2.6. String Interpolation

2.6.1. Mod Operator

  • Since Python 1.0

  • positional

  • keyword

  • %s - str

  • %d - int

  • %f - float

name = 'Alice'
age = 30
pi = 3.141592653589793

'Hello %s' % name                   # Hello Alice
'Hello %d' % name                   # TypeError: %d format: a number is required, not str
'Hello %f' % name                   # TypeError: must be real number, not str

'I have %s years' % age             # 'I have 42 years'
'I have %d years' % age             # 'I have 42 years'
'I have %f years' % age             # 'I have 42.000000 years'

'Number PI is %s' % pi              # 'Number PI is 3.141592653589793'
'Number PI is %f' % pi              # 'Number PI is 3.141593'
'Number PI is %d' % pi              # 'Number PI is 3'

name = 'Alice'
age = 30

'%s has %s years' % (name, age))      # Alice has 30 years
'%s has %s years' % (age, name))      # 30 has Alice years

pi = 3.141592653589793

def square(value):
    return value ** 2

'PI squared is %f' % square(pi)      # 'PI squared is 9.869604'

data = {
    'name': 'Alice',
    'age': 30,
}

'%(name)s has %(age)d years' % data
# 'Alice has 30 years'

'%(name)s has %(age)d years' % {'name': 'Alice', 'age': 30}
# 'Alice has 30 years'

name = 'Alice'
age = 30

'Hello %(name)s' % locals()
# 'Hello Alice'

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'

2.6.4. Assignments

# %% About
# - Name: String Interpolation Mod
# - Difficulty: easy
# - Lines: 1
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Read a number from user
# 2. User will input `int` and will not try to input invalid data
# 3. Define `result: bool` with parity check of input number
# 4. Number is even, when divided modulo (`%`) by 2 reminder equal to 0
# 5. Do not use `if` statement
# 6. Run doctests - all must succeed

# %% Polish
# 1. Wczytaj liczbę od użytkownika
# 2. Użytkownika poda `int` i nie będzie próbował wprowadzać niepoprawnych danych
# 3. Zdefiniuj `result: bool` z wynikiem sprawdzania parzystości liczby wprowadzonej
# 4. Liczba jest parzysta, gdy dzielona modulo (`%`) przez 2 ma resztę równą 0
# 5. Nie używaj instrukcji `if`
# 6. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `int()`
# - `%`
# - `==`
# - `%` has different meaning for `int` and `str`
# - `%` on `str` is overloaded as a string formatting
# - `%` on `int` is overloaded as a modulo division

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> assert 'result' in globals(), \
'Variable `result` is not defined; assign result of your program to it.'

>>> assert result is not Ellipsis, \
'Variable `result` has an invalid value; assign result of your program to it.'

>>> assert type(result) is bool, \
'Variable `result` has an invalid type; expected: `bool`.'

>>> result
True
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
from unittest.mock import Mock

# %% Types
result: bool

# %% Data
input = Mock(side_effect=['4'])  # Simulate users response: '4'
number = input('What is your number?: ')

# %% Result
result = ...