Python Number Formatting: Decimals, Commas, % and Alignment
A format specifier controls how an f-string shows a number: decimal places, width, alignment and separators. It goes after a colon, as in {price:.2f}.
Read String Formatting first. The later examples also use try and except, and the task loops over pairs with unpacking.
Show two decimal places
:.2f shows a number with exactly two digits after the decimal point.
price = 3.5
print(f"Price: ${price:.2f}")
print(price)
The first line is Price: $3.50. The second is 3.5: formatting makes text
for display and does not change price.
Format or round()?
round() changes the number. A format specifier only changes how it looks.
Use round() when you will keep calculating, and :.2f when you print.
total = 12.5
print(round(total, 2))
print(f"{total:.2f}")
round(total, 2) prints 12.5, because the number has no second digit to
show. Only :.2f guarantees 12.50.
Line up columns with width and alignment
A number before the type sets the width: the minimum number of characters.
< aligns left, > aligns right and ^ centres.
print(f"[{'Ada':<6}][{8:>4}]")
print(f"[{'Sam':<6}][{12:>4}]")
print(f"[{'hi':^6}]")
The brackets show the padding. Names line up on the left and scores on the
right. The order inside the braces is alignment, width, then decimals:
{price:>8.2f} means right-aligned, 8 wide, two decimal places.
Thousands separators and percentages
, adds a comma every three digits. % multiplies by 100 and adds a percent
sign.
crowd = 1250000
win_rate = 0.375
print(f"{crowd:,} people")
print(f"Win rate: {win_rate:.1%}")
This prints 1,250,000 people, then Win rate: 37.5%.
When a specifier does not fit the value
A number specifier needs a number. Text read from input() is a string, so
convert it first.
amount = "4.5"
try:
print(f"{amount:.2f}")
except ValueError as error:
print("Error:", error)
print(f"{float(amount):.2f}")
The first attempt raises ValueError. After float() it prints 4.50.
Note: This lesson covers the common specifiers. The full format mini-language also has fill characters, signs and other number bases.
See also: Data Validation.
Your turn
Print each item on its own line: the name left-aligned in 8 characters, then the price right-aligned in 7 characters with two decimal places.
items = [("Pie", 4.5), ("Coffee", 12), ("Cake", 3.25)]
for name, price in items:
print(name, price) # change this to use width, alignment and :.2f
Expected output
Pie 4.50
Coffee 12.00
Cake 3.25