16.6. OOP Init

• __init__() - initializer method

• It's a first method run after object is initiated

• All classes has default __init__()

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.

16.6.1. Without Arguments

Initializer method without arguments:

>>> class User:
...     def __init__(self):
...         print('hello')
>>>
>>>
>>> mark = User()
hello
>>>
>>> melissa = User()
hello

16.6.2. Required Parameters

>>> class User:
...     def __init__(self, firstname, lastname):
...         print(f'Hello {firstname} {lastname}')
>>>
>>>
>>> mark = User()
Traceback (most recent call last):
TypeError: User.__init__() missing 2 required positional arguments: 'firstname' and 'lastname'

>>> class User:
...     def __init__(self, firstname, lastname):
...         print(f'Hello {firstname} {lastname}')
>>>
>>>
>>> mark = User('Mark', 'Watney')
Hello Mark Watney
>>>
>>> mark = User(firstname='Mark', lastname='Watney')
Hello Mark Watney

16.6.3. Optional Parameters

>>> class User:
...     def __init__(self, firstname=None, lastname=None):
...         print(f'Hello {firstname} {lastname}')
>>>
>>>
>>> mark = User('Mark', 'Watney')
Hello Mark Watney
>>>
>>> mark = User('Mark')
Hello Mark None
>>>
>>> mark = User()
Hello None None

16.6.4. Constant Attributes

>>> class User:
...     def __init__(self):
...         self._authenticated = False

>>> mark = User()
>>> melissa = User()

>>> vars(mark)
{'_authenticated': False}
>>>
>>> vars(melissa)
{'_authenticated': False}

16.6.5. Variable Attributes

>>> class User:
...     def __init__(self, a, b):
...         self.firstname = a
...         self.lastname = b

>>> mark = User('Mark', 'Watney')
>>> melissa = User(a='Melissa', b='Lewis')

>>> vars(mark)
{'firstname': 'Mark', 'lastname': 'Watney'}

>>> vars(melissa)
{'firstname': 'Melissa', 'lastname': 'Lewis'}

16.6.6. Better Names

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

>>> mark = User('Mark', 'Watney')
>>> vars(mark)
{'firstname': 'Mark', 'lastname': 'Watney'}

>>> mark = User(firstname='Mark', lastname='Watney')
>>> vars(mark)
{'firstname': 'Mark', 'lastname': 'Watney'}

16.6.7. Combine Attributes

• You can combine firstname and lastname into self.name

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.name = f'{firstname} {lastname}'
>>>
>>>
>>> mark = User('Mark', 'Watney')
>>> vars(mark)
{'name': 'Mark Watney'}

>>> print(mark.name)
Mark Watney
>>>
>>> print(mark.firstname)
Traceback (most recent call last):
AttributeError: 'User' object has no attribute 'firstname'
>>>
>>> print(mark.lastname)
Traceback (most recent call last):
AttributeError: 'User' object has no attribute 'lastname'

16.6.8. Validation

>>> class User:
...     def __init__(self, firstname, lastname, age):
...         if not 0 <= age <= 130:
...             raise ValueError('Age must be between 0 and 130')
...         self.firstname = firstname
...         self.lastname = lastname
...         self.age = age

>>> mark = User('Mark', 'Watney', age=42)

>>> mark = User('Mark', 'Watney', age=-1)
Traceback (most recent call last):
ValueError: Age must be between 0 and 130

16.6.9. 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)

16.6.10. 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)

16.6.11. 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')

16.6.12. 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)
>>>
>>>
>>> mark = Hero('Mark', position_x=10, position_y=20)
>>> vars(mark)
{'name': 'Mark', 'position': (10, 20), 'health': 74}

16.6.13. 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)
>>>
>>>
>>> mark = Hero('Mark', health_min=50)
>>> vars(mark)
{'name': 'Mark', 'position': (0, 0), 'health': 74}

16.6.14. Assignments

# %% 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

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

# %% About
# - Name: OOP Init Hello
# - Difficulty: easy
# - Lines: 2
# - Minutes: 2

# %% 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

# %% Tests
"""
>>> 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)
{}
"""

# Define initialization method
# Method should print 'hello' at object creation
class User:
    ...

# %% 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

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

# %% About
# - Name: OOP Init Fields
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2

# %% 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

# %% Tests
"""
>>> 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}
"""

# Define initialization method
# Method should set `_authenticated` attribute to `False`
class User:
    ...

# %% 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

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

# %% About
# - Name: OOP Init Parameters
# - Difficulty: easy
# - Lines: 3
# - Minutes: 2

# %% 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

# %% Tests
"""
>>> 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__')

>>> mark = User('Mark', 'Watney')
>>> vars(mark)
{'firstname': 'Mark', 'lastname': 'Watney'}

>>> mark = User(firstname='Melissa', lastname='Lewis')
>>> vars(mark)
{'firstname': 'Melissa', 'lastname': 'Lewis'}
"""

# 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
class User:
    ...