8.2. Adapter

8.2.1. Rationale

• EN: Adapter

• PL: Adapter

• Type: class and object

8.2.2. Use Cases

• Convert an interface of an object to a different form

• Like power socket adapter for US and EU

• Refactoring of a large application

• Working with legacy code / database

8.2.3. Problem

• BlackAndWhite3rdPartyFilter is from external library

• Does not conform to Filter interface

• Do not have apply() method

• Need manual call of init() at initialization

• Need manual call of render()

from abc import ABCMeta, abstractmethod
from dataclasses import dataclass


class Image:
    pass


class Filter(metaclass=ABCMeta):
    @abstractmethod
    def apply(self, image: Image) -> None:
        pass


class VividFilter(Filter):
    def apply(self, image: Image) -> None:
        print('Applying Vivid Filter')


class BlackAndWhite3rdPartyFilter:
    def init(self):
        """Required by 3rd party library"""

    def render(self, image: Image):
        print('Applying BlackAndWhite Filter')


@dataclass
class ImageView:
    __image: Image

    def apply(self, filter: Filter):
        filter.apply(self.__image)


if __name__ == '__main__':
    image_view = ImageView(Image())
    image_view.apply(BlackAndWhite3rdPartyFilter())
    # Traceback (most recent call last):
    # AttributeError: 'BlackAndWhite3rdPartyFilter' object has no attribute 'apply'

8.2.4. Design

8.2.5. Implementation

• Inheritance is simpler

• Composition is more flexible

• Favor Composition over Inheritance

Figure 8.1. Please mind, that on Picture there is a Caramel filter but in code BlackAndWhite3rdPartyFilter

Inheritance:

from abc import ABCMeta, abstractmethod
from dataclasses import dataclass


class Image:
    pass


class Filter(metaclass=ABCMeta):
    @abstractmethod
    def apply(self, image: Image) -> None:
        pass


class VividFilter(Filter):
    def apply(self, image: Image) -> None:
        print('Applying Vivid Filter')


class BlackAndWhite3rdPartyFilter:
    def init(self):
        """Required by 3rd party library"""

    def render(self, image: Image):
        print('Applying BlackAndWhite Filter')


@dataclass
class BlackAndWhiteAdapter:
    __filter: BlackAndWhite3rdPartyFilter

    def apply(self, image: Image) -> None:
        self.__filter.init()
        self.__filter.render(image)


@dataclass
class ImageView:
    __image: Image

    def apply(self, filter: Filter):
        filter.apply(self.__image)


if __name__ == '__main__':
    image_view = ImageView(Image())
    image_view.apply(BlackAndWhiteAdapter(BlackAndWhite3rdPartyFilter()))

Composition:

from abc import ABCMeta, abstractmethod
from dataclasses import dataclass


class Image:
    pass


class Filter(metaclass=ABCMeta):
    @abstractmethod
    def apply(self, image: Image) -> None:
        pass


class VividFilter(Filter):
    def apply(self, image: Image) -> None:
        print('Applying Vivid Filter')


class BlackAndWhite3rdPartyFilter:
    def init(self):
        """Required by 3rd party library"""

    def render(self, image: Image):
        print('Applying BlackAndWhite Filter')


@dataclass
class BlackAndWhiteAdapter(BlackAndWhite3rdPartyFilter, Filter):
    def apply(self, image: Image) -> None:
        self.init()
        self.render(image)


@dataclass
class ImageView:
    __image: Image

    def apply(self, filter: Filter):
        filter.apply(self.__image)


if __name__ == '__main__':
    image_view = ImageView(Image())
    image_view.apply(BlackAndWhiteAdapter())

8.2.6. Use Cases

>>> def otherrange(a, b, c):  # function with bad API
...     current = a
...     result = []
...     while current < b:
...         result.append(current)
...         current += c
...     return result
>>>
>>>
>>> def myrange(start, stop, step):  # adapter
...     return otherrange(a=start, b=stop, c=step)
>>>
>>>
>>> myrange(start=10, stop=20, step=2)
[10, 12, 14, 16, 18]

8.2.7. Assignments