F-Strings and String Formatting in Python

The problem f-strings solve

Suppose you have a name and a score and want to print a friendly message. Beginners often write:

name = "Ada"
score = 95
print("Hello " + name + ", you scored " + str(score) + " points")

This works, but it is fiddly: you must add spaces by hand, glue pieces with +, and wrap the number in str() or Python complains. F-strings make the same line far cleaner.

This lesson assumes you know the basics of working with text and strings and Python input and output.

Your first f-string

name = "Ada"
score = 95
print(f"Hello {name}, you scored {score} points")
# Hello Ada, you scored 95 points

Two things make this an f-string:

1. The letter f immediately before the opening quote.
2. The {name} and {score} placeholders. Python replaces each one with the variable's value, converting numbers to text automatically. No +, no str().

Expressions inside the braces

You are not limited to plain variable names. Any Python expression works inside the braces:

price = 8
print(f"Two cost {price * 2} pounds")     # Two cost 16 pounds

word = "python"
print(f"Shout: {word.upper()}!")          # Shout: PYTHON!

print(f"3 plus 4 is {3 + 4}")             # 3 plus 4 is 7

Python evaluates whatever is between the braces, then drops the result into the string. This is why {price * 2} shows 16, not the literal text.

Formatting numbers

The most useful feature is the format specifier, written after a colon inside the braces. It controls how a value is displayed.

Decimal places

pi = 3.14159
print(f"Pi is about {pi:.2f}")     # Pi is about 3.14
print(f"Pi is about {pi:.4f}")     # Pi is about 3.1416

:.2f means "format as a float with 2 digits after the decimal point". Python rounds the displayed value for you. This is perfect for money and measurements.

Thousands separators

big = 1234567
print(f"Population: {big:,}")      # Population: 1,234,567

The :, inserts commas to group thousands, making large numbers readable.

Percentages

ratio = 0.732
print(f"Score: {ratio:.0%}")       # Score: 73%

:.0% multiplies by 100, adds a percent sign, and shows no decimals.

Padding and aligning text

Format specifiers can also set a width and alignment, which is ideal for tidy tables:

items = [("Apple", 3), ("Banana", 12), ("Cherry", 5)]
for name, qty in items:
    print(f"{name:<8}{qty:>3}")

Output:

Apple     3
Banana   12
Cherry    5

• {name:<8} left-aligns the name in a field 8 characters wide.
• {qty:>3} right-aligns the quantity in a field 3 characters wide, so the numbers line up neatly.

This loop uses tuple unpacking, which you can revisit in Python tuples and sets.

Worked example: a shop receipt

Let's combine these tools into a small program that prints a clean receipt.

cart = [
    ("Notebook", 2, 3.50),
    ("Pen", 5, 0.99),
    ("Eraser", 1, 1.20)
]

total = 0
print(f"{'Item':<10}{'Qty':>4}{'Price':>8}")
print("-" * 22)

for name, qty, price in cart:
    line_cost = qty * price
    total += line_cost
    print(f"{name:<10}{qty:>4}{line_cost:>8.2f}")

print("-" * 22)
print(f"{'TOTAL':<10}{'':>4}{total:>8.2f}")

Output:

Item       Qty   Price
----------------------
Notebook     2    7.00
Pen          5    4.95
Eraser       1    1.20
----------------------
TOTAL              13.15

Step by step:

• The header uses left and right alignment so columns line up.
• line_cost = qty * price calculates each row's cost; total += line_cost adds it to the running total.
• {line_cost:>8.2f} combines width (8), alignment (right) and two decimal places — all in one specifier.
• "-" * 22 repeats a dash 22 times to draw a divider, a handy string trick.

Every column lines up because each value sits in a fixed-width field. Try doing that with + joins and you will appreciate f-strings instantly.

Common mistakes

• Forgetting the f. Without it, "{name}" prints the literal braces instead of the value.
• Putting the colon in the wrong place. Format instructions go after the value and a colon: {value:.2f}, never {.2f:value}.
• Confusing display with storage. {pi:.2f} only changes how pi looks; the variable still holds the full number.
• Quote clashes. Inside a double-quoted f-string, use single quotes for any inner strings.

Try it yourself

1. Ask the user for a temperature in Celsius and print it converted to Fahrenheit with one decimal place using an f-string.
2. Print a multiplication table for the number 7 where every product is right-aligned in a field 4 wide.
3. Build a receipt for your own list of three items and show the total with a thousands separator and two decimals.

When you are comfortable, deepen your text skills with the Python string methods deep dive.