String Formatting

C-Style Formatting (1)

Good old C: %[flags][width][.precision]type}

Good ol’ C

Python
Program
int i = 42;
float f = 3.14159265359;
printf("%07d, %8.4f\n", i, f);
Output
0000042,   3.1416
Same in Python, using the % operator
>>> '%07d' % 42
'0000042'
>>> '%07d, %8.4f' % (42, 3.14159265359)
'0000042,   3.1416'

C-Style Formatting: Conversions

Frequently used conversions …

s

String

c

Single character

d

Integer (decimal)

o

Integer (octal)

x

Integer (hexadecimal lowercase)

X

Integer (hexadecimal uppercase)

f

Floating point

exponential format (lowercase)

F

Floating point

exponential format (uppercase)

\%

The % sign itself

C-Style Formatting: Flags

Frequently used flags …

#

Octal or hex integer conversions: 0x... prefixes

0

Pad with ‘0’ characters

-

Left alignment

+

Print sign even if positive

`` `` (space)

Print space in place of sign if positive

C-Style Formatting: Examples

>>> '%#5X' % 47
' 0X2F'
>>> '%5X' % 47
'   2F'
>>> '%#5.4X' % 47
'0X002F'
>>> '%#5o' % 25
' 0o31'
>>> '%+d' % 42
'+42'

>>> '% d' % 42
' 42'
>>> '%+2d' % 42
'+42'
>>> '% 4d' % 42
'  42'
>>> '% 4d' % -42
' -42'
>>> '%04d' % 42
'0042'

The format Method

Problems with C-style formatting

  • Not flexible enough (as always)

  • Positional parameters only

  • Parameter position must match occurence in format string

A better (?) way of formatting
>>> '0 {0:05d}, 1 {1:8.2f}, 0 again {0}'.format(42, 1.5)
'0 00042, 1     1.50, 0 again 42'
>>> 'a {a:05d}, b {b:8.2f}, a again {a}'.format(a=42, b=1.5)
'a 00042, b     1.50, a again 42'

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