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Format Mini-Language in f-strings

The Format Mini-Language is powerful. And often overlooked. It affords precise control over how values are displayed, far beyond variable interpolation.

The Format Mini-Language follows the pattern: {value:format_spec}. format_spec can include alignment, padding, precision, type codes, etc.

Common Format Specifiers

Alignment and Padding

Control text alignment within a fixed width field:

# Basic alignment
name = "Python"
print(f"|{name:<12}|")  # Left align
print(f"|{name:>12}|")  # Right align
print(f"|{name:^12}|")  # Center align

# Custom padding characters
print(f"{name:=^20}")   # Center with '='
print(f"{name:*>15}")   # Right align with '*'

Output:

|Python      |
|      Python|
|   Python   |
=======Python=======
*********Python

Number Formatting

Format numbers with precision, thousands separators, and different bases:

value = 1234567.89
small = 0.00456

# Decimal places
print(f"Fixed 2 decimals: {value:.2f}")
print(f"Fixed 4 decimals: {small:.4f}")

# Thousands separator
print(f"With commas: {value:,.2f}")

# Scientific notation
print(f"Scientific: {value:.2e}")
print(f"General format: {small:.2g}")

# Different bases
num = 255
print(f"Hex: {num:x}, {num:#x}, {num:#X}")
print(f"Binary: {num:b}, {num:#b}")
print(f"Octal: {num:o}, {num:#o}")

Output:

Fixed 2 decimals: 1234567.89
Fixed 4 decimals: 0.0046
With commas: 1,234,567.89
Scientific: 1.23e+06
General format: 0.0046
Hex: ff, 0xff, 0XFF
Binary: 11111111, 0b11111111
Octal: 377, 0o377

Percentage and Sign Formatting

ratio = 0.8765
change = -0.0234
positive = 42.5

# Percentage
print(f"Completion: {ratio:.1%}")
print(f"Precise: {ratio:.3%}")

# Sign handling
print(f"Always sign: {positive:+.1f}")
print(f"Space for positive: {positive: .1f}")
print(f"Negative: {change:+.2%}")

Output:

Completion: 87.7%
Precise: 87.650%
Always sign: +42.5
Space for positive:  42.5
Negative: -2.34%

Zero Padding and Width

# Zero padding for numbers
day = 5
month = 12
print(f"Date: {month:02d}/{day:02d}")

# Fixed width with zeros
order_id = 42
print(f"Order: {order_id:08d}")

# Combining with other formats
pi = 3.14159
print(f"Pi: {pi:08.3f}")

Output:

Date: 12/05
Order: 00000042
Pi: 003.142

Debug Expressions

Supported in Python 3.8+

The = specifier shows both the expression and its value:

x = 10
y = 20
name = "Alice"

print(f"{x + y=}")
print(f"{name.upper()=}")
print(f"{len(name)=}")

Output:

x + y=30
name.upper()='ALICE'
len(name)=5

Datetime Formatting

Format datetime objects directly in f-strings:

from datetime import datetime

now = datetime.now()

print(f"Full date: {now:%Y-%m-%d %H:%M:%S}")
print(f"Just date: {now:%B %d, %Y}")
print(f"Just time: {now:%I:%M %p}")
print(f"Weekday: {now:%A}")
print(f"ISO format: {now:%Y%m%dT%H%M%S}")

Output:

Full date: 2025-01-23 14:30:45
Just date: January 23, 2025
Just time: 02:30 PM
Weekday: Thursday
ISO format: 20250123T143045

Advanced Combinations

You can combine multiple format specifiers for complex formatting:

# Header with dynamic width
width = 60
title = "Processing Results"
print(f"{title:=^{width}}")

# Table-like output
items = [
    ("Dataset A", 0.9234, 1000000),
    ("Dataset B", 0.8765, 2500000),
    ("Dataset C", 0.9876, 750000)
]

print(f"{'Name':<15} {'Accuracy':>10} {'Samples':>12}")
print("-" * 39)
for name, acc, samples in items:
    print(f"{name:<15} {acc:>10.2%} {samples:>12,}")

Output:

=====================Processing Results=====================
Name             Accuracy      Samples
---------------------------------------
Dataset A          92.34%    1,000,000
Dataset B          87.65%    2,500,000
Dataset C          98.76%      750,000

Format Specification Mini-Language

General form: [[fill]align][sign][#][0][width][grouping_option][.precision][type]

  • fill: Any character to use for padding
  • align: < (left), > (right), ^ (center), = (padding after sign)
  • sign: + (always show), - (only negative), (space for positive)
  • #: Alternate form (adds prefix for bases)
  • 0: Zero-padding
  • width: Minimum field width
  • grouping_option: , or _ for thousands separator
  • precision: Number of digits after decimal
  • type: s (string), d (decimal), f (fixed-point), e (exponential), % (percentage), etc.

All of this makes f-strings suitable for everything from simple interpolation to complex report formatting, all while maintaining readability and performance.