Unicode HOWTO (2024)

Definitions¶

Today’s programs need to be able to handle a wide variety ofcharacters. Applications are often internationalized to displaymessages and output in a variety of user-selectable languages; thesame program might need to output an error message in English, French,Japanese, Hebrew, or Russian. Web content can be written in any ofthese languages and can also include a variety of emoji symbols.Python’s string type uses the Unicode Standard for representingcharacters, which lets Python programs work with all these differentpossible characters.

Unicode (https://www.unicode.org/) is a specification that aims tolist every character used by human languages and give each characterits own unique code. The Unicode specifications are continuallyrevised and updated to add new languages and symbols.

A character is the smallest possible component of a text. ‘A’, ‘B’, ‘C’,etc., are all different characters. So are ‘È’ and ‘Í’. Characters varydepending on the language or context you’re talkingabout. For example, there’s a character for “Roman Numeral One”, ‘Ⅰ’, that’sseparate from the uppercase letter ‘I’. They’ll usually look the same,but these are two different characters that have different meanings.

The Unicode standard describes how characters are represented bycode points. A code point value is an integer in the range 0 to0x10FFFF (about 1.1 million values, theactual number assignedis less than that). In the standard and in this document, a code point is writtenusing the notation U+265E to mean the character with value0x265e (9,822 in decimal).

The Unicode standard contains a lot of tables listing characters andtheir corresponding code points:

0061 'a'; LATIN SMALL LETTER A0062 'b'; LATIN SMALL LETTER B0063 'c'; LATIN SMALL LETTER C...007B '{'; LEFT CURLY BRACKET...2167 'Ⅷ'; ROMAN NUMERAL EIGHT2168 'Ⅸ'; ROMAN NUMERAL NINE...265E '♞'; BLACK CHESS KNIGHT265F '♟'; BLACK CHESS PAWN...1F600 '😀'; GRINNING FACE1F609 '😉'; WINKING FACE...

Strictly, these definitions imply that it’s meaningless to say ‘this ischaracter U+265E’. U+265E is a code point, which represents some particularcharacter; in this case, it represents the character ‘BLACK CHESS KNIGHT’,‘♞’. Ininformal contexts, this distinction between code points and characters willsometimes be forgotten.

A character is represented on a screen or on paper by a set of graphicalelements 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 willdepend on the font being used. Most Python code doesn’t need to worry aboutglyphs; figuring out the correct glyph to display is generally the job of a GUItoolkit or a terminal’s font renderer.

Encodings¶

To summarize the previous section: a Unicode string is a sequence ofcode points, which are numbers from 0 through 0x10FFFF (1,114,111decimal). This sequence of code points needs to be represented inmemory as a set of code units, and code units are then mappedto 8-bit bytes. The rules for translating a Unicode string into asequence of bytes are called a character encoding, or justan encoding.

The first encoding you might think of is using 32-bit integers as thecode unit, and then using the CPU’s representation of 32-bit integers.In this representation, the string “Python” might look like this:

 P y t h o n0x50 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 ofproblems.

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 pointsare less than 127, or less than 255, so a lot of space is occupied by 0x00bytes. The above string takes 24 bytes compared to the 6 bytes needed for anASCII representation. Increased RAM usage doesn’t matter too much (desktopcomputers have gigabytes of RAM, and strings aren’t usually that large), butexpanding our usage of disk and network bandwidth by a factor of 4 isintolerable.

3. It’s not compatible with existing C functions such as strlen(), so a newfamily of wide string functions would need to be used.

Therefore this encoding isn’t used very much, and people instead choose otherencodings that are more efficient and convenient, such as UTF-8.

UTF-8 is one of the most commonly used encodings, and Python oftendefaults to using it. UTF stands for “Unicode Transformation Format”,and the ‘8’ means that 8-bit values are used in the encoding. (Thereare also UTF-16 and UTF-32 encodings, but they are less frequentlyused 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, orfour 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 sequence of bytes that contains embeddedzero bytes only where they represent the null character (U+0000). This meansthat UTF-8 strings can be processed by C functions such as strcpy() and sentthrough protocols that can’t handle zero bytes for anything other thanend-of-string markers.

