feat: Enhanced Singleton and Factory pattern example with thread-safety, dynamic registration, and logging

This PR enhances the existing implementation of the Singleton and Factory Method design patterns by making them more robust, extensible, and aligned with real-world usage.
It improves the structure, documentation, and maintainability of the code — turning a simple pattern demo into a clean, production-ready educational example.

Author: Shreekant-Bharti

advanced/01_design_patterns/singleton_factory.py

@@ -1,36 +1,90 @@
-# Singleton Pattern
+"""
+Title: Singleton and Factory Design Pattern Example
+Author: Python Design Patterns Contributors
+Description:
+    Demonstrates the Singleton and Factory Method design patterns in Python
+    with a Logger and Animal creation system.
+Date: October 2025
+"""
+
+from threading import Lock
+from typing import Type, Dict
+
+
+# =============================================================================
+# 1. SINGLETON PATTERN (Thread-safe)
+# =============================================================================
 class SingletonMeta(type):
+    """A thread-safe implementation of Singleton."""
     _instance = None
+    _lock: Lock = Lock()
+
     def __call__(cls, *args, **kwargs):
-        if not cls._instance:
-            cls._instance = super().__call__(*args, **kwargs)
+        with cls._lock:
+            if cls._instance is None:
+                cls._instance = super().__call__(*args, **kwargs)
         return cls._instance
 
+
 class Logger(metaclass=SingletonMeta):
-    def log(self, msg):
+    """A simple logger class using Singleton pattern."""
+    def log(self, msg: str) -> None:
         print(f"[LOG]: {msg}")
 
-logger1 = Logger()
-logger2 = Logger()
-assert logger1 is logger2 
+
+# =============================================================================
+# 2. FACTORY PATTERN
+# =============================================================================
 class Animal:
-    def speak(self):
-        pass
+    """Base Animal class."""
+    def speak(self) -> str:
+        raise NotImplementedError("Subclasses must implement 'speak' method.")
+
 
 class Dog(Animal):
-    def speak(self):
+    def speak(self) -> str:
         return "Woof!"
 
+
 class Cat(Animal):
-    def speak(self):
+    def speak(self) -> str:
         return "Meow!"
 
-def animal_factory(animal_type: str) -> Animal:
-    if animal_type == "dog":
-        return Dog()
-    elif animal_type == "cat":
-        return Cat()
-    raise ValueError("Unknown animal type")
 
-pet = animal_factory("dog")
-print(pet.speak())  
+class AnimalFactory:
+    """Factory for creating Animal instances dynamically."""
+    _registry: Dict[str, Type[Animal]] = {}
+
+    @classmethod
+    def register_animal(cls, name: str, animal_cls: Type[Animal]) -> None:
+        cls._registry[name.lower()] = animal_cls
+
+    @classmethod
+    def create(cls, name: str) -> Animal:
+        if name.lower() not in cls._registry:
+            raise ValueError(f"Unknown animal type: {name}")
+        return cls._registry[name.lower()]()
+
+
+# Register animals dynamically
+AnimalFactory.register_animal("dog", Dog)
+AnimalFactory.register_animal("cat", Cat)
+
+
+# =============================================================================
+# 3. DEMONSTRATION
+# =============================================================================
+if __name__ == "__main__":
+    logger = Logger()
+    logger.log("Application started.")
+
+    pet_type = "dog"
+    pet = AnimalFactory.create(pet_type)
+    logger.log(f"Created a {pet_type} that says: {pet.speak()}")
+
+    # Validate Singleton behavior
+    logger2 = Logger()
+    assert logger is logger2
+    logger.log("Verified Singleton: Both logger instances are identical.")
+
+    logger.log("Application finished successfully.")