7.7. Proxy¶
EN: Proxy
PL: Pełnomocnik
Type: object
7.7.1. Pattern¶
Create a proxy, or agent for a remote object
Agent takes message and forwards to remote object
Proxy can log, authenticate or cache messages
7.7.2. Problem¶
Creating Ebook object is costly, because we have to read it from the disk and store it in memory
It will load all ebooks in our library, just to select one
from dataclasses import dataclass, field
@dataclass
class Ebook:
filename: str
def __post_init__(self):
self._load()
def _load(self) -> None:
print(f'Loading the ebook {self.filename}')
def show(self) -> None:
print(f'Showing the ebook {self.filename}')
def get_filename(self) -> None:
return self.filename
@dataclass
class Library:
ebooks: dict[str, Ebook] = field(default_factory=dict)
def add(self, ebook: Ebook) -> None:
self.ebooks[ebook.get_filename()] = ebook
def open(self, filename: str) -> None:
self.ebooks.get(filename).show()
if __name__ == '__main__':
library: Library = Library()
filenames: list[str] = ['ebook-a.pdf', 'ebook-b.pdf', 'ebook-c.pdf'] # Read from database
for filename in filenames:
library.add(Ebook(filename))
library.open('ebook-a.pdf')
# Loading the ebook ebook-a.pdf
# Loading the ebook ebook-b.pdf
# Loading the ebook ebook-c.pdf
# Showing the ebook ebook-a.pdf
7.7.3. Solution¶
Lazy evaluation
Open/Close Principle
from abc import ABC, abstractmethod
from dataclasses import dataclass, field
class Proxy:
pass
class Ebook(ABC):
@abstractmethod
def show(self) -> None:
pass
@abstractmethod
def get_filename(self) -> None:
pass
@dataclass
class RealEbook(Ebook):
filename: str
def __post_init__(self):
self.load()
def load(self) -> None:
print(f'Loading the ebook {self.filename}')
def show(self) -> None:
print(f'Showing the ebook {self.filename}')
def get_filename(self) -> None:
return self.filename
@dataclass
class EbookProxy(Ebook):
filename: str
ebook: RealEbook | None = None
def show(self) -> None:
if self.ebook is None:
self.ebook = RealEbook(self.filename)
self.ebook.show()
def get_filename(self) -> None:
return self.filename
@dataclass
class Library:
ebooks: dict[str, RealEbook] = field(default_factory=dict)
def add(self, ebook: RealEbook) -> None:
self.ebooks[ebook.get_filename()] = ebook
def open(self, filename: str) -> None:
self.ebooks.get(filename).show()
if __name__ == '__main__':
library: Library = Library()
filenames: list[str] = ['ebook-a.pdf', 'ebook-b.pdf', 'ebook-c.pdf'] # Read from database
for filename in filenames:
library.add(EbookProxy(filename))
library.open('ebook-a.pdf')
# Loading the ebook ebook-a.pdf
# Showing the ebook ebook-a.pdf
Proxy with Authorization and Logging:
from abc import ABC, abstractmethod
from dataclasses import dataclass, field
class Proxy:
pass
class Ebook(ABC):
@abstractmethod
def show(self) -> None:
pass
@abstractmethod
def get_filename(self) -> None:
pass
@dataclass
class RealEbook(Ebook):
filename: str
def __post_init__(self):
self.load()
def load(self) -> None:
print(f'Loading the ebook {self.filename}')
def show(self) -> None:
print(f'Showing the ebook {self.filename}')
def get_filename(self) -> None:
return self.filename
@dataclass
class EbookProxy(Ebook):
filename: str
ebook: RealEbook | None = None
def show(self) -> None:
if self.ebook is None:
self.ebook = RealEbook(self.filename)
self.ebook.show()
def get_filename(self) -> None:
return self.filename
@dataclass()
class LoggingEbookProxy(Ebook):
filename: str
ebook: RealEbook | None = None
def show(self) -> None:
if self.ebook is None:
self.ebook = RealEbook(self.filename)
print('Logging')
self.ebook.show()
def get_filename(self) -> None:
return self.filename
@dataclass
class Library:
ebooks: dict[str, RealEbook] = field(default_factory=dict)
def add(self, ebook: RealEbook) -> None:
self.ebooks[ebook.get_filename()] = ebook
def open(self, filename: str) -> None:
self.ebooks.get(filename).show()
if __name__ == '__main__':
library: Library = Library()
filenames: list[str] = ['ebook-a.pdf', 'ebook-b.pdf', 'ebook-c.pdf'] # Read from database
for filename in filenames:
library.add(LoggingEbookProxy(filename))
library.open('ebook-a.pdf')
# Loading the ebook ebook-a.pdf
# Logging
# Showing the ebook ebook-a.pdf