3.2: Formatted Strings

Learning Objectives

By the end of this section you should be able to

• Use f-strings to simplify output with multiple values.
• Format numbers with leading zeros and fixed precision.

F-strings

A formatted string literal (or f-string) is a string literal that is prefixed with "f" or "F". A replacement field is an expression in curly braces ({}) inside an f-string. Ex: The string f"Good morning, {first} {last}!" has two replacement fields: one for a first name, and one for a last name. F-strings provide a convenient way to combine multiple values into one string.

Checkpoint: Printing an f-string

Concepts in Practice \(\PageIndex{1}\): Basic f-strings

What is the output of the following code?

animal = "dog"
says = "bark"
print(f"My {animal} says {says} {says} {says}")

1. My dog bark bark bark bark
2. My dog says bark bark bark
3. Error

Answer

b. The same variable, says, is used in three replacement fields.

Concepts in Practice \(\PageIndex{2}\): Basic f-strings

What is the output of the following code?

temp = "hot"
food = "potato"
print("{temp} {food}")

1. {temp} {food}
2. hot potato
3. Error

Answer

a. The string is not an f-string, so the brace characters (around temp and food) are printed literally.

Concepts in Practice \(\PageIndex{3}\): Basic f-strings

How can the following code be rewritten using an f-string?

print(x, "+", y, "=", x + y)

1. print(f"x + y = {x+y}")
2. print(f"{x + y} = {x + y}")
3. print(f"{x} + {y} = {x+y}")

Answer

c. Using an f-string allows the entire output to be specified without commas and extra quote marks.

Formatting numbers

Programs often need to display numbers in a specific format. Ex: When displaying the time, minutes are formatted as two-digit integers. If the hour is 9 and the minute is 5, then the time is "9:05" (not "9:5").

In an f-string, a replacement field may include a format specifier introduced by a colon. A format specifier defines how a value should be formatted for display. Ex: In the string f"{hour}:{minute:02d}", the format specifier for minute is 02d.

Table 3.3 Example format specifiers. The table shows common ways that numbers can be formatted. Many more formatting options are available and described in Python's Format Specification Mini-Language.

Format	Description	Example	Result
d	Decimal integer (default integer format).	f"{12345678:d}"	'12345678'
,d	Decimal integer, with comma separators.	f"{12345678:,d}"	'12,345,678'
10d	Decimal integer, at least 10 characters wide.	f"{12345678:10d}"	' 12345678'
010d	Decimal integer, at least 10 characters wide, with leading zeros.	f"{12345678:010d}"	'0012345678'
f	Fixed-point (default is 6 decimal places).	f"{math.pi:f}"	'3.141593'
.4f	Fixed-point, rounded to 4 decimal places.	f"{math.pi:.4f}"	'3.1416'
8.4f	Fixed-point, rounded to 4 decimal places, at least 8 characters wide.	f"{math.pi:8.4f}"	' 3.1416'
08.4f	Fixed-point, rounded to 4 decimal places, at least 8 characters wide, with leading zeros.	f"{math.pi:08.4f}"	'003.1416'

Concepts in Practice \(\PageIndex{4}\): Formatting numbers

What f-string formats a date as MM/DD/YYYY?

1. f"{month:02d}/{day:02d}/{year}"
2. f"{month:2d}/{day:2d}/{year}"
3. f"{month}/{day}/{year}"

Answer

a. The format specifier 02d appends a 0 to the front of the integer when the integer is only one digit.

Concepts in Practice \(\PageIndex{5}\): Formatting numbers

What statement displays the variable money rounded to two decimal places?

1. print(f"{money:2d}")
2. print(f"{money:2f}")
3. print(f"{money:.2f}")

Answer

c. The format specifier ".2f" (with a period) means fixed-point, rounded to two decimal places.

Concepts in Practice \(\PageIndex{6}\): Formatting numbers

What format specifier displays a floating-point number with comma separators and rounded to two decimal places?

1. .2,f
2. ,.2f
3. ,d.2f

Answer

b. This format specifier is useful for displaying large amounts of money.

Try It: Mad lib (f-string)

A mad lib is a funny story that uses words provided by a user. The following mad lib is based on four words (user input in bold):

    Enter a name: Buster
    Enter a noun: dog
    Enter an adjective: super
    Verb ending in -ing: swimming
    
    Buster, the super dog, likes to go swimming.

Most of the code for this mad lib is already written. Complete the code below by writing the f-string.

Interactive Code

name = input("Enter a name: ")
noun = input("Enter a noun: ")
adjective = input("Enter an adjective: ")
verb_ing = input("Verb ending in -ing: ")

print()
print(f"")  # TODO: edit this line only

Input

Buster
dog
super
swimming

Answer

name = input("Enter a name: ")
noun = input("Enter a noun: ")
adjective = input("Enter an adjective: ")
verb_ing = input("Verb ending in -ing: ")

print()
print(f"{name}, the {adjective} {noun}, likes to go {verb_ing}.")

Try It: Wage calculator

You just landed a part-time job and would like to calculate how much money you will earn. Write a program that inputs the time you start working, the time you stop working, and your hourly pay rate (example input in bold):

    Starting hour: 9
    Starting minute: 30
    Stopping hour: 11
    Stopping minute: 0
    Hourly rate: 15

Based on the user input, your program should calculate and display the following results:

    Worked 9:30 to 11:00
    Total hours: 1.5
    Payment: $22.50

For this exercise, you need to write code that (1) calculates the total payment and (2) formats the three output lines. Use f-strings and format specifiers to display two-digit minutes, one decimal place for hours, and two decimal places for payment. The input code has been provided as a starting point.

Assume the use of a 24-hour clock. Ex: 16:15 is used instead of 4:15pm.

Interactive Code

start_hour = int(input("Starting hour: "))
start_min = int(input("Starting minute: "))
stop_hour = int(input("Stopping hour: "))
stop_min = int(input("Stopping minute: "))
rate = float(input("Hourly rate: "))
print()

Input

9
30
11
0
15

Answer

start_hour = int(input("Starting hour: "))
start_min = int(input("Starting minute: "))
stop_hour = int(input("Stopping hour: "))
stop_min = int(input("Stopping minute: "))
rate = float(input("Hourly rate: "))
print()

start = start_hour*60 + start_min
stop = stop_hour*60 + stop_min
hours = (stop - start) / 60
payment = hours * rate

print(f"Worked {start_hour}:{start_min:02d} to {stop_hour}:{stop_min:02d}")
print(f"Total hours: {hours:.1f}")
print(f"Payment: ${payment:.2f}")