Unicode (and Python)

Juan Manuel Gimeno Illa

[email protected]

November 2008

J.M.Gimeno ([email protected]) Unicode November 2008 1 / 21

Outline

1 Before Unicode

2 UnicodeUnicode ConceptsEncodings

3 Python’s Unicode SupportUnicode String TypeSource Code Encoding

4 Bibliography

Before Unicode

In the beginning, computing was mainly centered in North Americaand done in English. Characters were stored one-per-byte by usingeither

I ASCII (7 bits)I EBCDIC (8 bits)

In other parts of the world, different ways of storing their characterswere invented

I Japan: various flavours of JIS encodingsI Russian: KOI8I India: ISCI standard

Also, there were some proprietary encodings defined by operatingsystem vendors

Before Unicode

ISO-8859-*

For the huge number of people in America, Europe, and the MiddleEast who use relatively small alphabets, there was ISO-8859

I left ASCII as ASCII (range 0 to 127)I used the range 128 through 255 for different purposes

1-4 Different accented characters (e.g. latin-1)

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

But you could only be using one at a time, so one couldn’t easily mixGreek and Cyrillic in the same file.

Before Unicode

ISO-8859-*

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

Before Unicode

ISO-8859-*

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

Before Unicode

ISO-8859-*

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

Before Unicode

ISO-8859-*

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

Before Unicode

ISO-8859-*

5 Cyrillic

6 Arabic

7 Greek

8 Hebrew

9 Turkish

10 Nordic languages

Before Unicode

Huston, Huston, . . .

Clearly this was an very unsatisfactory situation

ISO-2022 provided a partial solution allowing to shift encodings inthe middle of a string

I it was difficult to useI so it wasn’t widespread

What was needed was an universal way to refer to all the differentcharacters in all the alphabets

I ISO/IEC 10646I Unicode

Before Unicode

Unicode Unicode Concepts

Unicode’s Solution

One encoding for all scripts of the world

ASCII compatibility (even Latin-1)

Includes character meta dataI Case mapping informationI Character category information

Accounts for scripts using different orientations

Enables sorting and normalization support

Unicode’s Terminology

Grapheme This is what users regard as a character- Andre

Code points This is an Unicode encoding of the string- AndrU+00E9 (LATIN SMALL LETTER E WITH ACUTE)- Andre’=AndreU+0301 (COMBINING ACUTE ACCENT)

Code Units This is what the implementation stores (e.g. UTF-8- Andre0xCC 0x81

This can be explored in Linux using the program gucharmap

Unicode Organization

Unicode currently defines just under 100000 code points but it hasspace for upto 1114112

They are organized into 17 planes of 216 = 65536 characters,numbered 0 to 16

Plane 0 is called Basic Multilingual Plane (BMP) and contains prettywell everything usefulThe characters in BMP are laid out more or less West to East

I ASCII characters from 0 to 127I Latin-1 characters from 128 to 255I Then moving East in Europe (Greek, Cyrillic)I Next Middle East (Arabic, Hebrew)I Then the Indus (scripts of India)I Next Southeast Asia (Thai, Laotian and so on)I and ending with China, Japan and Korea

Planes 1 to 16 are sometimes called astral planes that include exotic,rare and historically important characters (old italic, byzantinemusical symbols, etc.)

Code Points

Each code point (“character”) gets a number and a name

The number is usually given in hexadecimal and prefixed by U+

(Note that it is not a 16 bit number due to the astral planes !!!)

Unicode includes tables with useful character properties (metadata)such as

I this is a numberI this is uppercaseI this is punctuation

The standard also providesI a helpful picture of a reasonably typical renditionI rules for line-breakingI hyphenationI sorting

Code Points

Unicode Encodings

Encodings

Along with the code points, Unicode also defines methods for storingthem in byte sequences in a computer

There are three approaches named UTF-8, UTF-16 and UTF-32

UTF stands for Unicode Transformation Format or UCSTransformation Format where UCS stands for Unicode CharacterSet

The characters we will use in the explanations are:

Number Name PlaneU+0026 (38) AMPERSAND BMPU+0416 (1046) CYRILLIC CAPITAL LETTER ZHE BMPU+4E2D (20013) HAN IDEOGRAPH 4E2E BMPU+10346 (66374) GOTHIC LETTER FAIHU Astral

Unicode Encodings

Encodings

Unicode Encodings

Encodings

Unicode Encodings

Encodings

Unicode Encodings

UTF-32

The simplest way to storing characters: you use 32 bits (4 bytes) tostore each character

So we store 38, 1046, 20013 and 66374 as 32 bit integers

For Latin-1 characters it wastes too much space

Problems with C strings because most bytes are zero (use wchar t)

There are lots of ways of storing 4 byte integers among 4 bytes(remember big-endian and little-endian?)

So if you send one of these 4-byte integers to another machineproblems occur if they use different orderings

Solutions:

Explicitness UTF-32BE and UTF-32LE encodingsByte Order Mark (BOM) Character U+FEFF (ZERO WIDTH

NO-BREAK SPACE) and the guarantee that U+FFFEwill never be a character

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-32

Solutions:

Unicode Encodings

UTF-16UTF-16 stores Unicode characters in 16 bit chunks

I all the BMP characters appear as themselvesI some trickery is needed for the astral plane ones

There are two blocks of code points in the BMP called surrogate blocksHigh surrogates from U+D800 to U+DBFFLow surrogates from U+DC00 to U+DFFFAstral plane characters are splitted into two characters

I first, 0x10000 = 216 is subtracted from the code pointI next, its 20 bits are splitted using the low surrogate for the low ten bits

and the high for the high ones

This gives 20 bits or 220 characters that fits the 16 = 24 astral planeswith 216 characters eachSo U+10346 is represented as the 16-bits integers 0xD800 0xDF46It also has ordering problems so the UTF-16BE, UTF-16LE or use ofthe BOMNightmare in C: embedded zeros and not same size as wchar tThe most efficient way to store asian characters

Unicode Encodings

UTF-8 was invented by Ken Thompson on September 2, 1992, on aplacemat in a New Jersey diner with Rob Pike.

It works like this:I characters whose value is less that 128 (ASCII) are encoded as

themselves in one byteI the rest will have its bits ripped apart and deal out into several (from

two to four) bytes as follows:F The first byte has a bunch of high-order one bits telling how many

bytes are used to encode the character, followed by a zero bitF The rest of the bytes each begin with a single one byte followed by a

zero bitF The bits of the character are dealt out in the space left over after these

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

signalling bits

Unicode Encodings

The following table summarizes the rules:

Hex range Binary UTF-8000000–00007F 0zzzzzzz 0zzzzzzz

000080–0007FF 00000yyy yyzzzzzz 110yyyyy 10zzzzzz

000800–00FFFF xxxxyyyy yyzzzzzz 1110xxxx 10yyyyyy 10zzzzzz

010000–10FFFF 000wwwxx xxxxyyyy yyzzzzzz 11110www 10xxxxxx 10yyyyyy 10zzzzzz

Our examples result in:

Character Binary UTF-8U+0026 00100110 00100110

U+0416 00000100 00010110 11010000 10010110

U+4E2D 01001110 00101101 11100100 10111000 10101101