String formatting: Part 2

Table of Contents

In part 1 we learned about formatting strings, sometimes called the formatting spec (short for specification), which are used to tell Python how a value should be displayed. And we learned how to use them in the format() function.

Today we are going to learn some of the other ways to use formatting strings.

The format() function

First lets do a quick review of the format() function.

Syntax:

format(VALUE, SPEC)

VALUE	the value to format
SPEC	the formatting specification

In this example, the VALUE is "Monday".

The SPEC is ".3s" which means:

• a precision of 3 should be used

• with a string presentation

format("Monday", ".3s")

'Mon'

The str.format() method

The .format() method on str objects also uses formatting strings. Here is how we would do the same thing with the .format() method.

"{0:.3s}".format("Monday")

'Mon'

The syntax is:

{POSITION:SPEC}.format(VALUE)

{}	curly braces placeholders which indicate where the value should go
POSITION	argument position
:	a colon is used to indicate that the SPEC follows
SPEC	the formatting specification
VALUE	the value to format

You might be wondering where that 0 for POSITION come from.

Well you see, one of the advantages to using .format() is that you can pass it multiple values. When you do that, each argument has its own position.

"{0:.3s} {1:.2f}".format("Monday", 5.2423)

'Mon 5.24'

If you don’t provide a position, each argument will be substituted in the same order that the placeholders ({}) appear.

"{:.3s} {:.2f}".format("Monday", 5.2423)

'Mon 5.24'

Or you can include the position to change the order in which they appear.

"{1:.2f} {0:.3s}".format("Monday", 5.2423)

'5.24 Mon'

Another advantage of .format() is that anything not in curly braces is interpreted literally.

"The price on {:.3s} is: ${:.2f}".format("Monday", 5.2423)

'The price on Mon is: $5.24'

You can also send keyword arguments to .format(). Then, instead of using position numbers, you use variable names.

blueprint = "The price on {day:.3s} is: ${cost:.2f}"
blueprint.format(
  day="Monday",
  cost=5.2423,
)

'The price on Mon is: $5.24'

Exercises

Exercise 38 (Format Email Address)

Given the following values:

user = "joe"
domain = "gmail"
extension = "com"

Use .format() to print the string:

"joe@gmail.com"

Bonus: Write a format_email() function that takes three arguments, name, domain, and extension and returns a formatted email address string.

Solution to Exercise 38 (Format Email Address)

Click to show

>>> "{user}@{domain}.{extension}".format(user="joe", domain="gmail", extension="com")
'joe@gmail.com'

Click to show: Bonus

def format_email(user, domain, extension):
  return "{}@{}.{}".format(user, domain, extension)

print(format_email("joe", "gmail", "com"))

Click to show: Bonus alternate

def format_email(user, domain, extension):
  return "{user}@{domain}.{extension}".format(user=user, domain=domain, extension=extension)

print(format_email("joe", "gmail", "com"))

f-strings

Since we so frequently need to format strings, Python provides a shortcut called f-strings, which have the letter f immediately before the opening single or double quote.

F-strings works just the same as .format(), except variables are used instead of keyword arguments.

day = "Monday"
cost = 5.2423

f"The price on {day:.3s} is: ${cost:.2f}"

'The price on Mon is: $5.24'

Exercise

Exercise 39 (Street Address)

Given the following values:

street = "1600 Pennsylvania Ave NW"
city = "Washington"
state = "DC"
zip = "20500"

Use an f-string to print the following:

1600 Pennsylvania Ave NW
Washington, DC 20500

Bonus: Write a format_address() function that takes the arguments street, city, state and zip and returns a formatted address string.

Solution to Exercise 39 (Street Address)

Click to show

>>> street = "1600 Pennsylvania Ave NW"
>>> city = "Washington"
>>> state = "DC"
>>> zip = "20500"
>>> print(f"{street}\n{city}, {state} {zip}")
1600 Pennsylvania Ave NW
Washington, DC 20500

Click to show: Bonus

def format_address(street, city, state, zip):
  return f"{street}\n{city}, {state} {zip}"

print(format_address(
  "1600 Pennsylvania Ave NW",
  "Washington",
  "DC",
  "20500",
))