6.14. OOP Abstract Interface

• Python don't have interfaces, although you can achieve similar effect

• Since Python 3.8 there are Protocols, which effectively are interfaces

• Interfaces cannot be instantiated

• Interfaces can be implemented

• Implemented class must define all interface methods (implement interface)

• Only public method declaration

interface

Software entity with public methods and attribute declaration

implement

Class implements interface if has all public fields and methods from interface

How do you specify and enforce an interface spec in Python?

An interface specification for a module as provided by languages such as C++ and Java describes the prototypes for the methods and functions of the module. Many feel that compile-time enforcement of interface specifications helps in the construction of large programs.

Python 3.0 adds an abc module that lets you define Abstract Base Classes (ABCs). You can then use isinstance() and issubclass() to check whether an instance or a class implements a particular ABC. The collections.abc module defines a set of useful abstract base classes such as Iterable, Container, and MutableMapping.

For Python, many of the advantages of interface specifications can be obtained by an appropriate test discipline for components.

A good test suite for a module can both provide a regression test and serve as a module interface specification and a set of examples. Many Python modules can be run as a script to provide a simple "self test". Even modules which use complex external interfaces can often be tested in isolation using trivial "stub" emulations of the external interface. The doctest and unittest modules or third-party test frameworks can be used to construct exhaustive test suites that exercise every line of code in a module.

An appropriate testing discipline can help build large complex applications in Python as well as having interface specifications would. In fact, it can be better because an interface specification cannot test certain properties of a program. For example, the append() method is expected to add new elements to the end of some internal list; an interface specification cannot test that your append() implementation will actually do this correctly, but it's trivial to check this property in a test suite.

Writing test suites is very helpful, and you might want to design your code to make it easily tested. One increasingly popular technique, test-driven development, calls for writing parts of the test suite first, before you write any of the actual code. Of course Python allows you to be sloppy and not write test cases at all.

Note

Source 2

6.14.1. Problem

>>> class DatabaseCache:
...     def insert(self, key, value): ...
...     def select(self, key): ...
...     def delete(self): ...
>>>
>>>
>>> class LocmemCache:
...     def store(self, value, key): ...
...     def retrieve(self, key): ...
...     def purge(self): ...
>>>
>>>
>>> class FilesystemCache:
...     def write(self, key, value): ...
...     def read(self, key): ...
...     def remove(self): ...

Each of those classes has different names for methods which eventually does the same job. This is lack of consistency and common interface:

>>> cache = DatabaseCache()
>>> cache.insert('firstname', 'Mark')
>>> cache.insert('lastname', 'Watney')
>>> cache.select('firstname')
>>> cache.select('lastname')
>>> cache.delete()

>>> cache = LocmemCache()
>>> cache.store('firstname', 'Mark')
>>> cache.store('lastname', 'Watney')
>>> cache.retrieve('firstname')
>>> cache.retrieve('lastname')
>>> cache.purge()

>>> cache = FilesystemCache()
>>> cache.write('firstname', 'Mark')
>>> cache.write('lastname', 'Watney')
>>> cache.read('firstname')
>>> cache.read('lastname')
>>> cache.remove()

6.14.2. Solution

• S.O.L.I.D.

• DIP - Dependency Inversion Principle

• Always depend on an abstraction not con

The principle states: High-level modules should not import anything from low-level modules. Both should depend on abstractions (e.g., interfaces). Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.

—SOLID, Dependency Inversion Principle, Robert C. Martin

>>> class ICache:
...     def set(self, key: str, value: str) -> None: raise NotImplementedError
...     def get(self, key: str) -> str: raise NotImplementedError
...     def clear(self) -> None: raise NotImplementedError
>>>
>>>
>>> class DatabaseCache(ICache):
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def clear(self) -> None: ...
>>>
>>>
>>> class FilesystemCache(ICache):
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def clear(self) -> None: ...
>>>
>>>
>>> class LocmemCache(ICache):
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def clear(self) -> None: ...

