Apprendre Singleton Pattern with Metaclasses | Metaclasses in Real-World Scenarios

The singleton pattern is a design approach that restricts a class to have only one instance throughout the program's lifetime. This is especially useful when you need a single shared resource, such as a configuration manager, database connection, or logging service. By ensuring that only one instance exists, you can avoid inconsistent state, reduce resource usage, and provide a single point of access to shared data.


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class SingletonMeta(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            instance = super().__call__(*args, **kwargs)
            cls._instances[cls] = instance
        return cls._instances[cls]


class Configuration(metaclass=SingletonMeta):
    def __init__(self, value):
        self.value = value


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config_a = Configuration("A")
config_b = Configuration("B")

print(config_a is config_b)  # True
print(config_a.value)        # A
print(config_b.value)        # A

While the singleton pattern can solve problems involving global state or shared resources, it comes with trade-offs.

Pros:

Guarantees that only one instance exists;
Provides a global point of access;
Can help manage shared resources efficiently.

Cons:

Makes unit testing harder due to global state;
Can hide dependencies and increase coupling;
May introduce unexpected behavior if mutable state is shared.

You should use singletons sparingly, and only when a single instance is truly necessary.

1. What does the singleton pattern guarantee?

2. How does a metaclass enforce singleton behavior?

3. Fill in the blank: The call method in a metaclass controls ____ creation.

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Section 3. Chapitre 3

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              12345678910111213
            
class SingletonMeta(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            instance = super().__call__(*args, **kwargs)
            cls._instances[cls] = instance
        return cls._instances[cls]


class Configuration(metaclass=SingletonMeta):
    def __init__(self, value):
        self.value = value


              123456
            
config_a = Configuration("A")
config_b = Configuration("B")

print(config_a is config_b)  # True
print(config_a.value)        # A
print(config_b.value)        # A