Strings and f-strings

Messages, names, passwords, news headlines — much of the data programs work with is text. Imagine you are making automatic birthday cards for your classmates: “Happy birthday, Elvin! You are 13 today.” Only the name and the age change from card to card. In this lesson you will learn to work with text and build templates like this in a single line.

Strings

A string (str) is a sequence of characters written in quotes: letters, digits, spaces and symbols. You can join strings with +, repeat them with * and find their length with the len() function.

greeting = 'Hello'
name = 'Aysel'
message = greeting + ', ' + name + '!'
print(message)
print(len(message))
print('Ha' * 3)

Hello, Aysel!
13
HaHaHa

len() counts spaces and punctuation too, which is why 'Hello, Aysel!' has a length of 13. Some characters are written in a special way: \n inside a string starts a new line. If your text contains an apostrophe, wrap the string in double quotes — then Python will not mistake the apostrophe for the end of the string.

print('First line\nSecond line')
print("It's easy!")

First line
Second line
It's easy!

Indexes and slices

Every character in a string has its own number — its index. In Python counting starts at 0: the first character is [0], the second [1], and so on. Negative indexes count from the end: [-1] is the last character.

A slice takes part of a string: word[start:end]. The start index is included, but the end index is not. If you leave one of the numbers out, Python goes from the very beginning or to the very end.

word = 'Python'
print(word[0])
print(word[5])
print(word[-1])
print(word[0:2])
print(word[2:])
print(word[:4])

P
n
n
Py
thon
Pyth

String methods

A method is a function that belongs to a type; you call it with a dot after the value: name.upper(). Methods do not change the original string — they return a new one. The most useful ones are:

city = '  baku  '
clean = city.strip()
print(clean.title())
print(clean.upper())

text = 'I like tea. Tea is tasty.'
print(text.lower().count('tea'))
print(text.replace('tea', 'coffee'))
print('tea' in text)
print(text.startswith('I'))

Baku
BAKU
2
I like coffee. Tea is tasty.
True
True

Notice that replace is case-sensitive, so only the lowercase tea was replaced. For count we first turned the whole text to lowercase with lower() — methods can be chained like this. The in operator checks whether one string appears inside another and gives True or False.

f-strings: templates

The easiest way to put values into text is an f-string. Write the letter f before the opening quote and put a variable or any expression in curly braces {}. Python replaces each pair of braces with its value.

name = 'Elvin'
age = 13
print(f'Happy birthday, {name}! You are {age} today.')
print(f'Next year you will be {age + 1}.')

price = 7.5
print(f'Price: {price:.2f} AZN')

Happy birthday, Elvin! You are 13 today.
Next year you will be 14.
Price: 7.50 AZN

{price:.2f} shows the number with exactly 2 digits after the decimal point — perfect for amounts of money. See why this matters in a real receipt:

item = 'Notebook'
qty = 3
unit_price = 2.35
total = qty * unit_price
print(total)
print(f'{item}: {qty} x {unit_price} = {total:.2f} AZN')

7.050000000000001
Notebook: 3 x 2.35 = 7.05 AZN

The odd 7.050000000000001 in the first line comes from the way computers store decimal numbers in binary. Formatting hides this tiny “tail”.

raw = '  aYSEL  '
# 1) name = raw without spaces, in title case
# 2) print name, its length and its first 3 letters in capitals

Aysel
5
AYS

item = 'Notebook'
qty = 4
unit_price = 1.35
# total = ...
# print the receipt line with an f-string

NOTEBOOK: 4 x 1.35 = 5.40 AZN