>>> cache: ICache = DatabaseCache()
>>> cache.set('firstname', 'Mark')
>>> cache.set('lastname', 'Watney')
>>> cache.get('firstname')
>>> cache.get('lastname')
>>> cache.clear()

>>> cache: ICache = FilesystemCache()
>>> cache.set('firstname', 'Mark')
>>> cache.set('lastname', 'Watney')
>>> cache.get('firstname')
>>> cache.get('lastname')
>>> cache.clear()

>>> cache: ICache = LocmemCache()
>>> cache.set('firstname', 'Mark')
>>> cache.set('lastname', 'Watney')
>>> cache.get('firstname')
>>> cache.get('lastname')
>>> cache.clear()

6.14.3. Interface Names

• Cache

• CacheInterface

• CacheIface

• ICache

>>> class Cache:
...     ...

>>> class CacheInterface:
...     ...

>>> class CacheIface:
...     ...

>>> class ICache:
...     ...

6.14.4. Alternative Notation

>>> class ICache:
...     def set(self, key: str, value: str) -> None:
...         raise NotImplementedError
...
...     def get(self, key: str) -> str:
...         raise NotImplementedError
...
...     def clear(self) -> None:
...         raise NotImplementedError

Interfaces do not have any implementation, so you can write them as one-liners. It is a bit more easier to read. You will also focus more on method names and attribute types.

>>> class ICache:
...     def set(self, key: str, value: str) -> None: raise NotImplementedError
...     def get(self, key: str) -> str: raise NotImplementedError
...     def clear(self) -> None: raise NotImplementedError

Sometimes you may get a shorter code, but it will not raise an error in case of implementing class do not cover the name.

>>> class ICache:
...     def set(self, key: str, value: str) -> None: pass
...     def get(self, key: str) -> str: pass
...     def clear(self) -> None: pass

As of three dots (...) is a valid Python object (Ellipsis) you can write that:

>>> class ICache:
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def clear(self) -> None: ...

6.14.5. Not Existing Notation

• This currently does not exists in Python

• In fact it is not even a valid Python syntax

• But it could greatly improve readability

How nice it would be to write:

>>> @interface 
... class Cache:
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def is_valid(self, key: str) -> bool: ...

>>> class Cache(interface=True): 
...     def set(self, key: str, value: str) -> None: ...
...     def get(self, key: str) -> str: ...
...     def is_valid(self, key: str) -> bool: ...

>>> interface Cache: 
...     def set(self, key: str, value: str) -> None
...     def get(self, key: str) -> str
...     def is_valid(self, key: str) -> bool

6.14.6. Use Case - 0x01

>>> class Account:
...     def login(self, username: str, password: str) -> None: ...
...     def logout(self) -> None: ...
>>>
>>>
>>> class Guest(Account):
...      ...
>>>
>>> class User(Account):
...      ...
>>>
>>> class Admin(Account):
...      ...

6.14.7. Use Case - 0x02

• Cache

File cache_iface.py:

>>> class ICache:
...     def get(self, key: str) -> str:
...         raise NotImplementedError
...
...     def set(self, key: str, value: str) -> None:
...         raise NotImplementedError
...
...     def clear(self) -> None:
...         raise NotImplementedError

File cache_impl.py:

>>> class DatabaseCache(ICache):
...     def get(self, key: str) -> str:
...         ...
...
...     def set(self, key: str, value: str) -> None:
...         ...
...
...     def clear(self) -> None:
...         ...
>>>
>>>
>>> class LocmemCache(ICache):
...     def get(self, key: str) -> str:
...         ...
...
...     def set(self, key: str, value: str) -> None:
...         ...
...
...     def clear(self) -> None:
...         ...
>>>
>>>
>>> class FilesystemCache(ICache):
...     def get(self, key: str) -> str:
...         ...
...
...     def set(self, key: str, value: str) -> None:
...         ...
...
...     def clear(self) -> None:
...         ...

File settings.py

>>> from myapp.cache_iface import ICache  
>>> from myapp.cache_impl import DatabaseCache  
>>> from myapp.cache_impl import LocmemCache  
>>> from myapp.cache_impl import FilesystemCache  
>>>
>>>
>>> DefaultCache = LocmemCache

File myapp.py:

>>> from myapp.settings import DefaultCache, ICache  
>>>
>>>
>>> cache: ICache = DefaultCache()
>>> cache.set('firstname', 'Mark')
>>> cache.set('lastname', 'Watney')
>>> cache.get('firstname')
>>> cache.get('lastname')
>>> cache.clear()

Note, that myapp doesn't know which cache is being used. It only depends on configuration in settings file.