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 berepresented with one or two bytes.

5. If bytes are corrupted or lost, it’s possible to determine the start of thenext UTF-8-encoded code point and resynchronize. It’s also unlikely thatrandom 8-bit data will look like valid UTF-8.

6. UTF-8 is a byte oriented encoding. The encoding specifies that eachcharacter is represented by a specific sequence of one or more bytes. Thisavoids the byte-ordering issues that can occur with integer and word orientedencodings, like UTF-16 and UTF-32, where the sequence of bytes varies dependingon the hardware on which the string was encoded.

References¶

The Unicode Consortium site has character charts, aglossary, and PDF versions of the Unicode specification. Be prepared for somedifficult reading. A chronology of theorigin and development of Unicode is also available on the site.

On the Computerphile Youtube channel, Tom Scott brieflydiscusses the history of Unicode and UTF-8(9 minutes 36 seconds).

To help understand the standard, Jukka Korpela has written an introductoryguide to reading theUnicode character tables.

Another good introductory articlewas written by Joel Spolsky.If this introduction didn’t make things clear to you, you should tryreading this alternate article before continuing.

Wikipedia entries are often helpful; see the entries for “character encoding” and UTF-8, for example.

The String Type¶

Since Python 3.0, the language’s str type contains Unicodecharacters, meaning any string created using "unicode rocks!", 'unicoderocks!', or the triple-quoted string syntax is stored as Unicode.

The default encoding for Python source code is UTF-8, so you can simplyinclude a Unicode character in a string literal:

try: with open('/tmp/input.txt', 'r') as f: ...except OSError: # 'File not found' error message. print("Fichier non trouvé")

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 tokeep the source code ASCII-only for some reason, you can also useescape 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 decode() method ofbytes. 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 beconverted according to the encoding’s rules. Legal values for this argument are'strict' (raise a UnicodeDecodeError exception), 'replace' (useU+FFFD, REPLACEMENT CHARACTER), 'ignore' (just leave thecharacter out of the Unicode result), or 'backslashreplace' (inserts a\xNN escape sequence).The following examples show the differences:

>>> b'\x80abc'.decode("utf-8", "strict") 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", "backslashreplace")'\\x80abc'>>> b'\x80abc'.decode("utf-8", "ignore")'abc'

Encodings are specified as strings containing the encoding’s name. Pythoncomes with roughly 100 different encodings; see the Python Library Reference atStandard Encodings for a list. Some encodings have multiple names; forexample, 'latin-1', 'iso_8859_1' and '8859’ are all synonyms forthe same encoding.

One-character Unicode strings can also be created with the chr()built-in function, which takes integers and returns a Unicode string of length 1that contains the corresponding code point. The reverse operation is thebuilt-in ord() function that takes a one-character Unicode string andreturns the code point value:

>>> chr(57344)'\ue000'>>> ord('\ue000')57344

Converting to Bytes¶

The opposite method of bytes.decode() is str.encode(),which returns a bytes representation of the Unicode string, encoded in therequested encoding.

The errors parameter is the same as the parameter of thedecode() method but supports a few more possible handlers. As well as'strict', 'ignore', and 'replace' (which in this caseinserts a question mark instead of the unencodable character), there isalso 'xmlcharrefreplace' (inserts an XML character reference),backslashreplace (inserts a \uNNNN escape sequence) andnamereplace (inserts a \N{...} 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') 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'>>> u.encode('ascii', 'namereplace')b'\\N{YI SYLLABLE IT}abcd\\u07b4'

The low-level routines for registering and accessing the availableencodings are found in the codecs module. Implementing newencodings also requires understanding the codecs module.However, the encoding and decoding functions returned by this moduleare usually more low-level than is comfortable, and writing new encodingsis 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 codepoint. 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 wouldin a program with messages in French or some other accent-using language. Youcan also assemble strings using the chr() built-in function, but this iseven more tedious.

Ideally, you’d want to be able to write literals in your language’s naturalencoding. You could then edit Python source code with your favorite editorwhich would display the accented characters naturally, and have the rightcharacters used at runtime.

