1 Whirlwind Introduction

You will find (or have found) that I tend to make use of whirlwind introductions. The f-string (or formatted string literal) lends itself well to such an introduction (i.e., looking at an example and working top down).

1.1 Case Study

Let us take a look a familiar code snippet (i.e., one from the lecture on user input).

def main():

    price = float(input("What is the price of your favorite lunch? "))
    frequency = int(input("How many times a week do you have that lunch? "))

    print(f"On average you spend {price * frequency:.2f}")


if __name__ == "__main__":
    main()

1.2 A Single Line

We only interested in a single line.

    print(f"On average you spend {price * frequency:.2f}")

Let us break this line down.

1. f" - the ‘f’ before the opening quote indicates that this is an f-string

2. "On average you spend - this is treated as a normal string literal

3. {price * frequency:.2f} - this is where the f-string magic happens

   1. price * frequency - the values stored in the price and frequency varaibles are retrieved and a multiplication is performed

   2. the colon separates the expression from the formatting specification

   3. the .2 indicates that we want two decimal places to be output

   4. the f indicates that we have a floating point value

2 A Few Quick Examples

Let us examine a few quick examples.

2.1 An Integer

Suppose that we wanted to output an integer (val) and right justify it within three spaces.

    val = 7
    print(f"{val:>3}")

The > indicates right alignment.

2.2 A Floating Point Number

Suppose that we wanted to output price_per_gallon and have it rounded to two decimal places.

    price_per_gallon = 7.552
    print(f"{price_per_gallon:>4.2f}")

The .2f handles the rounding and the >4 handles the right alignment.

2.3 A String

Suppose that we want to output color as a centered title.

    color = "blue"
    print(f"{color.title():^80}")

The ^ (caret) indicates center alignment.

3 What About the “format” Function?

The .format function predates f-strings and, while it does have a few use cases, is should be avoided when an f-string can be used. Let us rewrite our previous three (3) examples to use .format.

    val = 7
    print("{>3}".format(val))

    price_per_gallon = 7.552
    print("{4.2f}".format(price_per_gallon))

    color = "blue"
    print("{^80}".format(color.title()))

The formatting syntax (more aptly grammar) are the same (which makes sense considering f-string syntax was based on that of .format).

4 What About C “printf” Style output?