Formatting with f-strings

3 min read

You can already print text and numbers side by side. By the end of this lesson you'll put values straight into the middle of a sentence, show birr with two decimals and thousands separators, and line up a receipt in neat columns. The tool is called an f-string, and you'll use it in nearly every program from now on.

Values inside text

Joining text and numbers with + gets clumsy fast. The number has to be turned into text first with str() (lesson 1.6 explains why), and every space has to be typed by hand:

total = 450
print("Total: " + str(total) + " birr")

Ctrl or Cmd + Enter runs

Here's another way. Notice the f before the opening quote. What does it print?

What will this print?

Decide before you look. Guessing wrong is how this sticks.

item = "macchiato"
price = 45
print(f"One {item} costs {price} birr.")

Your prediction

Python prints

One macchiato costs 45 birr.

Inside an f-string, anything in curly braces {} is replaced by its value. {item} becomes macchiato and {price} becomes 45.

A string with an f in front is an f-stringf-stringA string with f before the opening quote. Anything inside {curly braces} in it is replaced by its value.. Python replaces each {name} with the value that name refers to, and turns numbers into text for you. Now leave out one letter. Predict this.

What will this print?

Decide before you look. Guessing wrong is how this sticks.

total = 450
print("Total: {total} birr")

Python prints

Total: {total} birr

Without the f, it's an ordinary string, and the braces are just characters. Python doesn't warn you, which is what makes this the most common f-string mistake. If you see braces in your output, look for a missing f.

Add the missing f so it prints Total: 450 birr:

total = 450
print("Total: {total} birr")

Ctrl or Cmd + Enter runs

The braces can hold any expression, not only a name. Python works it out and puts the result in the text.

cups = 3
price = 45
print(f"{cups} cups cost {cups * price} birr.")

Ctrl or Cmd + Enter runs

Formatting numbers

After a value in the braces, a colon starts a format specformat specInstructions after a colon inside an f-string's braces that say how to show the value, like :.2f for two decimal places.: instructions for how to show it. Three are worth knowing now:

:.2f shows exactly two decimal places.
:, puts a comma between thousands.
:,.2f does both, which is how you'll show birr.

What will this print?

Decide before you look. Guessing wrong is how this sticks.

print(f"{1234.5:,.2f}")

Your prediction

Python prints

1,234.50

, adds the thousands separator and .2f shows two decimals, adding a zero to make .5 into .50.

price = 1234567.891
print(f"{price:.2f}")
print(f"{price:,}")
print(f"{price:,.2f}")

Ctrl or Cmd + Enter runs

Change price to 2035 / 3, a share of a bill, and run it again. Only the display is rounded; the value itself doesn't change.

The f in .2f means "show as a decimal number", so it only works on numbers. Text that happens to contain digits isn't a number:

Common mistake

This breaks

price = "45"
print(f"{price:.2f}")

Python says

ValueError: Unknown format code 'f' for object of type 'str'

The fix

price = 45
print(f"{price:.2f}")

"45" in quotes is a str, and .2f can't show text with two decimals. You'll meet this again in the next lesson, where everything the user types arrives as text.

Lining up columns

A number after < or > sets a width. :<12 fills 12 characters with the value on the left; :>8 fills 8 with the value on the right. That's how receipts get straight columns.

item = "Macchiato"
price = 45
print(f"{item:<12}{price:>8.2f}")
item = "Sambusa"
price = 30
print(f"{item:<12}{price:>8.2f}")

Ctrl or Cmd + Enter runs

{price:>8.2f} combines both: 8 characters wide, right-aligned, two decimals. The prices line up on the decimal point, whatever their size. Try a price of 1250.

One last trick for finding bugs: put = after a name in the braces, and Python shows the name and its value.

price = 45
total = price * 3
print(f"{price=}")
print(f"{total=}")

Ctrl or Cmd + Enter runs

When a program gives a wrong answer, a few lines like these show you what each name refers to at that moment.

Exercise

A receipt in columns

Print a three-line receipt with the item names in a column 12 characters wide on the left, and prices with two decimals in a column 8 characters wide on the right, exactly like this. Use names for the prices.

Output

Macchiato      45.00
Sambusa        30.00
Water          20.00

Your code

macchiato = 45
sambusa = 30
water = 20

Ctrl or Cmd + Enter runs

Show a solution

This is one way to solve it, not the only one. If yours prints the same thing, it works.

macchiato = 45
sambusa = 30
water = 20
print(f"{'Macchiato':<12}{macchiato:>8.2f}")
print(f"{'Sambusa':<12}{sambusa:>8.2f}")
print(f"{'Water':<12}{water:>8.2f}")

Key takeaways

An f-string has f before the quote; each {expression} inside is replaced by its value.
Forget the f and the braces print as they are, with no error.
:.2f shows two decimals, :, adds thousands separators, and :,.2f does both.
:<12 and :>8 set a width and alignment, for columns.
f"{price=}" shows a name and its value, which helps when hunting bugs.