String Formatting with f-strings

f-strings — string literals prefixed with f — are the modern way to build text from values. Beyond inserting variables, their format specifiers control decimal places, padding, alignment, thousands separators, percentages and dates, all inline.

The basics

name = "Ana"
items = 3
price = 4.5

print(f"Hi {name}, you have {items} items.")
print(f"Total: {items * price}")
print(f"Shout: {name.upper()}!")

Hi Ana, you have 3 items.
Total: 13.5
Shout: ANA!

Anything between braces is a Python expression, evaluated and converted to a string. To include a literal brace, double it: f"{{not a field}}".

Format specifiers

After the expression, a colon introduces a format spec:

pi = 3.14159265
big = 1234567.891
ratio = 0.256

print(f"{pi:.2f}")
print(f"{big:,.2f}")
print(f"{big:_.0f}")
print(f"{ratio:.1%}")
print(f"{42:05d}")
print(f"{255:x} {255:b} {255:#x}")
print(f"{1e-7:.2e}")

3.14
1,234,567.89
1_234_568
25.6%
00042
ff 11111111 0xff
1.00e-07

Width and alignment

rows = [("Apples", 3, 1.2), ("Bananas", 12, 0.5), ("Cherries", 200, 0.05)]

print(f"{'Item':<10}{'Qty':>5}{'Price':>8}")
for item, qty, price in rows:
    print(f"{item:<10}{qty:>5}{price:>8.2f}")
print(f"{'centred':^20}|")
print(f"{'pad':*^11}")

Item        Qty   Price
Apples        3    1.20
Bananas      12    0.50
Cherries    200    0.05
      centred       |
****pad****

<, > and ^ align left, right and centre within the given width; a character before them sets the fill. This makes neat text tables easy.

Debugging with =

Adding = after an expression prints the expression itself along with its value — perfect for quick debugging:

width = 12
height = 5
print(f"{width=}, {height=}, {width * height=}")
print(f"{width / 7 = :.3f}")

width=12, height=5, width * height=60
width / 7 = 1.714

repr and str conversions

text = "line\nbreak"
print(f"{text}")
print(f"{text!r}")

line
break
'line\nbreak'

!r uses repr(), showing quotes and escape sequences — useful for spotting stray whitespace.

Dates and nested specs

from datetime import date

d = date(2026, 10, 6)
print(f"{d:%A, %d %B %Y}")
print(f"{d:%Y-%m-%d}")

decimals = 3
print(f"{2 / 3:.{decimals}f}")

Tuesday, 06 October 2026
2026-10-06
0.667

Objects can define their own format specs; dates use strftime codes (lesson 49). Specs can themselves contain fields, like {decimals} above.

Multi-line f-strings

order = {"id": 1042, "customer": "Rosa", "total": 7.8}

receipt = (
    f"Order #{order['id']}\n"
    f"Customer: {order['customer']}\n"
    f"Total: ${order['total']:.2f}"
)
print(receipt)

Order #1042
Customer: Rosa
Total: $7.80

Adjacent string literals are joined automatically, which keeps long f-strings readable. Since Python 3.12, you can even reuse the same quote type inside the braces, e.g. f"{order["id"]}".

Older formatting styles

You'll see two older styles in existing code. They still work, but f-strings are clearer:

name, score = "Ana", 91.5
print("%s scored %.1f" % (name, score))
print("{} scored {:.1f}".format(name, score))
print(f"{name} scored {score:.1f}")

Ana scored 91.5
Ana scored 91.5
Ana scored 91.5

One exception: when logging, pass values as arguments — logging.info("User %s logged in", name) — so the message is only formatted if it's actually logged.

Try it yourself