Poids mouche en Python
Le poids mouche est un patron de conception structurel qui permet à des programmes de limiter leur consommation de mémoire malgré un très grand nombre d’objets.
Ce patron est obtenu en partageant des parties de l’état d’un objet à plusieurs autres objets. En d’autres termes, le poids mouche économise de la RAM en mettant en cache les données identiques chez différents objets.
Complexité :
Popularité :
Exemples d’utilisation : Le poids mouche n’a qu’une seule utilité : minimiser l’utilisation de la mémoire. Si votre programme ne rencontre aucun problème de RAM, ignorez ce patron pour le moment.
Identification : Le poids mouche peut être reconnu par une méthode de création qui renvoie des objets du cache plutôt que d’en créer de nouveaux.
Exemple conceptuel
Dans cet exemple, nous allons voir la structure du Poids mouche. Nous allons répondre aux questions suivantes :
- Que contiennent les classes ?
- Quels rôles jouent-elles ?
- Comment les éléments du patron sont-ils reliés ?
main.py: Exemple conceptuel
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: Résultat de l’exécution
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
