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

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Concepts in Practice: Basic f-strings

1.

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

My dog says bark bark bark

Error

2.

What is the output of the following code?

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

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

3.

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}")

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.

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'

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.

Concepts in Practice: Formatting numbers

4.

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

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

f"{month:2d}/{day:2d}/{year}"

f"{month}/{day}/{year}"

5.

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}")

6.

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

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.

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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.

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