A computer stores everything as numbers, and text is no exception. Every character has a number of its own, and a string is stored as the list of numbers for its characters. The code that says which number goes with which character is called ASCII, short for American Standard Code for Information Interchange.

The ASCII codes

You do not need to learn the codes by heart, but it is worth knowing how they are laid out:

The letters are in alphabetical order, so B is 66, C is 67 and so on up to 90 for Z. Capital letters and small letters have different codes: A is 65 but a is 97. That is why Python treats "A" and "a" as different characters. Punctuation has codes too, mostly in the gaps between these ranges.

From a character to its code

ord takes one character and gives back its ASCII code:

code = ord("B")
print(code)

That prints 66. The code is a whole number, not text, so it can be used in a calculation straight away. To join it onto a string with +, wrap it in str() first, as with any other number.

ord works on one character only. ord("Hi") stops the program with a TypeError, because a two-character string does not have a single code. To get the code of one character out of a longer string, pick that character out first, as in ord(name[0]).

From a code to its character

chr goes the other way. It takes a number and gives back the character with that code:

letter = chr(66)
print(letter)

That prints B. The result is a one-character string, so it can be joined onto other strings with +:

word = ""

word = word + chr(67)
word = word + chr(97)
word = word + chr(116)

print(word)

word starts out as the empty string and gains one character on each line, so it spells out a word by the end. Look up 67, 97 and 116 in the table above and work out what it will be before you run it.

Doing sums with characters

Because the letters are numbered in order, adding to a code moves along the alphabet. Go from the character to its code with ord, do the sum, then go back to a character with chr:

letter = "c"

code = ord(letter)
next_code = code + 1
next_letter = chr(next_code)

print(letter + " is followed by " + next_letter)

Both conversions are needed. letter + 1 is an error, because Python cannot add a number to a string, and chr(letter) is an error too, because chr needs a number.

The three lines can also be written as one, chr(ord(letter) + 1), which is read from the inside out: the code of the letter, plus one, turned back into a character.

Adding up the codes of a word

ord fits into a loop like anything else. This one visits each letter of a name with the short form of the for loop and keeps a running total of their codes:

name = "Ada"
total = 0

for letter in name:
    total = total + ord(letter)

print(total)

A is 65, d is 100 and a is 97, so the total is 262. Change name to "ADA" and the total changes, because capital letters have smaller codes.

All together

Every string operation in this chapter, in one table:

In the substring rows, end_position is the position of the last character you want. The + 1 is there because a slice stops before its second number, so it is what makes the last character part of the result.