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Singleton

Keywords: share resources, single instance, global access

Introduction

In a coffee shop, there is only one cashier. Why?

  • If there were multiple cashiers, they would need extra space, which might not be efficient.
  • Having a single cashier helps maintain consistency in processing orders and payments.
  • It ensures that resources like the cash register are used optimally.

This setup mirrors the Singleton pattern in software design, where a single instance of a class is used to manage shared resources.

Problem Statement

Imagine you and I walk into a coffee shop, and every time we order, we have to deal with a different cashier. That would be confusing, right?

  • We might get inconsistent service.
  • Payments could be processed differently each time.
  • The shop would need extra resources to manage multiple cashiers.

Similarly, in programming, if multiple instances of a class manage the same resource, it can lead to unexpected behaviors and inefficiencies.

Solution

The Singleton pattern ensures that a class has only one instance and provides a global access point to it. Just like how you and I interact with a single cashier in a coffee shop, this pattern ensures that every part of our program interacts with the same instance of a class.

Benefits

  • Saves resources by avoiding multiple instances.
  • Ensures consistency in managing a shared resource.
  • Provides a controlled way to access that resource.

Implementation

Diagram

graph LR;
    A[Customer] -->|Orders| B(Cashier - Singleton);
    A2[Customer] -->|Orders| B;
    A3[Customer] -->|Orders| B;
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Theoretical Explanation

  • The Singleton class restricts instantiation so that only one object exists.
  • It provides a static method to get that single instance.
  • This ensures that any part of the program interacting with it is using the same instance.

Code Implementation

class CashierMeta:
    _name: str

    def __new__(cls, name):
        if not hasattr(cls, "instance"):
            cls.instance = super(CashierMeta, cls).__new__(cls)
            cls.instance._name = name

        return cls.instance


class Cashier(CashierMeta):
    def __repr__(self):
        return f"Cashier name is {self._name}"


if __name__ == "__main__":
    cashier_name = "Minh Le Duc 00"
    cashier_00 = Cashier(cashier_name)
    print(cashier_00)  # Cashier name is Minh Le Duc 00

    # Create another instance
    cashier_01 = Cashier(cashier_name)
    if cashier_00 == cashier_01:
        print("Both cashiers share the same instance!!!")

Conclusion

Just like a coffee shop benefits from having a single cashier to manage all transactions efficiently, the Singleton pattern ensures that a single instance is used to manage shared resources in a program. This helps in saving memory, ensuring consistency, and improving maintainability.