CODESAMPLE
Multiton - Python
The Multiton pattern ensures that only a limited number of instances of a class exist. Unlike a Singleton which allows only one instance, a Multiton permits a predefined number. This is useful when managing a pool of resources or managing a small set of unique entities. The Python implementation uses a class-level dictionary to store instances, creating them on demand up to the specified maximum limit. The __new__ method is overridden to control instantiation. This is idiomatic Python as it leverages the language’s dynamic nature and uses dictionaries for efficient instance management, avoiding excessive class decoration or complex static initialization.
class Multiton:
"""
Multiton pattern implementation. Limits the number of instances of a class.
"""
_instances = {}
_max_instances = 3 # Define maximum number of instances
def __new__(cls, *args, **kwargs):
if len(cls._instances) < cls._max_instances:
instance = super().__new__(cls)
cls._instances[id(instance)] = instance
return instance
else:
# Return an existing instance if the limit is reached
for inst in cls._instances.values():
return inst
#Should not reach here, but handle case where no instances exist after exceeding limit.
return None
def __init__(self, name):
self.name = name
def __repr__(self):
return f"Multiton instance: {self.name}, id: {id(self)}"
# Example Usage:
if __name__ == "__main__":
instance1 = Multiton("Instance 1")
instance2 = Multiton("Instance 2")
instance3 = Multiton("Instance 3")
instance4 = Multiton("Instance 4") # Will return an existing instance
print(instance1)
print(instance2)
print(instance3)
print(instance4)
print(f"Number of instances: {len(Multiton._instances)}")