6.14.8. Use Case - 0x03

Figure 6.4. GIMP (GNU Image Manipulation Project) window with tools and canvas 1

Interface definition with all event handler specification:

>>> class ITool:
...     def on_mouse_over(self): raise NotImplementedError
...     def on_mouse_out(self): raise NotImplementedError
...     def on_mouse_leftbutton(self): raise NotImplementedError
...     def on_mouse_rightbutton(self): raise NotImplementedError
...     def on_key_press(self): raise NotImplementedError
...     def on_key_unpress(self): raise NotImplementedError

Implementation:

>>> class Pencil(ITool):
...     def on_mouse_over(self): ...
...     def on_mouse_out(self): ...
...     def on_mouse_leftbutton(self): ...
...     def on_mouse_rightbutton(self): ...
...     def on_key_press(self): ...
...     def on_key_unpress(self): ...
>>>
>>>
>>> class Pen(ITool):
...     def on_mouse_over(self): ...
...     def on_mouse_out(self): ...
...     def on_mouse_leftbutton(self): ...
...     def on_mouse_rightbutton(self): ...
...     def on_key_press(self): ...
...     def on_key_unpress(self): ...
>>>
>>>
>>> class Brush(ITool):
...     def on_mouse_over(self): ...
...     def on_mouse_out(self): ...
...     def on_mouse_leftbutton(self): ...
...     def on_mouse_rightbutton(self): ...
...     def on_key_press(self): ...
...     def on_key_unpress(self): ...
>>>
>>>
>>> class Eraser(ITool):
...     def on_mouse_over(self): ...
...     def on_mouse_out(self): ...
...     def on_mouse_leftbutton(self): ...
...     def on_mouse_rightbutton(self): ...
...     def on_key_press(self): ...
...     def on_key_unpress(self): ...

6.14.9. References

1

Download GIMP. Year: 2022. Retrieved: 2022-08-11. URL: https://anderbot.com/wp-content/uploads/2020/10/GIMP5.jpg

2

van Rossum, G. et al. How do you specify and enforce an interface spec in Python? Year: 2022. Retrieved: 2022-09-25. URL: https://docs.python.org/3/faq/design.html#how-do-you-specify-and-enforce-an-interface-spec-in-python

6.14.10. Assignments

Code 6.21. Solution

"""
* Assignment: OOP AbstractInterface Define
* Complexity: easy
* Lines of code: 13 lines
* Time: 8 min

English:
    1. Define interface `IrisInterface`
    2. Attributes: `sepal_length, sepal_width, petal_length, petal_width`
    3. Methods: `sum()`, `len()`, `mean()` in `IrisInterface`
    4. All methods and constructor must raise exception `NotImplementedError`
    5. Run doctests - all must succeed

Polish:
    1. Zdefiniuj interfejs `IrisInterface`
    2. Attributes: `sepal_length, sepal_width, petal_length, petal_width`
    3. Metody: `sum()`, `len()`, `mean()` w `IrisInterface`
    4. Wszystkie metody oraz konstruktor muszą podnosić wyjątek `NotImplementedError`
    5. Uruchom doctesty - wszystkie muszą się powieść

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

    >>> assert hasattr(IrisInterface, 'mean')
    >>> assert hasattr(IrisInterface, 'sum')
    >>> assert hasattr(IrisInterface, 'len')

    >>> assert isfunction(IrisInterface.mean)
    >>> assert isfunction(IrisInterface.sum)
    >>> assert isfunction(IrisInterface.len)

    >>> IrisInterface.__annotations__  # doctest: +NORMALIZE_WHITESPACE
    {'sepal_length': <class 'float'>,
     'sepal_width': <class 'float'>,
     'petal_length': <class 'float'>,
     'petal_width': <class 'float'>}

    >>> iris = IrisInterface(5.8, 2.7, 5.1, 1.9)
    Traceback (most recent call last):
    NotImplementedError
"""

