f-strings

Big idea

An f-string puts values straight into text: write f before the quote, and anything in { } is worked out and dropped in.

The idea

name = "Ada"
age = 36
print(f"{name} is {age} years old.")
print(f"Next year she'll be {age + 1}.")

output
Ada is 36 years old.
Next year she'll be 37.

No +, no str(), no commas. Any expression works inside the braces — maths, method calls, even another variable's len.

Forget the f, though, and the braces print literally:

total = 42
print("Your total is {total}")

output
Your total is 42

Watch it

Formatting numbers

After a colon inside the braces you can say how to show the value:

price = 3.14159
print(f"{price:.2f}")
print(f"{1234567:,}")
print(f"{0.256:.1%}")
print(f"{7:03}")

3.14
1,234,567
25.6%
007

• :.2f — a float with 2 decimal places (rounded).
• :, — commas between thousands.
• :.1% — as a percentage with 1 decimal place.
• :03 — at least 3 wide, padded with zeros.

Lining things up

A width with < (left), > (right), or ^ (centre) pads a value with spaces — perfect for tables:

item = "Apple"
cost = 0.5
print(f"{item:<8}{cost:>6.2f}")
item = "Bread"
cost = 2.25
print(f"{item:<8}{cost:>6.2f}")
item = "Cheese"
cost = 11
print(f"{item:<8}{cost:>6.2f}")

output
Apple     0.50
Bread     2.25
Cheese   11.00

{item:<8} makes the name 8 characters wide, padded on the right; {cost:>6.2f} makes the price 6 wide, right-aligned, with 2 decimals. Repeating the same print three times is clumsy — loops, coming up next, fix that.

Debugging trick

Put = after an expression and Python prints its text too:

x = 5
print(f"{x=}, {x * 2=}")    # x=5, x * 2=10

Think of it like

A form letter with blanks: "Dear {name}, your order of {count} items…". The f-string fills in every blank right before it's printed.

Key terms

• f-string — A string starting with f whose { } parts are filled in with values.
• Format specifier — The part after : in the braces that controls how a value looks, like .2f.