All systems use a particular way of encoding characters. The basic encoding that (almost) every system supports is the standard ASCII code. This is a 7-bits code, which can represent 128 different characters. Several of these (in particular those with the lowest-numbered encodings) are control characters that have a special function. Most of these special functions are only useful for archaic computer systems, but the tabulation, newline, and backspace characters are found amongst them. If you only use characters on a standard US keyboard, you are limited to ASCII characters.

Nowadays, many systems use Unicode. Unicode supports far more characters. There are different formats for storing characters in Unicode. The best-known is UTF-8, which uses one byte for each of the ASCII characters, but multiple bytes for all the other characters (a byte is a group of 8 bits, whereby each bit contains either a 1 or a zero). Other Unicode encodings use multiple bytes to store any character. Python, be default, works with UTF-8, which means that it also supports regular ASCII encodings.

ASCII

Below I display the ASCII table. The only characters I have left off are those which are control sequences. These have the numbers zero to 31, and 127. 32 is the space. I also display the hexadecimal code for each character next to the decimal code. (If you wonder why I even bother listing those hexadecimal codes: they become relevant in a later chapter.)

As you can see, each character has a number attached to it. To find out what a character’s number is in a program, you can use the ord() function. For instance, ord("A") returns the number of "A", which, as you can see, is 65. The counterpart of the ord() function is the chr() function. chr() gets a number as argument, and returns the character that belongs to that number. For instance, chr(65) is the letter "A".

DC HX      DC HX      DC HX      DC HX      DC HX      DC HX
32 20      48 30 0    64 40 @    80 50 P    96 60 `   112 70 p
33 21 !    49 31 1    65 41 A    81 51 Q    97 61 a   113 71 q
34 22 "    50 32 2    66 42 B    82 52 R    98 62 b   114 72 r
35 23 #    51 33 3    67 43 C    83 53 S    99 63 c   115 73 s
36 24 $    52 34 4    68 44 D    84 54 T   100 64 d   116 74 t
37 25 %    53 35 5    69 45 E    85 55 U   101 65 e   117 75 u
38 26 &    54 36 6    70 46 F    86 56 V   102 66 f   118 76 v
39 27 '    55 37 7    71 47 G    87 57 W   103 67 g   119 77 w
40 28 (    56 38 8    72 48 H    88 58 X   104 68 h   120 78 x
41 29 )    57 39 9    73 49 I    89 59 Y   105 69 i   121 79 y
42 2A *    58 3A :    74 4A J    90 5A Z   106 6A j   122 7A z
43 2B +    59 3B ;    75 4B K    91 5B [   107 6B k   123 7B {
44 2C ,    60 3C <    76 4C L    92 5C \   108 6C l   124 7C |
45 2D -    61 3D =    77 4D M    93 5D ]   109 6D m   125 7D }
46 2E .    62 3E >    78 4E N    94 5E ^   110 6E n   126 7E ~
47 2F /    63 3F ?    79 4F O    95 5F _   111 6F o

A comparison of strings which use only these characters use the numbers of the characters to determine which string is “smaller.” For instance, the string "orange" is smaller than the string "ordinal", because the first character that differs between them is the third one, which is "a" for "orange" and "d" for "ordinal", and since the number for "a" is lower than the number for "d", the string "orange" is considered to be smaller than the string "ordinal". This is, basically, an alphabetic comparison. If characters occur in a string that are not letters, you can check in the ASCII table which is considered lower. Notice how all the digits are lower than letters.

print( ord( 'A' ) )
print( ord( 'a' ) )
print( chr( 65 ) )
print( chr( 97 ) )
print( "orange" < "ordinal" )

You can use these numbers that are associated with characters to do all kinds of neat calculations. For instance, if I want to know which the twelfth letter after "g" is, I can calculate that as follows:

print( "The 12th letter after g is", chr( ord( "g" )+12 ) )

For another example of what you can do with character codes, here is a program that generates the ASCII table as a matrix:

print( ' ', end='' )
for i in range(16):
    if i < 10:
        print( ' '+chr( ord( '0' )+i ), end='' )
    else:
        print( ' '+chr( ord( 'A' )+i-10 ), end='' )
print()
for i in range( 2, 8 ):
    print( i, end='' )
    for j in range( 16 ):
        c = i*16+j
        print( ' '+chr( c ), end='' )
    print()

Note that I highly prefer you using the ord() and chr() functions if you want to juggle character encoding. If you want to refer to the character code of the letter "A", do not write 65, but write ord("A") instead. 65 is only meaningful to people who know ASCII encodings, and your programs should be meaningful to anybody. Moreover, while ASCII is a widely-used standard, there are still computers out there which use different encoding mechanisms, in which the code for "A" is not necessarily 65 (I am looking at you, IBM).

UTF-8

Python supports Unicode, in particular the most common Unicode encoding scheme UTF-8. This means that you can use all kinds of “weird” characters. I explained that in the naming of functions and variables you can use “letters,” which you probably assumed meant "A" to "Z" and "a" to "z". The funny thing is that it depends on the language codes of your computer what is considered a letter. For instance, if your computer tells Python that the language is German, then you can also use characters with umlauts. I strongly discourage using such letters in variable and function names, by the way. Not only are they hard to type, but they also make your program less portable.

In UTF-8, the regular characters which you find on a keyboard are represented in strings exactly as you would expect. However, “special” characters can be incorporated too, but look quite different. Since Python supports UTF-8, you have to be careful when you copy texts from, for instance, a word processor document. Word processors have the disturbing habit of changing characters into other characters, like turning straight quotes into round quotes. If you copy such round quotes into your program, Python will accept the characters, but will not interpret them as, for instance, string boundaries.

If you want to display Unicode characters, you can do so by using Unicode encodings. You have to know the UTF-8 number of the character that you want to display. If you know that, you can use a code \uxxxx, where xxxx is a hexadecimal number, to incorporate a Unicode character in a string. For example, the code below displays the capitals of the Greek alphabet:¹¹

alpha = "\u0391"
for i in range( 25 ):
    print( chr( ord( alpha )+i ), end=" " )

In general, you will not need to worry too much about character encodings. I recommend that you restrict yourself to ASCII whenever possible. In cases where you have to deal with Unicode characters, things usually work correctly automatically, since the standard Python functionalities support Unicode. Occasionally I have run into translation problems from Unicode to ASCII, in particular where files were concerned. It will be a while before you run into problems like that, and I will bring it up again in Chapter 17² and later.