Format numbers and dates inside the f-string

Everything after the colon inside { } is a format spec. Learn five of them and you stop writing round(), str() and manual padding to make output readable.

Numbers

revenue = 1234567.891
share = 0.3456

print(f"{revenue:,.2f}")
print(f"{share:.1%}")
print(f"{revenue:_.0f}")

1,234,567.89
34.6%
1_234_568

, adds thousands separators, .2f means two decimals, % multiplies by 100 and adds the sign, and _ uses underscores, the way you'd write the number in Python code.

Columns

< left-aligns, > right-aligns, and the number is the width:

rows = [("Tea", 2.5, 120), ("Coffee", 3.75, 86), ("Hot chocolate", 4.2, 15)]

for name, price, sold in rows:
    print(f"{name:<14}{price:>6.2f}{sold:>5}")

Tea             2.50  120
Coffee          3.75   86
Hot chocolate   4.20   15

A readable table, with no padding code at all.

Dates

A date takes the same codes as strftime, right in the braces:

from datetime import date

d = date(2026, 9, 29)
print(f"{d:%d %b %Y}")
print(f"{d:%A}")

29 Sep 2026
Tuesday

Debugging with =

Put = after an expression and the f-string prints the expression too:

share = 0.3456
print(f"{share=}")
print(f"{share * 100 = :.1f}")

share=0.3456
share * 100 = 34.6

Why it works

Each value's type decides what its format spec means: numbers understand , and f, dates understand %Y. The f-string passes the text after the colon to format(value, spec), which calls the type's __format__.

When not to use it

Format for display, not for storage: f"{price:.2f}" is a string, and doing maths on it later means parsing it back. Keep the number, format at the edge. And for money that must be exact, round with Decimal first; .2f rounds binary floats, which can surprise you on halves.