3.8. OOP Inheritance Patterns¶

no inheritance
single inheritance
multilevel inheritance
multiple inheritance (mixin classes)

single inheritance¶: One class inherits from one other class. Has one parent.
multilevel inheritance¶: One class inherits from other class, and yet another class inherits from it. This creates hierarchical structure.
multiple inheritance¶
mixin classes¶: One class derives from several other classes at once.

3.8.1. No Inheritance¶

>>> class Vehicle:
...     pass
>>>
>>>
>>> class Car:
...     pass

3.8.2. Single Inheritance¶

>>> class Vehicle:
...     pass
>>>
>>>
>>> class Car(Vehicle):
...     pass

3.8.3. Multilevel Inheritance¶

>>> class Vehicle:
...     pass
>>>
>>>
>>> class VehicleWithWindows(Vehicle):
...     pass
>>>
>>>
>>> class Car(VehicleWithWindows):
...     pass

3.8.4. Multiple Inheritance¶

HasEngine and HasWindows are Mixin Classes
Such classes are usually called: EngineMixin, WindowsMixin

>>> class Vehicle:
...     pass
>>>
>>> class HasEngine:
...     pass
>>>
>>> class HasWindows:
...     pass
>>>
>>>
>>> class Car(Vehicle, HasEngine, HasWindows):
...     pass
...

3.8.5. Composition¶

>>> class Vehicle:
...     pass
>>>
>>> class Engine:
...     pass
...
>>> class Windows:
...     pass
...
>>>
>>> class Car(Vehicle):
...     engine = Engine
...     windows = Windows

3.8.6. Aggregation¶

>>> class Vehicle:
...     pass
...
>>> class Engine:
...     pass
...
>>> class Windows:
...     pass
...
>>>
>>> class Car(Vehicle):
...     parts = [Engine, Windows]

3.8.7. Why Composition?¶

>>> class Mother:
...     pass
>>>
>>> class Father:
...     pass
>>>
>>>
>>> class Child:
...     mother: Mother
...     father: Father
...
...     def __init__(self, mother=Mother(), father=Father()):
...         self.mother = mother
...         self.father = father

>>> class StepFather:
...     pass
>>>
>>> me = Child(father=StepFather())

3.8.8. Use Case - 0x01¶

Following example is simple and easy to understand, but not totally accurate. Inheritance means, that a class is a specialized form of its base. This results in a subclass being an instance of a superclass. Which is weird when we think, that a Child might be its Parent in the same time.

No Inheritance:

>>> class Parent:
...     pass
>>>
>>>
>>> class Child:
...     pass

Single Inheritance:

>>> class Parent:
...     pass
>>>
>>>
>>> class Child(Parent):
...     pass

Multilevel Inheritance:

>>> class Grandparent:
...     pass
>>>
>>> class Parent(Grandparent):
...     pass
>>>
>>>
>>> class Child(Parent):
...     pass

Multiple Inheritance:

>>> class Mother:
...     pass
>>>
>>> class Father:
...     pass
>>>
>>>
>>> class Child(Mother, Father):
...     pass

Composition:

>>> class Mother:
...     pass
>>>
>>> class Father:
...     pass
>>>
>>> class Child:
...     mother = Mother
...     father = Father

Aggregation:

>>> class Mother:
...     pass
>>>
>>> class Father:
...     pass
>>>
>>> class Child:
...     parents = [Father, Mother]

3.8.9. Use Case - 0x02¶

>>> class Mother:
...     pass
>>>
>>> class Father:
...     pass
>>>
>>>
>>> class Child:
...     mother: Mother
...     father: Father
...
...     def __init__(self, mother=Mother(), father=Father()):
...         self.mother = mother
...         self.father = father

3.8.10. Use Case - 0x03¶

>>> class Vehicle:
...     engine: Engine
...     windows: Windows | None
>>>
>>> class Engine:
...     def engine_start(self): ...
...     def engine_stop(self): ...
...
>>> class Windows:
...     def window_open(self): ...
...     def window_close(self): ...
...
>>>
>>> class Car(Vehicle):
...     engine: Engine
...     windows: Windows
...
...     def __init__(self, windows=Windows(), engine=Engine()):
...         self.windows = windows
...         self.engine = engine
...
...     def engine_start(self):
...         if self.engine:
...             return self.engine.engine_start()
...
...     def engine_stop(self):
...         if self.engine:
...             return self.engine.engine_stop()
...
...     def window_open(self):
...         if self.windows:
...             return self.windows.windows_open()
...
...     def window_close(self):
...         if self.windows:
...             return self.windows.windows_close()

3.8.11. Use Case - 0x04¶

>>> class Encoder:
...     def encode(self, data):
...         ...
>>>
>>> class Decoder:
...     def decode(self, data):
...         ...
>>>
>>>
>>> class JSONSerializer:
...     encoder: Encoder
...     decoder: Decoder
...
...     def __init__(self,
...                  encoder: Encoder = Encoder(),
...                  decoder: Decoder = Decoder(),
...                  ) -> None:
...         self.encoder = encoder
...         self.decoder = decoder
...
...     def encode(self, data):
...        return self.encoder.encode(data)
...
...     def decode(self, data):
...         return self.decoder.decode(data)
>>>
>>>
>>> DATA = {'firstname': 'Mark', 'lastname': 'Watney'}

Now, if you want to serialize your data, just create an instance and call method .encode() on it.

