortable, and writing new encodings
|
|
is a specialized task, so the module won't be covered in this HOWTO.
|
|
|
|
|
|
Unicode Literals in Python Source Code
|
|
--------------------------------------
|
|
|
|
In Python source code, specific Unicode 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 eight hex digits,
|
|
not four::
|
|
|
|
>>> s = "a\xac\u1234\u20ac\U00008000"
|
|
... # ^^^^ two-digit hex escape
|
|
... # ^^^^^^ four-digit Unicode escape
|
|
... # ^^^^^^^^^^ eight-digit Unicode escape
|
|
>>> [ord(c) for c in s]
|
|
[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:`chr` 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 source code in UTF-8 by default, but you can use almost
|
|
any encoding if you declare the encoding being used. This is done by including
|
|
a special comment as either the first or second line of the source file::
|
|
|
|
#!/usr/bin/env python
|
|
# -*- coding: latin-1 -*-
|
|
|
|
u = 'abcdé'
|
|
print(ord(u[-1]))
|
|
|
|
The syntax is inspired by Emacs's notation for specifying variables local to a
|
|
file. Emacs supports many different variables, but Python only supports
|
|
'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.
|
|
|
|
If you don't include such a comment, the default encoding used will be UTF-8 as
|
|
already mentioned. See also :pep:`263` for more information.
|
|
|
|
|
|
Unicode Properties
|
|
------------------
|
|
|
|
The Unicode specification includes a database of information about code points.
|
|
For each defined code point, the information includes the character's
|
|
name, its category, the numeric value if applicable (Unicode has characters
|
|
representing the Roman numerals and fractions such as one-third and
|
|
four-fifths). There are also properties related to the code point's use in
|
|
bidirectional text and other display-related properties.
|
|
|
|
The following program displays some information about several characters, and
|
|
prints the numeric value of one particular character::
|
|
|
|
import unicodedata
|
|
|
|
u = chr(233) + chr(0x0bf2) + chr(3972) + chr(6000) + chr(13231)
|
|
|
|
for i, c in enumerate(u):
|
|
print(i, '%04x' % ord(c), unicodedata.category(c), end=" ")
|
|
print(unicodedata.name(c))
|
|
|
|
# Get numeric value of second character
|
|
print(unicodedata.numeric(u[1]))
|
|
|
|
When run, this prints:
|
|
|
|
.. code-block:: none
|
|
|
|
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
|
|
`the General Category Values section of the Unicode Character Database documentation <http://www.unicode.org/reports/tr44/#General_Category_Values>`_ for a
|
|
list of category codes.
|
|
|
|
|
|
Unicode Regular Expressions
|
|
---------------------------
|
|
|
|
The regular expressions supported by the :mod:`re` module can be provided
|
|
either as bytes or strings. Some of the special character sequences such as
|
|
``\d`` and ``\w`` have different meanings depending on whether
|
|
the pattern is supplied as bytes or a string. For example,
|
|
``\d`` will match the characters ``[0-9]`` in bytes but
|
|
in strings will match any character that's in the ``'Nd'`` category.
|
|
|
|
The string in this example has the number 57 written in both Thai and
|
|
Arabic numerals::
|
|
|
|
import re
|
|
p = re.compile('\d+')
|
|
|
|
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 bytes
|
|
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:`~io.TextIOBase.read` and
|
|
:meth:`~io.TextIOBase.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:`~codecs.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.
|