Python’s three string formatting syntaxes

[qingquan-li]

References:

• https://docs.python.org/3/tutorial/inputoutput.html#fancier-output-formatting
• https://realpython.com/python-string-formatting/
• https://realpython.com/python-f-strings/

1. Old String Formatting: % Operator

printf-style String Formatting

The % operator (modulo) can also be used for string formatting. Given 'string' % values, instances of % in string are replaced with zero or more elements of values. This operation is commonly known as string interpolation.

>>> import math
>>> print('The value of pi is approximately %5.3f.' % math.pi)
The value of pi is approximately 3.142.

>>> name = 'Bob'
>>> 'Hello, %s' % name
'Hello, Bob'

>>> name = 'Alice'
>>> age = 18
>>> 'My name is %s. I am %s.' % (name, age)
'My name is Alice. I am 18.'

2. The String format() Method: str.format()

str.format() >=Python 2.6

>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
We are the knights who say "Ni!"

>>> print('{0} and {1}'.format('spam', 'eggs'))
spam and eggs
>>> print('{1} and {0}'.format('spam', 'eggs'))
eggs and spam

>>> print('This {food} is {adjective}.'.format(
...     food='spam', adjective='absolutely horrible'))
This spam is absolutely horrible.

>>> for x in range(1, 11):
...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x))
... 
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
 6  36  216
 7  49  343
 8  64  512
 9  81  729
10 100 1000

3. Formatted String Literals: f-strings

>= Python 3.6

Formatted string literals (also called f-strings for short) let you include the value of Python expressions inside a string by prefixing the string with f or F and writing expressions as {expression}.

>>> import math
>>> print(f'The value of pi is approximately {math.pi:.3f}.')
The value of pi is approximately 3.142.

>>> animals = 'eels'
>>> print(f'My hovercraft is full of {animals}.')
My hovercraft is full of eels.
>>> print(f'My hovercraft is full of {animals!r}.')
My hovercraft is full of 'eels'.

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
...     print(f'{name:10} ==> {phone:10d}')
...
Sjoerd     ==>       4127
Jack       ==>       4098
Dcab       ==>       7678