7.2. Adapter

• EN: Adapter

• PL: Adapter

• Type: class and object

7.2.1. Pattern

• 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

• Niekompatybilne API dwóch systemów

• Wymagające różnych sposobów uwierzytelniania (OAuth2, BasicAuth)

• Tłumaczenie pomiędzy różnymi formatami danych (SOAP/XML, REST/JSON)

• Iteracyjne przepisywanie legacy systemu na nowy, ale tak, aby móc wciąż korzystać ze starego

7.2.2. 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 ABC, abstractmethod
from pathlib import Path


#%% 3rd Party
class AmazingFilter:
    def __init__(self):
        print('Setting up filter')

    def transform(self, content: bytes) -> bytes:
        print('Making your phot amazing')
        return content


#%% Abstract
class Filter(ABC):
    @abstractmethod
    def apply(self, content: bytes) -> bytes:
        ...


#%% Implementation
class SepiaFilter(Filter):
    def apply(self, content: bytes) -> bytes:
        print('Making photo in sepia')
        return content

class SharpenFilter(Filter):
    def apply(self, content: bytes) -> bytes:
        print('Making photo sharp')
        return content


#%% Main
class Image:
    path: Path
    content: bytes

    def __init__(self, path):
        self.path = Path(path)
        self.content = self.path.read_bytes()

    def apply(self, filter: Filter) -> None:
        self.content = filter.apply(self.content)


if __name__ == '__main__':
    img = Image('/tmp/myfile.png')
    img.apply(SepiaFilter())
    img.apply(SharpenFilter())

    img.apply(AmazingFilter())
    # AttributeError: 'AmazingFilter' object has no attribute 'apply'

7.2.3. Solution

• Inheritance is simpler

• Composition is more flexible

• Favor Composition over Inheritance

Figure 7.9. Please mind, that on Picture there is a Caramel filter but in code BlackAndWhite3rdPartyFilter

from abc import ABC, abstractmethod
from pathlib import Path


#%% 3rd Party
class AmazingFilter:
    def __init__(self):
        print('Setting up filter')

    def transform(self, content: bytes) -> bytes:
        print('Making your phot amazing')
        return content


#%% Abstract
class Filter(ABC):
    @abstractmethod
    def apply(self, content: bytes) -> bytes:
        ...


#%% Implementation
class SepiaFilter(Filter):
    def apply(self, content: bytes) -> bytes:
        print('Making photo in sepia')
        return content

class SharpenFilter(Filter):
    def apply(self, content: bytes) -> bytes:
        print('Making photo sharp')
        return content


# %% Adapter
class AmazingFilterAdapter(Filter):
    filter: AmazingFilter

    def __init__(self):
        self.filter = AmazingFilter()

    def apply(self, content: bytes) -> bytes:
        self.filter.__init__()
        return self.filter.transform(content)


#%% Main
class Image:
    path: Path
    content: bytes

    def __init__(self, path):
        self.path = Path(path)
        self.content = self.path.read_bytes()

    def apply(self, filter: Filter) -> None:
        self.content = filter.apply(self.content)


if __name__ == '__main__':
    img = Image('/tmp/myfile.png')
    img.apply(SepiaFilter())
    img.apply(SharpenFilter())
    img.apply(AmazingFilterAdapter())

7.2.4. Use Case - 0x01

>>> 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]

7.2.5. Assignments