2.6. String Formatting

2.6.1. Mod Operator

  • Since Python 1.0

  • positional

  • keyword

  • %s - str

  • %d - int

  • %f - float

One parameter:

>>> name = 'Alice'
>>>
>>> 'Hello %s' % name
'Hello Alice'

Many parameters as tuple:

>>> firstname = 'Alice'
>>> lastname = 'Apricot'
>>> age = 30
>>>
>>> 'Hello %s %s is %d years old' % (firstname, lastname, age)
'Hello Alice Apricot is 30 years old'

>>> data = ('Alice', 'Apricot', 30)
>>>
>>> 'Hello %s %s is %d years old' % data
'Hello Alice Apricot is 30 years old'

Many parameters as dict:

>>> data = {
...     'firstname': 'Alice',
...     'lastname': 'Apricot',
...     'age': 30,
... }
>>>
>>> 'Hello %(firstname)s %(lastname)s is %(age)d years old' % data
'Hello Alice Apricot is 30 years old'

2.6.2. Format Method

  • Since Python 3.0

  • Since Python 3.0: PEP 3101 -- Advanced String Formatting

name = 'Alice'
age = 30

'{} has {} years'.format(name, age)                     # 'Alice has 30 years'
'{0} has {1} years'.format(name, age)                   # 'Alice has 30 years'
'{1} has {0} years'.format(name, age)                   # '30 has Alice years'

name = 'Alice'
age = 30

'{a} has {b} years'.format(a=name, b=age)               # 'Alice has 30 years'
'{name} has {age} years'.format(name=name, age=age)     # 'Alice has 30 years'
'{age} has {name} years'.format(**locals())             # '30 has Alice years'

2.6.3. f-strings

  • Since Python 3.6

  • Preferred way

name = 'Alice'
pi = 3.141592653589793

def square(value):
    return value ** 2

f'Hello {name}'                      # 'Hello Alice'
f'PI squared is {square(pi)}'        # 'PI squared is 9.869604401089358'

from datetime import datetime


now = datetime.now()
iso = '%Y-%m-%dT%H:%M:%SZ'

f'Today is: {now:%Y-%m-%d}')              # 'Today is: 1969-07-21'
f'Today is: {now:{iso}}')                 # 'Today is: 1969-07-21T02:56:15Z'