Flyweight in Python
Flyweight is a structural design pattern that allows programs to support vast quantities of objects by keeping their memory consumption low.
The pattern achieves it by sharing parts of object state between multiple objects. In other words, the Flyweight saves RAM by caching the same data used by different objects.
Complexity:
Popularity:
Usage examples: The Flyweight pattern has a single purpose: minimizing memory intake. If your program doesn’t struggle with a shortage of RAM, then you might just ignore this pattern for a while.
Identification: Flyweight can be recognized by a creation method that returns cached objects instead of creating new.
Conceptual Example
This example illustrates the structure of the Flyweight design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.py: Conceptual example
import json from typing import Dict class Flyweight(): """ The Flyweight stores a common portion of the state (also called intrinsic state) that belongs to multiple real business entities. The Flyweight accepts the rest of the state (extrinsic state, unique for each entity) via its method parameters. """ def __init__(self, shared_state: str) -> None: self._shared_state = shared_state def operation(self, unique_state: str) -> None: s = json.dumps(self._shared_state) u = json.dumps(unique_state) print(f"Flyweight: Displaying shared ({s}) and unique ({u}) state.", end="") class FlyweightFactory(): """ The Flyweight Factory creates and manages the Flyweight objects. It ensures that flyweights are shared correctly. When the client requests a flyweight, the factory either returns an existing instance or creates a new one, if it doesn't exist yet. """ _flyweights: Dict[str, Flyweight] = {} def __init__(self, initial_flyweights: Dict) -> None: for state in initial_flyweights: self._flyweights[self.get_key(state)] = Flyweight(state) def get_key(self, state: Dict) -> str: """ Returns a Flyweight's string hash for a given state. """ return "_".join(sorted(state)) def get_flyweight(self, shared_state: Dict) -> Flyweight: """ Returns an existing Flyweight with a given state or creates a new one. """ key = self.get_key(shared_state) if not self._flyweights.get(key): print("FlyweightFactory: Can't find a flyweight, creating new one.") self._flyweights[key] = Flyweight(shared_state) else: print("FlyweightFactory: Reusing existing flyweight.") return self._flyweights[key] def list_flyweights(self) -> None: count = len(self._flyweights) print(f"FlyweightFactory: I have {count} flyweights:") print("\n".join(map(str, self._flyweights.keys())), end="") def add_car_to_police_database( factory: FlyweightFactory, plates: str, owner: str, brand: str, model: str, color: str ) -> None: print("\n\nClient: Adding a car to database.") flyweight = factory.get_flyweight([brand, model, color]) # The client code either stores or calculates extrinsic state and passes it # to the flyweight's methods. flyweight.operation([plates, owner]) if __name__ == "__main__": """ The client code usually creates a bunch of pre-populated flyweights in the initialization stage of the application. """ factory = FlyweightFactory([ ["Chevrolet", "Camaro2018", "pink"], ["Mercedes Benz", "C300", "black"], ["Mercedes Benz", "C500", "red"], ["BMW", "M5", "red"], ["BMW", "X6", "white"], ]) factory.list_flyweights() add_car_to_police_database( factory, "CL234IR", "James Doe", "BMW", "M5", "red") add_car_to_police_database( factory, "CL234IR", "James Doe", "BMW", "X1", "red") print("\n") factory.list_flyweights() Output.txt: Execution result
FlyweightFactory: I have 5 flyweights: Camaro2018_Chevrolet_pink C300_Mercedes Benz_black C500_Mercedes Benz_red BMW_M5_red BMW_X6_white Client: Adding a car to database. FlyweightFactory: Reusing existing flyweight. Flyweight: Displaying shared (["BMW", "M5", "red"]) and unique (["CL234IR", "James Doe"]) state. Client: Adding a car to database. FlyweightFactory: Can't find a flyweight, creating new one. Flyweight: Displaying shared (["BMW", "X1", "red"]) and unique (["CL234IR", "James Doe"]) state. FlyweightFactory: I have 6 flyweights: Camaro2018_Chevrolet_pink C300_Mercedes Benz_black C500_Mercedes Benz_red BMW_M5_red BMW_X6_white BMW_X1_red 