Code 6.22. Solution

"""
* Assignment: OOP AbstractInterface Implement
* Complexity: easy
* Lines of code: 12 lines
* Time: 5 min

English:
    1. Define class `Setosa` implementing `IrisInterface`
    2. Implement methods
    3. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Setosa` implementującą `IrisInterface`
    2. Zaimplementuj metody
    3. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * `vars(self).values()`
    * `mean = sum() / len()`

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

    >>> assert issubclass(Setosa, IrisInterface)
    >>> assert hasattr(Setosa, 'mean')
    >>> assert hasattr(Setosa, 'sum')
    >>> assert hasattr(Setosa, 'len')

    >>> assert isfunction(Setosa.mean)
    >>> assert isfunction(Setosa.sum)
    >>> assert isfunction(Setosa.len)

    >>> Setosa.__annotations__  # doctest: +NORMALIZE_WHITESPACE
    {'sepal_length': <class 'float'>,
     'sepal_width': <class 'float'>,
     'petal_length': <class 'float'>,
     'petal_width': <class 'float'>}

    >>> setosa = Setosa(5.1, 3.5, 1.4, 0.2)
    >>> setosa.len()
    4
    >>> setosa.sum()
    10.2
    >>> setosa.mean()
    2.55
"""

class IrisInterface:
    sepal_length: float
    sepal_width: float
    petal_length: float
    petal_width: float

    def __init__(self,
                 sepal_length: float,
                 sepal_width: float,
                 petal_length: float,
                 petal_width: float) -> None:
        raise NotImplementedError

    def mean(self) -> float:
        raise NotImplementedError

    def sum(self) -> float:
        raise NotImplementedError

    def len(self) -> int:
        raise NotImplementedError

Code 6.23. Solution

"""
* Assignment: OOP AbstractInterface Values
* Complexity: easy
* Lines of code: 12 lines
* Time: 8 min

English:
    1. Define class `Setosa` implementing `IrisInterface`
    2. Implement methods
    3. Note, that attribute `species` is a `str`, and in Python you cannot add `str` and `float`
    4. Create protected method `_get_values()` which returns values of `int` and `float` type attibutes
    5. Why this method is not in interface?
    6. Run doctests - all must succeed

Polish:
    1. Stwórz klasę `Setosa` implementującą `IrisInterface`
    2. Zaimplementuj metody
    3. Zwróć uwagę, że atrybut `species` jest `str`, a Python nie można dodawać `str` i `float`
    4. Stwórz metodę chronioną `_get_values()`, która zwraca wartości atrybutów typu `int` i `float`
    5. Dlaczego ta metoda nie jest w interfejsie?
    6. Uruchom doctesty - wszystkie muszą się powieść

Hints:
    * `var(self).values()`
    * `instanceof()` or `type()`
    * `mean = sum() / len()`

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

    >>> assert issubclass(Setosa, IrisInterface)
    >>> assert hasattr(Setosa, 'mean')
    >>> assert hasattr(Setosa, 'sum')
    >>> assert hasattr(Setosa, 'len')

    >>> assert isfunction(Setosa.mean)
    >>> assert isfunction(Setosa.sum)
    >>> assert isfunction(Setosa.len)

    >>> Setosa.__annotations__  # doctest: +NORMALIZE_WHITESPACE
    {'sepal_length': <class 'float'>,
     'sepal_width': <class 'float'>,
     'petal_length': <class 'float'>,
     'petal_width': <class 'float'>,
     'species': <class 'str'>}

    >>> setosa = Setosa(5.1, 3.5, 1.4, 0.2, 'setosa')
    >>> setosa.len()
    4
    >>> setosa.sum()
    10.2
    >>> setosa.mean()
    2.55
"""

class IrisInterface:
    sepal_length: float
    sepal_width: float
    petal_length: float
    petal_width: float
    species: str

    def __init__(self,
                 sepal_length: float,
                 sepal_width: float,
                 petal_length: float,
                 petal_width: float,
                 species: str) -> None:
        raise NotImplementedError

    def mean(self) -> float:
        raise NotImplementedError

    def sum(self) -> float:
        raise NotImplementedError

    def len(self) -> int:
        raise NotImplementedError