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 "MISE A JOUR TERMINEE"
PRINT "PARAMETRES ENREGISTRES"
Those messages should contain accents (terminée, paramètre, enregistrés) 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 Organization for
Standardization, 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 128 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
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:
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:
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.
- It’s not portable; different processors order the bytes differently.
- 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.
- It’s not compatible with existing C functions such as
strlen(), so a new
family of wide string functions would need to be used.
- 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:
- If the code point is < 128, each byte is the same as the value of the code
point.
- If the code point is 128 or greater, the Unicode string can’t be represented
in this encoding. (Python raises a
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:
- If the code point is < 128, it’s represented by the corresponding byte value.
- 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:
- It can handle any Unicode code point.
- A Unicode string is turned into a sequence 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.
- A string of ASCII text is also valid UTF-8 text.
- UTF-8 is fairly compact; the majority of commonly used characters can be
represented with one or two bytes.
- 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.