String Formatting#

The format Method#

String objects have a format method. This method allows for converting numbers and other data to strings.

a = 4
b = 5
c = a + b
nice_string = 'The result of {} plus {} is {}.'.format(a, b, c)
print(nice_string)

The result of 4 plus 5 is 9.

Calling the format method of a string replaces all pairs {} by the arguments passed to the format method. Next to integers also floats and strings can be passed to format. The original string is not modified because it’s immutable. Instead, format returns a new string object.

The arguments of format can also be accessed by providing their index: the first argument has index 0, the second has index 1, and so on.

a = 4
b = a + a
print('The result of {0} plus {0} is {1}.'.format(a, b))

The result of 4 plus 4 is 8.

For more complex output (or more readable code) keyword arguments can be passed to format.

print('Mister {name} is {age} years old.'.format(name='Muller', age='42'))

Mister Muller is 42 years old.

Converting numbers to strings sometimes requires additional parameters: How many digits to use for floats? Shall numbers in several lines be aligned horizontally? There is a whole ‘mini language’ for writing formatting options. Here we provide only some examples. For details see Python documentation on format string syntax.

# print integer as float with 2 decimal places
print('The result is {:.2f}.'.format(3))

# right-align integers in fixed-width area
print('number of apples:  {:3d}'.format(2))
print('number of oranges: {:3d}'.format(145))

# same works for floats and strings
print('percentage of apples:  {:7.2f}'.format(2.1234))
print('percentage of oranges: {:7.2f}'.format(80.8976455))
print('percentage of bananas: {:7}'.format('unknown'))

The result is 3.00.
number of apples:    2
number of oranges: 145
percentage of apples:     2.12
percentage of oranges:   80.90
percentage of bananas: unknown

If indices or names are used for refering to format’s arguments, they have to be placed on the left-hand side of the collon: {name:3.2f}.

Formatted String Literals (f-Strings)#

Starting with version 3.6 of Python there is a more comfortable way to format strings. It’s very similar to formatting via format method, but requires less code and increases readability. The two major differences are:

  • string literals have to be prefixed by f,

  • the curly braces may contain a Python expression (an object name, for instance).

a = 123
print(f'Here you see {a}.')

b = 4.56789
print(f'Formatting works as above: {b:.2f} is a rounded float.')

Here you see 123.
Formatting works as above: 4.57 is a rounded float.

For details see formatted string literals in the Python documentation.