Strings

1What you already know

A string is a sequence of characters in quotes. Single, double or triple quotes — you chose them back in the print chapter. And because it is a sequence, all six common features apply:

word = 'hello'

print(len(word))
print(word[0])
print(word[-1])
print(word[1:4])
print('hi' + 'there')
print('ha' * 3)
print('ell' in word)

5
h
o
ell
hithere
hahaha
True

2Methods: a string's own tricks

A method is a function that belongs to a value. You write it after the value, with a dot in between: 'hello'.upper(). Same parentheses as always, so you already know it is a function — it just happens to belong to that string.

String methods come in four kinds, sorted by the question you are asking. Try each one, and keep half an eye on the amber box on the right.

The trouble with showing this is that spaces are invisible. So print each answer between square brackets — then you can see exactly where the string ends:

typed = '  Ravi  '

print('[' + typed.strip() + ']')
print('[' + typed.lstrip() + ']')
print('[' + typed.rstrip() + ']')
print('[' + typed + ']')

[Ravi]
[Ravi  ]
[  Ravi]
[  Ravi  ]

replace() is the one that takes two things: what to look for, and what to put in its place. It changes every match it finds, not just the first one:

greeting = 'good morning'

print(greeting.replace('o', '0'))
print(greeting.replace('morning', 'evening'))
print(greeting)

g00d m0rning
good evening
good morning

All three os went, because replace() does not stop at the first one. And look at the last line: greeting is still 'good morning'. Like every method on this page, replace() hands you a new string and leaves the original exactly as it was.

3The is-methods: asking, not changing

These all begin with is, and they all answer True or False. They never change anything — they only look and report.

4find() and index() — the same, until they fail

Both tell you where something is. When the thing is there, they give exactly the same answer:

word = 'hello'

print(word.find('l'))
print(word.index('l'))

2
2

The difference only appears when the thing is not there — and it is a big one:

word = 'hello'

print(word.find('z'))   # not there -> -1, and the program carries on
print(word.index('z'))  # not there -> the program stops here

-1
Traceback (most recent call last):
  File "finding.py", line 4, in <module>
    print(word.index('z'))  # not there -> the program stops here
          ^^^^^^^^^^^^^^^
ValueError: substring not found

Sometimes you do not need to know where something is — only whether the string begins or ends with it. That is what startswith() and endswith() are for, and they answer True or False:

name = 'Ramesh Kumar'
filename = 'report.txt'

print(name.startswith('Ram'))
print(name.endswith('Kumar'))
print(filename.endswith('.txt'))
print(name.startswith('ram'))

True
True
True
False

5split(), partition() and join()

These three move between a string and a list. split() and partition() break a string apart; join() glues it back together.

letters = 'a,b,c'.split(',')
parts = 'good morning'.partition(' ')
joined = '-'.join(['a', 'b', 'c'])

print(letters)
print(parts)
print(joined)

['a', 'b', 'c']
('good', ' ', 'morning')
a-b-c

6Try it at the prompt

7The original never changes

This is the most important sentence in the lesson. A string method never edits the string it was called on. It builds a new string and gives that back.

word = 'hello'

print(word.upper())   # a new string comes back
print(word)           # the original is untouched

HELLO
hello

Look at that carefully. We shouted at 'hello', and it politely handed back a capitalised copy. Then we asked for 'hello' again — and it is still small, still itself, entirely unbothered.

8in — is it in there?

This one is not a string method at all — it is the membership operator from the sequences lesson, and it works on lists and tuples too. Ask whether a character (or a smaller piece of text) appears inside a string. The answer comes back as a boolean — True or False. Unlike find(), it does not tell you where — only whether.

word = 'hello'

print('e' in word)
print('z' in word)
print('ell' in word)

True
False
True

9Escape sequences — the backslash

An escape sequence is a short code you write inside a string. It always begins with a backslash (\), followed by one or more characters. You type two symbols, but Python does not read them one by one — it treats the pair as one single character, and each pair has one fixed job.

They exist because some characters cannot simply be typed. Here is the smallest example. You want to print It's mine, so you write it in single quotes:

print('It's mine')

  File "quotes.py", line 1
    print('It's mine')
                    ^
SyntaxError: unterminated string literal (detected at line 1)

Python is not being difficult. It read the second quote — the one in It's — and decided the string ended there. Everything after it made no sense.

The fix is the backslash. Write \' and Python knows that quote is part of the text, not the end of it.

print('It\'s mine')       # \' means: a plain quote, not the end
print("It's mine")        # or just use the other kind of quotes
print("He said \"hi\"")

It's mine
It's mine
He said "hi"

The same backslash does a second job. Some characters cannot be typed inside a string at all — you cannot press Enter in the middle of one, and you cannot type a Tab and expect it to survive. So Python gives you short codes for them.

\n and \t are the two you will actually use. One breaks a line, the other lines things up in columns:

print('Name\tClass\tMarks')
print('Ramesh\t11\t87')
print('line one\nline two')

Name	Class	Marks
Ramesh	11	87
line one
line two

newline = '\n'
tab = '\t'
backslash = '\\'

print(len(newline))
print(len(tab))
print(len(backslash))
print(len('a\nb'))

1
1
1
3

The trap: Windows file paths

Windows writes paths with backslashes, like C:\notes\table.txt. Put that straight into a string and Python reads \n and \t as escape sequences — and your path falls apart.

path = 'C:\notes\table.txt'
print(path)              # \n became a new line, \t became a tab!

safe = 'C:\\notes\\table.txt'
print(safe)              # doubled backslashes: correct, but ugly

raw = r'C:\notes\table.txt'
print(raw)               # an r in front: much easier

C:
otes	able.txt
C:\notes\table.txt
C:\notes\table.txt

10Recap

And the escape sequences — every one of them a single character: