Python String Formatting Using format() Method

Introduction

Python 2.6 introduced the string.format() method with a slightly different syntax from the existing % operator.

Background

Older '%' string formatter.

Prior to python 2.6, a simpler string formatter '%' was available.
It was easy to use but the number of arguments that it could accept were limited.

For example:

>>> ex = (1,2,3)
>>> print "hi there %s" %ex #this would throw a TypeError

To make it work:

>>> print "hi there %s" % (ex,) #looks ugly, though!

.format() doesn't have such issues.

Something that the modulo operator ( % ) can't do:

>>> ex = (12,45,22222,103,6)
>>> print '{0} {2} {1} {2} {3} {2} {4} {2}'.format(*ex)
>>> 12 22222 45 22222 103 22222 6 22222

Moreover, for a long list of arguments, it is easy that a variable could be missed.

>>> print "%s, %s, %s, %s, %s, %s, %s, %s" % (a,b,c,d,e,f,g,h)

Using the Format Method

Accessing Arguments by Position

>>> '{0}, {1}, {2}'.format('a', 'b', 'c')
'a, b, c'
>>> '{}, {}, {}'.format('a', 'b', 'c')  # 2.7+ only
'a, b, c'
>>> '{2}, {1}, {0}'.format('a', 'b', 'c')
'c, b, a'
>>> '{2}, {1}, {0}'.format(*'abc')      # unpacking argument sequence
'c, b, a'
>>> '{0}{1}{0}'.format('abra', 'cad')   # arguments' indices can be repeated
'abracadabra'

Accessing Arguments by Name

>>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
'Coordinates: 37.24N, -115.81W'

Passing a Dictionary

>>> coord = {'latitude': '37.24N', 'longitude': '-115.81W'}
>>> 'Coordinates: {latitude}, {longitude}'.format(**coord)
'Coordinates: 37.24N, -115.81W' 

Passing a List

>>> coord = ['37.24N', '-115.81W']
>>> 'Coordinates: {}, {}'.format(*coord) #can be also 'Coordinates: {0}, {1}'.format(*coord)
>>> 'Coordinates: 37.24N, -115.81W' 

Converting Values to Different Bases

You can use the following letters to convert a number to their bases - decimal, hex, octal, binary.

>>> # format also supports binary numbers
>>> "int: {0:d};  hex: {0:x};  oct: {0:o};  bin: {0:b}".format(42)
'int: 42;  hex: 2a;  oct: 52;  bin: 101010'
>>> # with 0x, 0o, or 0b as prefix:
>>> "int: {0:d};  hex: {0:#x};  oct: {0:#o};  bin: {0:#b}".format(42)
'int: 42;  hex: 0x2a;  oct: 0o52;  bin: 0b101010'

Use Format as a Function

You can use .format as a function which allows for some separation of text and formatting from code.
For example, at the beginning of your program, you could include all your formats and then use later.

# defining formats
>>> email_f = "Your email address is {email}".format

# use somewhere else
>>> print(email_f(email="bob@example.com"))

Named Arguments

The newer format method can be used as a templating engine and use named arguments, instead of requiring a strict order.

>>> madlib = " I {verb} the {object} off the {place} ".format(object="cheese", verb="took", place="table")
>>> I took the cheese off the table

Escaping Braces

If you need to print braces when using format, use braces twice!

>>> print(" The {} set is often represented as {{0}} ".format("empty"))
>>> The empty set is often represented as {0}

Aligning the Text and Specifying a Width

>>> print '{:<30}'.format('left aligned')
>>> 'left aligned                  '
>>> print '{:>30}'.format('right aligned')
>>> '                 right aligned'
>>> print '{:^30}'.format('centered')
>>> '           centered           '
>>> print '{:*^30}'.format('centered')  # use '*' as a fill char
>>> '***********centered***********'

Using the comma(,) as a Thousands Separator

Introduced in Python 3.1 - PEP

>>> print '{:,}'.format(1234567890) #backported to version 2.7
>>> '1,234,567,890'

Expressing a Percentage

>>> points = 19.5
>>> total = 22
>>> print 'Correct answers: {:.2%}'.format(points/total)
>>> 'Correct answers: 88.64%'

Another Point

Since format is a function, it can be used as an argument in other functions.

>>> li = [12,45,78,784,2,69,1254,4785,984]
>>> print map('the number is {}'.format, li)
>>> ['the number is 12', 'the number is 45', 'the number is 78', 'the number is 784', 'the number is 2', 'the number is 69', 'the number is 1254', 'the number is 4785', 'the number is 984']

Points of Interest

format is around 1x faster than using the % operator. But for some cases the % operator maybe faster too.
One issue with format is the extra function call overhead, which consumes somewhere around 50-150ns.
Results using timeit:

>>>timeit( "%d and %.1f" % (4,2.2) )
1000000 loops, best of 3: 633 ns per loop

>>>timeit( "{} and {:.1f}".format(4,2.2) )
1000000 loops, best of 3: 585 ns per loop