>>> json = JSONSerializer()
>>> result = json.encode(DATA)

If you want to use your better version of encoder (for example which can encode datetime object. You can create a class which inherits from default Encoder and overwrite .encode() method.

>>> class MyBetterEncoder(Encoder):
...     def encode(self):
...         ...
>>>
>>> json = JSONSerializer(encoder=MyBetterEncoder)
>>> result = json.encode(DATA)

3.8.12. Use Case - 0x05¶

>>> from datetime import date
>>> import json

>>> DATA = {'firstname': 'Mark', 'lastname': 'Watney'}
>>>
>>> json.dumps(DATA)
'{"firstname": "Mark", "lastname": "Watney"}'

>>> DATA = {'firstname': 'Mark', 'lastname': 'Watney', 'birthdate': date(1969, 7, 21)}
>>>
>>> json.dumps(DATA)
Traceback (most recent call last):
TypeError: Object of type date is not JSON serializable

>>> class Encoder(json.JSONEncoder):
...     def default(self, x):
...         if isinstance(x, date):
...             return x.isoformat()
...
>>>
>>> DATA = {'firstname': 'Mark', 'lastname': 'Watney', 'birthdate': date(1969, 7, 21)}
>>>
>>> json.dumps(DATA, cls=Encoder)
'{"firstname": "Mark", "lastname": "Watney", "birthdate": "1969-07-21"}'

3.8.13. Further Reading¶

3.8.14. Assignments¶

Code 3.64. Solution¶

"""
* Assignment: OOP InheritancePatterns Simple
* Type: class assignment
* Complexity: easy
* Lines of code: 4 lines
* Time: 3 min

English:
    1. Create class `Account`
    2. Create class `User` which inherits from `Account`
    3. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Account`
    2. Stwórz klasę `User`, która dziedziczy po `Account`
    3. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isclass

    >>> assert isclass(Account)
    >>> assert isclass(User)
    >>> assert issubclass(User, Account)
"""

Code 3.65. Solution¶

"""
* Assignment: OOP InheritancePatterns NoInheritance
* Complexity: easy
* Lines of code: 8 lines
* Time: 3 min

English:
    1. Create classes `MyAccount`, `Account`, `User`,  `Admin`
    2. Do not use inheritance
    3. Assignment demonstrates syntax, so do not add any attributes and methods
    4. Run doctests - all must succeed

Polish:
    1. Stwórz klasy `MyAccount`, `Account`, `User`,  `Admin`
    2. Nie używaj dziedziczenia
    3. Zadanie demonstruje składnię, nie dodawaj żadnych atrybutów i metod
    4. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isclass

    >>> assert isclass(Account)
    >>> assert isclass(User)
    >>> assert isclass(Admin)
    >>> assert isclass(MyAccount)
"""

Code 3.66. Solution¶

"""
* Assignment: OOP InheritancePatterns Multilevel
* Complexity: easy
* Lines of code: 8 lines
* Time: 3 min

English:
    1. Create class `MyAccount` from classes `Account`, `User`, `Admin`
    2. Use multilevel inheritance
    3. Assignment demonstrates syntax, so do not add any attributes and methods
    4. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `MyAccount` z klas `Account`, `User`, `Admin`
    2. Użyj wielopoziomowego dziedziczenia
    3. Zadanie demonstruje składnię, nie dodawaj żadnych atrybutów i metod
    4. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isclass

    >>> assert isclass(Account)
    >>> assert isclass(User)
    >>> assert isclass(Admin)
    >>> assert isclass(MyAccount)
    >>> assert issubclass(MyAccount, Account)
    >>> assert issubclass(MyAccount, User)
    >>> assert issubclass(MyAccount, Admin)

    >>> assert issubclass(Account, object)
    >>> assert issubclass(User, Account)
    >>> assert issubclass(Admin, User)
    >>> assert issubclass(MyAccount, Admin)

    >>> assert len(Account.__subclasses__()) == 1
    >>> assert len(User.__subclasses__()) == 1
    >>> assert len(Admin.__subclasses__()) == 1
    >>> assert len(MyAccount.__subclasses__()) == 0
"""

Code 3.67. Solution¶

"""
* Assignment: OOP InheritancePatterns Multiple
* Complexity: easy
* Lines of code: 8 lines
* Time: 3 min

English:
    1. Create class `MyAccount` from classes `Account`, `User`, `Admin`
    2. Use mixins classes
    3. You can modify given classes
    4. Assignment demonstrates syntax, so do not add any attributes and methods
    5. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `MyAccount` z klas `Account`, `User`, `Admin`
    2. Użyj klas domieszkowych (mixin)
    3. Możesz modyfikować dane klasy
    4. Zadanie demonstruje składnię, nie dodawaj żadnych atrybutów i metod
    5. Uruchom doctesty - wszystkie muszą się powieść

Tests:
    >>> import sys; sys.tracebacklimit = 0
    >>> from inspect import isclass

    >>> assert isclass(Account)
    >>> assert isclass(User)
    >>> assert isclass(Admin)
    >>> assert isclass(MyAccount)
    >>> assert issubclass(MyAccount, Account)
    >>> assert issubclass(MyAccount, User)
    >>> assert issubclass(MyAccount, Admin)

    >>> assert len(Account.__subclasses__()) == 1
    >>> assert len(User.__subclasses__()) == 1
    >>> assert len(Admin.__subclasses__()) == 1
    >>> assert len(MyAccount.__subclasses__()) == 0
"""