Python f-Strings: A Quick Reference

Python f-strings make it easy to insert values into text and control how those values are displayed.

An f-string begins with the letter f immediately before the quotation mark:

name = "Alex"
print(f"Hello, {name}!")

Output:

Hello, Alex!

Anything inside { } is evaluated by Python and inserted into the string.

1. Putting Variables Inside Strings

Instead of joining strings together manually, place variables inside braces.

name = "Maria"
age = 19

print(f"{name} is {age} years old.")

Output:

Maria is 19 years old.

This is usually easier to read than string concatenation.

Instead of:

print(name + " is " + str(age) + " years old.")

you can write:

print(f"{name} is {age} years old.")

2. Expressions Inside f-Strings

The contents of { } do not have to be simple variables. Python can evaluate expressions there.

x = 8
y = 5

print(f"{x} + {y} = {x + y}")

Output:

8 + 5 = 13

Another example:

price = 12.50
quantity = 4

print(f"Total: ${price * quantity}")

Output:

Total: $50.0

3. Formatting Decimal Numbers

A very common use of f-strings is controlling the number of digits displayed after the decimal point.

Use:

:.2f

to display two digits after the decimal point.

price = 7.5

print(f"Price: ${price:.2f}")

Output:

Price: $7.50

The general pattern is:

{value:.nf}

where n is the number of digits you want after the decimal point.

For example:

number = 3.1415926

print(f"{number:.1f}")
print(f"{number:.2f}")
print(f"{number:.4f}")

Output:

3.1
3.14
3.1416

Notice that Python rounds the displayed value.

4. Percentages

The % formatting option converts a decimal value into a percentage.

score = 0.8734

print(f"Score: {score:.1%}")

Output:

Score: 87.3%

Here:

.1%

means to display one digit after the decimal point.

5. Adding Commas to Large Numbers

Use a comma in the format specification to make large numbers easier to read.

population = 1234567

print(f"Population: {population:,}")

Output:

Population: 1,234,567

You can combine commas with decimal formatting:

amount = 1234567.891

print(f"${amount:,.2f}")

Output:

$1,234,567.89

6. Aligning Text

f-strings can align values within a fixed amount of space.

Suppose we want each value to take up 10 character positions.

Left aligned

name = "Python"

print(f"{name:<10}|")

Output:

Python    |

The < means left align.

Right aligned

print(f"{name:>10}|")

Output:

    Python|

The > means right align.

Centered

print(f"{name:^10}|")

Output:

  Python  |

The ^ means center.

7. Lining Up Numbers

Right alignment is particularly useful when displaying columns of numbers.

print(f"{12.5:8.2f}")
print(f"{3.75:8.2f}")
print(f"{125.9:8.2f}")

Output:

   12.50
    3.75
  125.90

In:

{12.5:8.2f}

• 8 means use a field that is 8 characters wide.
• .2f means display 2 digits after the decimal point.

8. Creating Simple Tables

Alignment becomes especially useful when producing tables.

item1 = "Coffee"
price1 = 3.5

item2 = "Sandwich"
price2 = 8.25

print(f"{'Item':<15}{'Price':>8}")
print(f"{item1:<15}${price1:>7.2f}")
print(f"{item2:<15}${price2:>7.2f}")

Output:

Item              Price
Coffee           $   3.50
Sandwich         $   8.25

The text is left aligned while the numbers are right aligned.

9. Formatting Integers with Leading Zeros

Sometimes you want numbers to always have the same number of digits.

number = 7

print(f"{number:03d}")

Output:

007

Here:

• 0 means fill unused positions with zeros.
• 3 means use three positions.
• d means format the value as an integer.

Another example:

for number in range(1, 4):
    print(f"File{number:03d}.txt")

Output:

File001.txt
File002.txt
File003.txt

10. Useful f-String Patterns

11. A Complete Example

name = "Jordan"
hours = 12.5
rate = 18.75

pay = hours * rate

print(f"Employee: {name}")
print(f"Hours:    {hours:8.2f}")
print(f"Rate:    ${rate:8.2f}")
print(f"Pay:     ${pay:8.2f}")

Output:

Employee: Jordan
Hours:       12.50
Rate:       $   18.75
Pay:        $  234.38

Key Idea

The basic f-string pattern is:

f"some text {expression}"

Formatting instructions come after a colon:

f"{expression:format}"

For example:

f"{price:.2f}"

means:

Insert the value of price and display it as a floating-point number with two digits after the decimal point.

For introductory Python programming, the most useful formats to remember are:

f"{value}"
f"{value:.2f}"
f"{value:>10}"
f"{value:<10}"
f"{value:8.2f}"

These cover most of the formatting needed for calculations, money, and simple tables.