Strings

Strings are immutable sequences of characters: indexing and slicing, joining, repeating and membership, traversal, and the string methods.

Key ideas

Indexes run from 0 to len(s) − 1 from the left and from −1 backwards from the right; an index past the end is an IndexError, but a slice past the end just stops.
s[a:b:c] takes characters from index a up to but not including b, in steps of c; s[::-1] gives the string reversed.
Strings are immutable: s[0] = 'X' is a TypeError, and every method gives back a new string.
+ joins strings, * repeats one, and in and not in check whether one string is part of another.
find() gives −1 when the text is not there, but index() raises a ValueError; count() counts occurrences that do not overlap.
split() with no argument splits at runs of spaces into a list; join() puts a separator between strings; partition() gives a tuple of three.
capitalize() makes only the first character a capital (the rest small); title() makes the first letter of every word a capital.

s = 'computer'	s[0] = 'c', s[-1] = 'r', s[2:5] = 'mpu', s[:3] = 'com', s[::2] = 'cmue'
Reverse and length	s[::-1] = 'retupmoc'; len(s) = 8
Searching	'banana'.count('a') = 3; 'banana'.find('n') = 2; 'banana'.find('z') = −1
Changing	'banana'.replace('a', 'o') = 'bonono'; ' hi '.strip() = 'hi'
split and join	'to be or'.split() = ['to', 'be', 'or']; '-'.join(['a', 'b']) = 'a-b'
partition()	'a,b,c'.partition(',') = ('a', ',', 'b,c')
Case	'my BOOK'.capitalize() = 'My book'; 'my BOOK'.title() = 'My Book'
Tests	'abc1'.isalnum() is True; 'abc1'.isalpha() is False; '42'.isdigit() is True

What does this program print? Give (a) the first line (b) the second.
s = 'science'
print(s[:-4])
print(s[::-1])

Working: index: s0 c1 i2 e3 n4 c5 e6

Answer: (a) sci; (b) ecneics