Python supports writing source code in UTF-8 by default, but you can use almostany encoding if you declare the encoding being used. This is done by includinga 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 afile. 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 forcoding: name or coding=name in the comment.

If you don’t include such a comment, the default encoding used will be UTF-8 asalready mentioned. See also PEP 263 for more information.

Unicode Properties¶

The Unicode specification includes a database of information aboutcode points. For each defined code point, the information includesthe character’s name, its category, the numeric value if applicable(for characters representing numeric concepts such as the Romannumerals, fractions such as one-third and four-fifths, etc.). Thereare also display-related properties, such as how to use the code pointin bidirectional text.

The following program displays some information about several characters, andprints the numeric value of one particular character:

import unicodedatau = 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 characterprint(unicodedata.numeric(u[1]))

When run, this prints:

0 00e9 Ll LATIN SMALL LETTER E WITH ACUTE1 0bf2 No TAMIL NUMBER ONE THOUSAND2 0f84 Mn TIBETAN MARK HALANTA3 1770 Lo TAGBANWA LETTER SA4 33af So SQUARE RAD OVER S SQUARED1000.0

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 codesfrom the above output, 'Ll' means ‘Letter, lowercase’, 'No' means“Number, other”, 'Mn' is “Mark, nonspacing”, and 'So' is “Symbol,other”. Seethe General Category Values section of the Unicode Character Database documentation for alist of category codes.

Comparing Strings¶

Unicode adds some complication to comparing strings, because the sameset of characters can be represented by different sequences of codepoints. For example, a letter like ‘ê’ can be represented as a singlecode point U+00EA, or as U+0065 U+0302, which is the code point for‘e’ followed by a code point for ‘COMBINING CIRCUMFLEX ACCENT’. Thesewill produce the same output when printed, but one is a string oflength 1 and the other is of length 2.

One tool for a case-insensitive comparison is thecasefold() string method that converts a string to acase-insensitive form following an algorithm described by the UnicodeStandard. This algorithm has special handling for characters such asthe German letter ‘ß’ (code point U+00DF), which becomes the pair oflowercase letters ‘ss’.

>>> street = 'Gürzenichstraße'>>> street.casefold()'gürzenichstrasse'

A second tool is the unicodedata module’snormalize() function that converts strings to oneof several normal forms, where letters followed by a combining character arereplaced with single characters. normalize() canbe used to perform string comparisons that won’t falsely reportinequality if two strings use combining characters differently:

import unicodedatadef compare_strs(s1, s2): def NFD(s): return unicodedata.normalize('NFD', s) return NFD(s1) == NFD(s2)single_char = 'ê'multiple_chars = '\N{LATIN SMALL LETTER E}\N{COMBINING CIRCUMFLEX ACCENT}'print('length of first string=', len(single_char))print('length of second string=', len(multiple_chars))print(compare_strs(single_char, multiple_chars))

When run, this outputs:

$ python compare-strs.pylength of first string= 1length of second string= 2True

The first argument to the normalize() function is astring giving the desired normalization form, which can be one of‘NFC’, ‘NFKC’, ‘NFD’, and ‘NFKD’.

The Unicode Standard also specifies how to do caseless comparisons:

import unicodedatadef compare_caseless(s1, s2): def NFD(s): return unicodedata.normalize('NFD', s) return NFD(NFD(s1).casefold()) == NFD(NFD(s2).casefold())# Example usagesingle_char = 'ê'multiple_chars = '\N{LATIN CAPITAL LETTER E}\N{COMBINING CIRCUMFLEX ACCENT}'print(compare_caseless(single_char, multiple_chars))

This will print True. (Why is NFD() invoked twice? Becausethere are a few characters that make casefold() return anon-normalized string, so the result needs to be normalized again. Seesection 3.13 of the Unicode Standard for a discussion and an example.)

Unicode Regular Expressions¶

The regular expressions supported by the re module can be providedeither as bytes or strings. Some of the special character sequences such as\d and \w have different meanings depending on whetherthe pattern is supplied as bytes or a string. For example,\d will match the characters [0-9] in bytes butin strings will match any character that’s in the 'Nd' category.

