Extension: Singleton Pattern

A design pattern is a reusable solution to a common programming problem. The Singleton pattern ensures a class has exactly one instanceand provides a global point of access to it.

When to Use a Singleton

  • Application configuration (one config object for the whole app)
  • Database connection pool (one shared connection manager)
  • Logging service (one log file writer)
  • Hardware interfaces (one printer spooler, one GPU context)

The Problem Without Singleton

config1 = Config()
config2 = Config()
config1.debug = True
print(config2.debug)  # → False — different object!

Without Singleton, you can accidentally create multiple independent configs that fall out of sync.

Implementing with __new__

Python calls __new__ to create an object (before __init__initialises it). By overriding __new__, we intercept the creation step.

class Config:
    _instance = None
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance
a = Config(); b = Config()
print(a is b)  # → True
→
True

One More Gotcha: __init__ Still Runs

Even if __new__ returns the same object each time, Python still calls __init__ on every Config(). Without a guard, later calls can reset shared state back to default values.

class Config:
    _instance = None
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance
    def __init__(self):
        if hasattr(self, "_initialized"):
            return
        self.debug = False
        self.version = "1.0"
        self._initialized = True

Identity vs Equality

  • a == b — are they equal? (can be overridden with __eq__)
  • a is b — are they the same object in memory? (identity)
  • For Singleton, a is b must be True

Your Task

  1. Add _instance = None to Config
  2. Fix __new__ so it creates the object only once and always returns that same object
  3. Stop later calls to Config() from resetting debug and version

After your fix, a is b and b is c should both be True, and c.debug should still be Trueafter you set a.debug = True.

Section 1/6When to Use a Singleton

When to Use a Singleton

Explore this concept

Extension: Singleton Pattern

A design pattern is a reusable solution to a common programming problem. The Singleton pattern ensures a class has exactly one instanceand provides a global point of access to it.

When to Use a Singleton

  • Application configuration (one config object for the whole app)
  • Database connection pool (one shared connection manager)
  • Logging service (one log file writer)
  • Hardware interfaces (one printer spooler, one GPU context)

Output:

Click "Check" to run your code.