When you call str.format(), each replacement field may contain a format specifier that tells Python exactly how to rander the corrresponding argument. The specifier is introduced by a colon : and is composed of the following optional components in order:

{[name][!conversion][:[[fill]align][sign][#][0][width][grouping][.precision][type]]}

Alignment & Width

>>> '{:<10}'.format('left')
'left      '
>>> '{:>10}'.format('right')
'     right'
>>> '{:^10}'.format('center')
'  center  '

Think of < and > as arrows pointing to the side where the content should stick.

Fill Character

Any character placed before the alignment symbol becomes the padding character.

>>> '{:*^12}'.format('Python')
'***Python***'

Numeric Options

• + : always show sign
• - : show sign only for negatives (default)
• space : leading space for positives
• # : alternate form (prefix 0b, 0o, 0x)
• 0 : zero-padding (same as fill='0' align='=')

>>> '{:+05d}'.format(42)
'+0042'
>>> '{:#x}'.format(255)
'0xff'

Precision & Types

>>> '{:.2f}'.format(3.1415926)
'3.14'
>>> '{:.2e}'.format(123456789)
'1.23e+08'
>>> '{:,.0f}'.format(1e6)
'1,000,000'

String Truncation

For strings, .precision limits the number of characters displayed.

>>> '{:.5}'.format('全国二级计算机考试')
'全国二级计'

Multiple Representations of One Value

>>> n = 276
>>> '{0:b} {0:c} {0:d} {0:o} {0:x} {0:X}'.format(n)
'100010100 Ť 276 424 114 72'

Putting It All Together

>>> '{:0>+8.2f}'.format(-3.141)
'-0003.14'

The specifier above means:

• 0 – use 0 as fill character
• > – right-align
• + – always show sign
• 8 – total width 8 characters
• .2f – fixed-point with 2 digits after decimal