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.

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.
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'`

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.

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

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.

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()

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.