17.6. OOP Init
__init__()is an initializer methodIt's a first method run after object is initiated
All classes has default
__init__()which does nothing by defaultYou can create your own initializer method and override the default
- constructor
Method called when Python creates an instance of a class. Constructor is responsible for creating an object. Constructor returns an instance.
- initializer
Method called when Python creates an instance of a class. Initializer is responsible for initializing empty object with values. Initializer should return
None.
17.6.1. Initialization
__init__()is an initializer methodIt's a first method run after object is initiated
All classes has default
__init__()which does nothing by defaultYou can create your own initializer method and override the default
Initializer method without arguments:
>>> class User:
... def __init__(self):
... print('hello')
>>>
>>>
>>> alice = User()
hello
>>>
>>> bob = User()
hello
17.6.2. Parameters and Arguments
Parameters - specified in method signature definition
Arguments - passed to the method on call
Required parameters - if not passed,
TypeErrorwill be raisedOptional parameters - if not passed, the default value will be used
Positional argument - passed as
valueto the methodKeyword argument - passed as
name=valueto the method
>>> class User:
... def __init__(self, a, b=None):
... print(f'Hello {a=} {b=}')
>>>
>>>
>>> alice = User()
Traceback (most recent call last):
TypeError: User.__init__() missing 1 required positional argument: 'a'
>>>
>>> alice = User('alice', 'secret')
Hello a='alice' b='secret'
>>>
>>> alice = User(a='alice', b='secret')
Hello a='alice' b='secret'
However, all that will not save in the object, and all variables
will be destroyed upon __init__() completion.
>>> vars(alice)
{}
17.6.3. Attributes Initialization
Init can initialize instance attributes with a value
>>> class User:
... def __init__(self):
... self.authenticated = False
>>>
>>>
>>> alice = User()
>>>
>>> vars(alice)
{'authenticated': False}
17.6.4. Attributes Parametrization
You can set initial value for instance attributes based on arguments passed to the
__init__()on object creation
>>> class User:
... def __init__(self, a, b):
... self.username = a
... self.password = b
>>>
>>>
>>> alice = User('alice', 'secret')
>>> alice = User(a='alice', b='secret')
Only when you assign a value to self.something, then it will
be stored as a instance attribute:
>>> vars(alice)
{'username': 'alice', 'password': 'secret'}
17.6.5. Convention
Use the same parameter names as instance attribute names
>>> class User:
... def __init__(self, username, password):
... self.username = username
... self.password = password
>>>
>>>
>>> alice = User('alice', 'secret')
>>> alice = User(username='alice', password='secret')
>>>
>>> vars(alice)
{'username': 'alice', 'password': 'secret'}
17.6.6. Attribute Validation
For example minimum
passwordlength validation
>>> class User:
... def __init__(self, username, password):
... if len(password) < 8:
... raise ValueError('Password must be at least 8 characters')
... self.username = username
... self.password = password
>>>
>>>
>>> alice = User('alice', 'secret')
Traceback (most recent call last):
ValueError: Password must be at least 8 characters
>>>
>>> alice = User('alice', 'secret123')
>>>
>>> vars(alice)
{'username': 'alice', 'password': 'secret123'}
17.6.7. Use Case - 1
>>> class Point:
... def __init__(self, x, y):
... self.x = x
... self.y = y
>>>
>>>
>>> a = Point(10, 20)
>>> b = Point(10, y=20)
>>> c = Point(x=10, y=20)
>>> d = Point(y=20, x=10)
17.6.8. Use Case - 2
>>> class Point:
... def __init__(self, x, y, z=0):
... self.x = x
... self.y = y
... self.z = z
>>> a = Point(10, 20)
>>> b = Point(10, y=20)
>>> c = Point(x=10, y=20)
>>> d = Point(y=20, x=10)
>>> e = Point(10, 20, 30)
>>> f = Point(10, 20, z=30)
>>> g = Point(10, y=20, z=30)
>>> h = Point(10, z=30, y=20)
>>> i = Point(x=10, y=20, z=30)
>>> j = Point(x=10, z=30, y=20)
>>> k = Point(y=20, x=10, z=30)
>>> l = Point(y=20, z=30, x=10)
>>> m = Point(z=30, x=10, y=20)
>>> n = Point(z=30, y=20, x=10)
17.6.9. Use Case - 3
>>> class Iris:
... def __init__(self, sepal_length, sepal_width,
... petal_length, petal_width, species):
... self.sepal_length = sepal_length
... self.sepal_width = sepal_width
... self.petal_length = petal_length
... self.petal_width = petal_width
