Python Singleton Metaclass: Ensure One Instance

Python Metaprogramming: Exercise-3 with Solution

Singleton Metaclass:

Write a Python program to create a metaclass SingletonMeta that ensures a class only has one instance (singleton pattern).

Sample Solution:

Python Code :

# Define a metaclass SingletonMeta
class SingletonMeta(type):
    # Dictionary to store instances of classes
    _instances = {}

    # Override the __call__ method of the metaclass
    def __call__(cls, *args, **kwargs):
        # Check if the class is not already instantiated
        if cls not in cls._instances:
            # If not, create a new instance and store it in _instances dictionary
            cls._instances[cls] = super().__call__(*args, **kwargs)
        # Return the existing instance if already instantiated
        return cls._instances[cls]

# Create a class SingletonClass using SingletonMeta as its metaclass
class SingletonClass(metaclass=SingletonMeta):
    pass

# Test the singleton class
# Create two instances of SingletonClass
instance1 = SingletonClass()
instance2 = SingletonClass()
# Check if both instances refer to the same object
print(instance1 is instance2)  # True 

Output:

True

Explanation:

• SingletonMeta Metaclass:
• It defines a metaclass named "SingletonMeta".
• It maintains a dictionary '_instances' to store instances of classes created using this metaclass.
• call Method:
• This method is called whenever an instance of a class with "SingletonMeta" as its metaclass is created.
• It checks if an instance of the class already exists in '_instances'.
• If not, it creates a new instance using super().__call__(*args, **kwargs) and stores it in '_instances'.
• It returns the existing instance if it's already instantiated.
• SingletonClass:
• It's a class that uses 'SingletonMeta' as its metaclass.
• As a result, instances of 'SingletonClass' will be singletons, meaning only one instance will exist throughout the program.
• Testing SingletonClass:
• Two instances ('instance1' and 'instance2') of "SingletonClass" are created.
• The 'is' operator checks if both instances refer to the same object.
• Since the Singleton pattern ensures that only one instance exists, 'instance1 is instance2' returns 'True'.

Python Code Editor :

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