String Formatting in Python

Turning values into well-presented text is something every program does: invoices, reports, log messages, emails, table output. Python offers three formatting systems — f-strings (the modern standard), str.format(), and the old % operator — plus string.Template for user-supplied templates.

This lesson covers all of them with the format specification mini-language: widths, alignment, precision, thousands separators, percentages, dates, number bases and debugging with =.

f-Strings

Prefix a string with f and put expressions in braces: f"{name} is {age + 1}". Any expression works, including method calls and conditional expressions. Since Python 3.12 you can reuse the same quote type inside the braces and write multi-line expressions. Use {{ and }} for literal braces.

The Format Specification Mini-Language

After a colon, a format spec controls output: [[fill]align][sign][width][,][.precision][type].

• Alignment: < left, > right, ^ centre, with an optional fill character ({x:*^10}).
• Numbers: .2f fixed decimals, , or _ thousands separators, + always show sign, e scientific, % percentage.
• Integers in other bases: b binary, o octal, x/X hex, # adds a prefix; 08 pads with zeros.
• Dates: datetime objects accept strftime codes, e.g. {today:%d %b %Y}.
• Conversions: !r uses repr(), !s uses str(); {x=} shows the expression and value.

str.format(), % and Template

"{} scored {:.1f}".format(name, score) uses the same mini-language and is useful when the template is stored separately from the values. The % operator ("%s is %d" % (name, age)) is legacy but still appears in older code and logging. string.Template("Hello $name") is the safe choice when end users write templates, because it cannot evaluate arbitrary expressions.

Examples

name, marks = "Asha", [78, 92, 85]
print(f"{name} has {len(marks)} marks, best {max(marks)}")
print(f"Average: {sum(marks) / len(marks):.2f}")
print(f"{name.upper()} {'passed' if min(marks) >= 40 else 'failed'}")
print(f"Set literal looks like {{1, 2}}")

Asha has 3 marks, best 92
Average: 85.00
ASHA passed
Set literal looks like {1, 2}

from datetime import date

print(f"|{'left':<10}|{'right':>10}|{'mid':^10}|{'fill':*^10}|")
print(f"{1234567.891:,.2f}  {1234567:_}  {-42:+}  {42:+}")
print(f"{0.4567:.1%}  {12345.678:.2e}  {7:03d}")
print(f"{255:b} {255:o} {255:x} {255:X} {255:#x} {5:08b}")
d = date(2026, 9, 27)
print(f"{d:%d %B %Y} | {d:%a %d/%m/%y}")
value = 3.14159
print(f"{value=:.2f}  {name!r}" if (name := "Ravi") else "")

|left      |     right|   mid    |***fill***|
1,234,567.89  1_234_567  -42  +42
45.7%  1.23e+04  007
11111111 377 ff FF 0xff 00000101
27 September 2026 | Sun 27/09/26
value=3.14  'Ravi'

from string import Template

template = "{name:<8}|{score:>6.1f}|{grade:^5}"
for row in [("Asha", 91.456, "A"), ("Ravi", 67.0, "C")]:
    print(template.format(name=row[0], score=row[1], grade=row[2]))

print("%s is %d years old and %.1f%% done" % ("Meera", 24, 87.25))

t = Template("Dear $name, your order $$${amount} has shipped.")
print(t.substitute(name="Asha", amount="2,999"))
print(Template("Hi $name, $missing").safe_substitute(name="Ravi"))

Asha    |  91.5|  A
Ravi    |  67.0|  C
Meera is 24 years old and 87.2% done
Dear Asha, your order $2,999 has shipped.
Hi Ravi, $missing