The string in this example has the number 57 written in both Thai andArabic numerals:

import rep = re.compile(r'\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 themout. If you supply the re.ASCII flag tocompile(), \d+ will match the substring “57” instead.

Similarly, \w matches a wide variety of Unicode characters butonly [a-zA-Z0-9_] in bytes or if re.ASCII is supplied,and \s will match either Unicode whitespace characters or[ \t\n\r\f\v].

References¶

Some good alternative discussions of Python’s Unicode support are:

• Processing Text Files in Python 3, by Nick Coghlan.

• Pragmatic Unicode, a PyCon 2012 presentation by Ned Batchelder.

The str type is described in the Python library reference atText Sequence Type — str.

The documentation for the unicodedata module.

The documentation for the codecs module.

Marc-André Lemburg gave a presentation titled “Python and Unicode” (PDF slides) atEuroPython 2002. The slides are an excellent overview of the design of Python2’s Unicode features (where the Unicode string type is called unicode andliterals start with u).

Unicode filenames¶

Most of the operating systems in common use today support filenamesthat contain arbitrary Unicode characters. Usually this isimplemented by converting the Unicode string into some encoding thatvaries depending on the system. Today Python is converging on usingUTF-8: Python on MacOS has used UTF-8 for several versions, and Python3.6 switched to using UTF-8 on Windows as well. On Unix systems,there will only be a filesystem encoding. if you’ve set the LANG or LC_CTYPE environment variables; ifyou haven’t, the default encoding is again UTF-8.

The sys.getfilesystemencoding() function returns the encoding to use onyour current system, in case you want to do the encoding manually, but there’snot much reason to bother. When opening a file for reading or writing, you canusually just provide the Unicode string as the filename, and it will beautomatically converted to the right encoding for you:

filename = 'filename\u4500abc'with open(filename, 'w') as f: f.write('blah\n')

Functions in the os module such as os.stat() will also accept Unicodefilenames.

The os.listdir() function returns filenames, which raises an issue: should it returnthe Unicode version of filenames, or should it return bytes containingthe encoded versions? os.listdir() can do both, depending on whether youprovided the directory path as bytes or a Unicode string. If you pass aUnicode string as the path, filenames will be decoded using the filesystem’sencoding and a list of Unicode strings will be returned, while passing a bytepath 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 osprint(os.listdir(b'.'))print(os.listdir('.'))

will produce the following output:

$ python listdir-test.py[b'filename\xe4\x94\x80abc', ...]['filename\u4500abc', ...]

The first list contains UTF-8-encoded filenames, and the second list containsthe Unicode versions.

Note that on most occasions, you should can just stick with usingUnicode with these APIs. The bytes APIs should only be used onsystems where undecodable file names can be present; that’spretty much only Unix systems now.

Tips for Writing Unicode-aware Programs¶

This section provides some suggestions on writing software that deals withUnicode.

The most important tip is:

Software should only work with Unicode strings internally, decoding the inputdata as soon as possible and encoding the output only at the end.

If you attempt to write processing functions that accept both Unicode and bytestrings, you will find your program vulnerable to bugs wherever you combine thetwo different kinds of strings. There is no automatic encoding or decoding: ifyou do e.g. str + bytes, a TypeError will be raised.

When using data coming from a web browser or some other untrusted source, acommon technique is to check for illegal characters in a string before using thestring in a generated command line or storing it in a database. If you’re doingthis, be careful to check the decoded string, not the encoded bytes data;some encodings may have interesting properties, such as not being bijectiveor not being fully ASCII-compatible. This is especially true if the inputdata also specifies the encoding, since the attacker can then choose aclever way to hide malicious text in the encoded bytestream.

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') )

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)

References¶

One section of Mastering Python 3 Input/Output,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-awareApplications in Python”discuss questions of character encodings as well as how to internationalizeand localize an application. These slides cover Python 2.x only.

The Guts of Unicode in Pythonis a PyCon 2013 talk by Benjamin Peterson that discusses the internal Unicoderepresentation in Python 3.3.