... self.species = species
>>> setosa = Iris(5.1, 3.5, 1.4, 0.2, 'setosa')
>>> virginica = Iris(
... sepal_length=5.8,
... sepal_width=2.7,
... petal_length=5.1,
... petal_width=1.9,
... species='virginica')
17.6.10. Use Case - 4
>>> from random import randint, seed
>>> seed(0)
>>>
>>>
>>> class Hero:
... def __init__(self, name, position_x, position_y):
... self.name = name
... self.position = (position_x, position_y)
... self.health = randint(50,100)
>>>
>>>
>>> alice = Hero('Alice', position_x=10, position_y=20)
>>> vars(alice)
{'name': 'Alice', 'position': (10, 20), 'health': 74}
17.6.11. Use Case - 5
>>> from random import randint, seed
>>> seed(0)
>>>
>>>
>>> class Hero:
... def __init__(self, name, health_min=10, health_max=100):
... self.name = name
... self.position = (0, 0)
... self.health = randint(health_min,health_max)
>>>
>>>
>>> alice = Hero('Alice', health_min=50)
>>> vars(alice)
{'name': 'Alice', 'position': (0, 0), 'health': 74}
17.6.12. Assignments
# %% About
# - Name: OOP Init Hello
# - Difficulty: easy
# - Lines: 2
# - Minutes: 2
# %% 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. Modify `User` class:
# - Define initialization method
# - Method should print 'hello' at object creation
# 2. Non-functional requirements:
# - Do not store any values in the class
# - Run doctests - all must succeed
# %% Polish
# 1. Zmodyfikuj klasę `User`:
# - Zdefiniuj metodę inicjalizacyjną
# - Metoda powinna wypisać 'hello' przy tworzeniu obiektu
# 2. Wymagania niefunkcjonalne:
# - Nie przechowuj żadnych wartości w klasie
# - Uruchom doctesty - wszystkie muszą się powieść
# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> from inspect import isclass
>>> assert isclass(User)
>>> assert hasattr(User, '__init__')
>>> result = User()
hello
>>> vars(result)
{}
"""
# %% 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
# %% Types
from typing import Callable
User: type
__init__: Callable[[object], None]
# %% Data
# %% Result
class User:
...
# %% About
# - Name: OOP Init Fields
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2
# %% 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. Modify `User` class:
# - Define initialization method
# - Method should set `authenticated` attribute to `False`
# 2. Non-functional requirements:
# - Run doctests - all must succeed
# %% Polish
# 1. Zmodyfikuj klasę `User`:
# - Zdefiniuj metodę inicjalizacyjną
# - Metoda powinna ustawić atrybut `authenticated` na `False`
# 2. Wymagania niefunkcjonalne:
# - Uruchom doctesty - wszystkie muszą się powieść
# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> from inspect import isclass
>>> assert isclass(User)
>>> assert hasattr(User, '__init__')
>>> result = User()
>>> vars(result)
{'authenticated': False}
"""
# %% 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
# %% Types
from typing import Callable
User: type
__init__: Callable[[object], None]
authenticated: bool
# %% Data
# %% Result
class User:
...
# %% About
# - Name: OOP Init Parameters
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2
# %% 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. Modify `User` class:
# - Define initialization method with parameters `firstname` and `lastname`
# - Method should set `firstname` attribute to the value of `firstname` parameter
# - Method should set `lastname` attribute to the value of `lastname` parameter
# 2. Non-functional requirements:
# - Run doctests - all must succeed
# %% Polish
# 1. Zmodyfikuj klasę `User`:
# - Zdefiniuj metodę inicjalizacyjną z parametrami `firstname` i `lastname`
# - Metoda powinna ustawić atrybut `firstname` na wartość parametru `firstname`
# - Metoda powinna ustawić atrybut `lastname` na wartość parametru `lastname`
# 2. Wymagania niefunkcjonalne:
# - Uruchom doctesty - wszystkie muszą się powieść
# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'
>>> from inspect import isclass
>>> assert isclass(User)
>>> assert hasattr(User, '__init__')
>>> alice = User('Alice', 'Apricot')
>>> vars(alice)
{'firstname': 'Alice', 'lastname': 'Apricot'}
>>> bob = User(firstname='Bob', lastname='Blackthorn')
>>> vars(bob)
{'firstname': 'Bob', 'lastname': 'Blackthorn'}
"""
# %% 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
# %% Types
from typing import Callable
User: type
__init__: Callable[[object], None]
firstname: str
lastname: str
# %% Data
# %% Result
class User:
...