Format Specifiers

Python's format specification mini-language controls how values are displayed in:

• f-strings
• str.format()
• the built-in format() function

The basic syntax is:

{value:format_spec}

Example:

value = 42.5
print(f"{value:.1f}")   # 42.5

Part	Meaning
value	object being formatted
:	begins the format specifier
.1f	formatting instructions

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Basic Alignment

Format specifiers can align text within a specified width.

Symbol	Meaning
<	left align
>	right align
^	center align

text = "test"

print(f"{text:<10}")   # 'test      '
print(f"{text:>10}")   # '      test'
print(f"{text:^10}")   # '   test   '

These correspond to the string alignment methods from the previous chapter:

Method	Format Spec
ljust(10)	:<10
rjust(10)	:>10
center(10)	:^10

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Fill Characters

A fill character may be placed before the alignment symbol.

print(f"{'test':*<10}")   # test******
print(f"{'test':->10}")   # ------test
print(f"{'test':_^10}")   # ___test___

The = alignment places padding between the sign and the digits.

print(f"{-42:0=8}")     # -0000042
print(f"{42:0=+8}")     # +0000042

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Width

Width specifies the minimum field size.

for word in ["a", "abc", "abcdefgh"]:
    print(f"[{word:5}]")

Output:

[a    ]
[abc  ]
[abcdefgh]

Values longer than the width are not truncated.

Default alignment depends on the value type:

# Strings: left-aligned
print(f"{'abc':10}")       # 'abc       '

# Numbers: right-aligned
print(f"{123:10}")         # '       123'

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Precision

Precision controls decimal places for floats or maximum length for strings. Precision rounds floating-point values.

Float Precision

pi = 3.14159265

print(f"{pi:.0f}")   # 3
print(f"{pi:.2f}")   # 3.14
print(f"{pi:.4f}")   # 3.1416

String Precision

text = "Hello, World!"

print(f"{text:.5}")      # Hello
print(f"{text:10.5}")    # Hello

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Numeric Formatting

Numbers support several formatting options.

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Sign Control

print(f"{42:+}")     # +42
print(f"{-42:+}")    # -42

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Thousands Separators

big = 1234567890

print(f"{big:,}")    # 1,234,567,890
print(f"{big:_}")    # 1_234_567_890

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Integer Bases

num = 255

print(f"{num:b}")     # binary
print(f"{num:x}")     # hexadecimal
print(f"{num:o}")     # octal

With prefixes:

print(f"{num:#b}")    # 0b11111111
print(f"{num:#X}")    # 0xFF

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Floating-Point Types

Fixed Point

pi = 3.14159265

print(f"{pi:f}")       # 3.141593
print(f"{pi:.2f}")     # 3.14
print(f"{pi:10.2f}")   #       3.14

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Scientific Notation

large = 1234567.89

print(f"{large:e}")      # 1.234568e+06
print(f"{large:.2e}")    # 1.23e+06
print(f"{large:.2E}")    # 1.23E+06

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Percentage

ratio = 0.8567

print(f"{ratio:%}")      # 85.670000%
print(f"{ratio:.1%}")    # 85.7%
print(f"{ratio:.0%}")    # 86%

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Datetime Formatting

Datetime objects support strftime codes inside format specifiers. These codes are part of the datetime.strftime syntax, not the format mini-language itself.

from datetime import datetime

dt = datetime(2025, 3, 15)

print(f"{dt:%Y-%m-%d}")     # 2025-03-15
print(f"{dt:%B %d, %Y}")    # March 15, 2025
print(f"{dt:%A}")           # Saturday

Time components:

dt = datetime(2025, 1, 12, 14, 30, 45)

print(f"{dt:%H:%M:%S}")   # 14:30:45
print(f"{dt:%I:%M %p}")   # 02:30 PM

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Common Patterns

Currency Formatting

price = 1234.5

print(f"${price:,.2f}")     # $1,234.50
print(f"${price:>10,.2f}")  #  $1,234.50

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Table Alignment

data = [("Alice", 95.5), ("Bob", 87.3)]

for name, score in data:
    print(f"{name:<10} {score:>6.1f}")

Output:

Alice       95.5
Bob         87.3

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ID Generation

for i in range(1, 4):
    print(f"ID-{i:04d}")

Output:

ID-0001
ID-0002
ID-0003

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Full Format Specification Grammar

The complete format specification pattern is:

[[fill]align][sign][#][0][width][,][.precision][type]

Example combining multiple components:

value = 42.5

print(f"{value:*>+10,.1f}")
# ****+42.5

Component	Meaning
fill	padding character
align	<, >, ^, =
sign	+, -, or space
width	minimum field size
,	thousands separator
precision	decimal digits
type	value representation

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Key Takeaways

• Format specifiers control how values are displayed.
• The syntax is {value:format_spec}.
• Alignment uses <, >, and ^.
• Width defines minimum field size.
• Precision controls decimal places or string length.
• Numeric types support bases, separators, and signs.
• Datetime formatting uses strftime codes.
• Format specifiers are commonly used in tables, reports, and numeric displays.