Master Python’s id() and Build a Singleton Pattern

Preface

Beyond the real world, the network world is bustling, with the CPU working nonstop. Two characters, c and d, represent Python objects instantiated from an Animal class.

# Define an Animal class
class Animal():
    pass

cat = Animal()  # c
dog = Animal()  # d

Memory Address

Using Python’s built‑in id() function, we can retrieve the memory address of an object, which is unique for each existing object.

# Define an Animal class
class Animal():
    pass

cat = Animal()
 dog = Animal()
print('c address:', id(cat))
print('d address:', id(dog))

The output shows different addresses for cat and dog, confirming that each instance occupies a distinct memory location.

Singleton Pattern

A singleton ensures a class has only one instance and provides a global access point. In Python, we can achieve this by overriding the __new__ method.

# Define an Animal class
class Animal():
    __instance = None

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = object.__new__(cls)
            return cls.__instance
        else:
            return cls.__instance

cat = Animal()
dog = Animal()
print('c address:', id(cat))
print('d address:', id(dog))

Both cat and dog now share the same memory address, demonstrating the singleton effect.

Singleton Pattern Effects

Advantages

Instance control – prevents creation of multiple copies, ensuring a single shared instance.

Flexibility – the class can modify the instantiation process as needed.

Disadvantages

Overhead – each request checks for an existing instance, which can add slight cost.

Potential confusion – developers must remember not to use new directly for singleton classes.

Object lifecycle – singletons may retain references, complicating memory management in some languages.

Choose the singleton pattern only when its benefits align with your design requirements.