Basic Concepts and Width Control
String formatting controls data display in Python programs. Imagine arranging data in a neat table where each column has consistent width. Python provides format() method and f-strings to control output display.
The basic replacement field syntax is {name : width}, where name determines the argument and width sets the minimum output width. When width is smaller than the original string length, Python still displays the complete string.
Format Types for Number Systems
| Format Type | Symbol | Example Input | Output | Usage |
|---|---|---|---|---|
| Decimal | d | 45 | 45 | Default for integer |
| Binary | b | 45 | 101101 | Bit representation |
| Octal | o | 45 | 55 | Unix systems |
| Hexadecimal | x/X | 45 | 2d/2D | Low-level programming |
# Width control with various formatsprint('|{0:15}|'.format('xxx')) # Output: |xxx |print('a = {0:6d}'.format(45)) # Output: a = 45print('a = {0:10b}'.format(45)) # Output: a = 101101print('a = {0:6x}'.format(45)) # Output: a = 2d# Width smaller than string - not truncatedprint('|{0:1}|'.format('xxx')) # Output: |xxx|Position Settings and Fill Characters
Alignment Options
| Symbol | Name | Behavior | Default for |
|---|---|---|---|
< | Left | Text on left, padding on right | String, objects |
> | Right | Text on right, padding on left | Numbers |
^ | Center | Text in center, even padding | - |
Fill Characters and Advanced Formatting
Fill character determines the character to fill empty space. Default uses space, but can be replaced with any character except { and }.
# Basic alignmentprint('|{0:<15}|'.format('xxx')) # Output: |xxx |print('|{0:>15}|'.format('xxx')) # Output: | xxx|print('|{0:^15}|'.format('xxx')) # Output: | xxx |# Custom fill charactersprint('|{0:-<15}|'.format('xxx')) # Output: |xxx------------|print('|{0:*>15}|'.format('xxx')) # Output: |************xxx|print('|{0:^^15}|'.format('xxx')) # Output: |^^^^^^xxx^^^^^^|# Decimal formattingx = 123.98print('x = {0:12f}'.format(x)) # Output: x = 123.980000print('x = {0:12e}'.format(x)) # Output: x = 1.239800e+02Argument Reference Methods
Python provides three ways to reference arguments in replacement fields, each with different rules.
Reference Method Comparison
| Method | Syntax | Advantages | Limitations |
|---|---|---|---|
| Numbered | {0}, {1} | Explicit control, can repeat | Must match argument count |
| Keyword | {name}, {age} | Easy to read, flexible | Names must match |
| Automatic | {}, {} | Simple syntax | Cannot mix with numbered |
# Numbered fields - explicit controlprint('{2} - {0} - {1}'.format('first', 'second', 'third'))# Output: third - first - second# Keyword fields - using namesprint('{name} is {age} years old'.format(name='Alice', age=25))# Output: Alice is 25 years old# Automatic numbering - sequential orderprint('{} + {} = {}'.format(5, 3, 8))# Output: 5 + 3 = 8# Error handlingtry: print('{a} - {b}'.format(a='xxx')) # Will errorexcept KeyError as e: print(f"KeyError: {e}") # Output: KeyError: 'b' try: print('{0} - {1}'.format('xxx')) # Will errorexcept IndexError as e: print(f"IndexError: {e}") # Output: IndexError: Replacement index 1 out of range for positional args tuplePrecision Control and Sign Specification
Precision for Decimal Numbers
Precision controls the number of decimal places or significant digits in number display.
| Format Type | Precision Behavior | Example |
|---|---|---|
f/F | Decimal places | {:.2f} → 123.99 |
e/E | Decimal places in exponential | {:.2e} → 1.24e+02 |
g/G | Significant digits | {:.3g} → 124 |
Sign Options
| Symbol | Behavior | Example |
|---|---|---|
+ | Always show sign | +100, -200 |
- | Only negative sign (default) | 100, -200 |
(space) | Space for positive | 100, -200 |
# Precision controlprint('x = {0:.3f}'.format(123.98765)) # Output: x = 123.988print('x = {0:12.2e}'.format(1.987e-10)) # Output: x = 1.99e-10print('x = {0:.3g}'.format(1.123456)) # Output: x = 1.12# Sign specificationnumbers = [-100, 200, -300]print('Default:', ['{:d}'.format(n) for n in numbers])# Output: Default: ['-100', '200', '-300']print('Always sign:', ['{:+d}'.format(n) for n in numbers])# Output: Always sign: ['-100', '+200', '-300']print('Space positive:', ['{: d}'.format(n) for n in numbers])# Output: Space positive: ['-100', ' 200', '-300']# Rounding behaviorprint('Tiny number f: {0:12f}'.format(1.1e-10)) # Output: Tiny number f: 0.000000print('Tiny number g: {0:12g}'.format(1.1e-10)) # Output: Tiny number g: 1.1e-10F-Strings and Dynamic Templates
F-Strings (Python 3.6+)
F-strings provide the simplest syntax with direct expression evaluation. Faster and more readable compared to format().
Dynamic Format Templates
Format templates can use variables for specification, useful when format is determined dynamically.
# F-strings - simple syntaxname, age = 'Alice', 25print(f'{name} is {age} years old') # Output: Alice is 25 years oldprint(f'Next year: {name} will be {age + 1}') # Output: Next year: Alice will be 26# Dynamic format templatesvalue, width, fill, align = 'data', 12, '-', '^'template = '|{val:{f}{a}{w}}|'result = template.format(val=value, w=width, f=fill, a=align)print(result) # Output: |----data----|# Comparison - same output, different methodsx = 123.456print('Old style: %8.2f' % x) # Output: Old style: 123.46print('Format method: {0:8.2f}'.format(x)) # Output: Format method: 123.46print(f'F-string: {x:8.2f}') # Output: F-string: 123.46# Complex f-string with format specimport mathradius = 5.7print(f'Area: {math.pi * radius**2:.2f} cm²') # Output: Area: 102.07 cm²Complete Format Specification Pattern
Format specification follows the pattern {name:fill align sign width .prec type} where each component is optional and has a specific function to control output display.