python string formatting

format() method allows you format string in any way you want.
Syntax: template.format(p1, p1, .... , k1=v1, k2=v2)
template is a string containing format codes, format()  method uses it’s argument to substitute value for each format codes. For e.g

>>> 'Sam has {0} red balls and {1} yellow balls'.format(12, 31)

1	>>> 'Sam has {0} red balls and {1} yellow balls'.format(12, 31)

{0}  and {1}  are format codes. The format code {0}  is replaced by the first argument of format()  i.e 12 , while {1}  is replaced by the second argument of format()  i.e 31 .
Expected Output:

Sam has 12 red balls and 31 yellow balls

1	Sam has 12 red balls and 31 yellow balls

This technique is okay for simple formatting but what if you want to specify precision in floating point number ? For such thing you need to learn more about format codes. Here is the full syntax of format codes.
Syntax: {[argument_index_or_keyword]:[width][.precision][type]}
type  can be used with format codes

Format codes	Description
d	for integers
f	for floating point numbers
b	for binary numbers
o	for octal numbers
x	for octal hexadecimal numbers
s	for string
e	for floating point in exponent format

Following examples will make things more clear.
Example 1:

>>> "Floating point {0:.2f}".format(345.7916732)

1	>>> "Floating point {0:.2f}".format(345.7916732)

Here we specify 2  digits of precision and f  is used to represent floating point number.
Expected Output:

Floating point 345.79

1	Floating point 345.79

Example 2:

>>> import math >>> "Floating point {0:10.3f}".format(math.pi)

1 2	>>> import math >>> "Floating point {0:10.3f}".format(math.pi)

Here we specify 3  digits of precision, 10  for width and f  for floating point number.
Expected Output:

Floating point 3.142

1	Floating point 3.142

Example 3:

"Floating point pi = {0:.3f}, with {1:d} digit precision".format(math.pi, 3)

1	"Floating point pi = {0:.3f}, with {1:d} digit precision".format(math.pi, 3)

here d  in {1:d} represents integer value.
Expected Output:

Floating point pi = 3.142, with 3 digit precision

1	Floating point pi = 3.142, with 3 digit precision

You need to specify precision only in case of floating point numbers if you specify precision for integer ValueError  will be raised.
Example 5:

'Sam has {1:d} red balls and {0:d} yellow balls'.format(12, 31)

1	'Sam has {1:d} red balls and {0:d} yellow balls'.format(12, 31)

Expected Output:

Sam has 31 red balls and 12 yellow balls

1	Sam has 31 red balls and 12 yellow balls

Example 6:

"In binary 4 is {0:b}".format(4) # b for binary, refer to Fig 1.1

1	"In binary 4 is {0:b}".format(4) # b for binary, refer to Fig 1.1

Expected Output:

In binary 4 is 100

1	In binary 4 is 100

Example 7:

array = [34, 66, 12] "A = {0}, B = {1}, C = {2}".format(*array)

1 2	array = [34, 66, 12] "A = {0}, B = {1}, C = {2}".format(*array)

Expected Output:

'A = 34, B = 66, C = 12'

1	'A = 34, B = 66, C = 12'

Example 8:

d = { 'hats' : 122, 'mats' : 42 }

1 2 3 4

d = { 'hats' : 122, 'mats' : 42 }

Expected Output:

"Sam had {hats} hats and {mats} mats".format(**d)

1	"Sam had {hats} hats and {mats} mats".format(**d)

format()  method also supports keywords arguments.

'Sam has {red} red balls and {green} yellow balls'.format(red = 12, green = 31)

1	'Sam has {red} red balls and {green} yellow balls'.format(red = 12, green = 31)

Note while using keyword arguments we need to use arguments inside {}  not numeric index.
You can also mix position arguments with keywords arguments

'Sam has {red} red balls, {green} yellow balls \ and {0} bats'.format(3, red = 12, green = 31)

1 2	'Sam has {red} red balls, {green} yellow balls \ and {0} bats'.format(3, red = 12, green = 31)

format()  method of formatting string is quite new and was introduced in python 2.6 . There is another old technique  you will see in legacy codes which allows you to format string using %  operator instead of format()  method.
Let’s take an example.

"%d pens cost = %.2f" % (12, 150.87612)

1	"%d pens cost = %.2f" % (12, 150.87612)

Here we are using template string on the left of % . Instead of {} for format codes we are using % . On the right side of % we use tuple to contain our values. %d and %.2f are called as format specifiers, they begin with % followed by character that represents the data type. For e.g %d  format specifier is a placeholder for a integer, similarly %.2f  is a placeholder for floating point number.
So %d  is replaced by the first value of the tuple i.e 12  and %.2f  is replaced by second value i.e 150.87612 .
Expected Output:

12 pens cost = 150.88

1	12 pens cost = 150.88

Some more examples
Example 1:
New: "{0:d} {1:d} ".format(12, 31)
Old: "%d %d" % (12, 31)
Expected Output:

12 31

1

12 31

Example 2:
New: "{0:.2f} {1:.3f}".format(12.3152, 89.65431)
Old "%.2f %.3f" % (12.3152, 89.65431)
Expected Output:

12.32 89.654

1	12.32 89.654

Example 3:
New: "{0:s} {1:o} {2:.2f} {3:d}".format("Hello", 71, 45836.12589, 45 )
Old:  "%s %o %.2f %d" % ("Hello", 71, 45836.12589, 45 )
Expected Output:

Hello 107 45836.13 45

1	Hello 107 45836.13 45