Singleton vs Multiple Instances

Singleton

Only one instance for the entire app — good for shared resources.

class AppConfig:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
            cls._instance.load_config()
        return cls._instance

config = AppConfig()  # whole app shares this one instance

When to Use Singleton

Scenario	Reason
Auth token management	All requests share one token, centralized refresh
Config management	Config only needs to be loaded once
Logger	Centralized log output
DB Connection Pool	Shared connection pool

When to Use Multiple Instances

Scenario	Reason
Parallel data processors	Each processor handles its own data
Multiple different DB connections	Connect to different databases
Multiple API clients	Connect to different services or accounts

# Multiple instances — each processor is independent
class DataProcessor:
    def __init__(self, data):
        self.data = data

    def process(self):
        return f"Processing {self.data}"

processor1 = DataProcessor("dataset_A")
processor2 = DataProcessor("dataset_B")

Summary

• Singleton: Shared resources, global state, configuration management
• Multiple instances: Parallel processing, resource isolation, different configurations