Implement singleton

Author: dancergraham

chapter03_decorator/readme.md

@@ -2,6 +2,6 @@
 
 **Decorator**: Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
 
-Not quite the same as python decorator syntax as in python you call the _decorated function_ and the decorating function is called first whereas the _decorating function_ must be called here.
+Not quite the same as python [decorator syntax](https://docs.python.org/3/reference/compound_stmts.html#grammar-token-decorators) as in python you call the _decorated function_ and the decorating function is called first whereas the _decorating function_ must be called here.
 
 I subclass `ABC` and used the `@abstractmethod` decorator from the `abc` module here but do not use any of this functionality - it just serves as documentation.

chapter04_factory/readme.md

@@ -2,6 +2,8 @@
 
 **Simple Factory**: A class which chooses which product class to instantiate and return, based upon method parameters.
 
+The Python standard library contains multiple references to factory objects, for instances in [dataclasses](https://docs.python.org/3/library/dataclasses.html?highlight=factory)
+
 **Factory Method**: Defines an interface for creating an object, but lets subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses.
 
 For instance the `PizzaStore` abstract class in this repo provides an abstract `create_pizza` interface for creating one product.  

chapter05_singleton/chocolate.py

@@ -0,0 +1,54 @@
+class SingletonError(Exception):
+    pass
+
+class Singleton(type):
+    _instances = {}
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instances[cls]
+        
+
+class ChocolateBoiler(metaclass=Singleton):
+    def __init__(self) -> None:
+        self.empty = True
+        self.boiled = False
+    
+    def fill(self):
+        if not self.empty:
+            raise SingletonError
+        self.empty = False
+        self.boiled = False
+        print("Full!")
+
+    def drain(self):
+        if self.empty or (not self.boiled):
+            raise SingletonError
+        self.empty = True
+        print("Empty!")
+
+    def boil(self):
+        self.boiled = True
+        print("boiled!")
+
+    def is_empty(self):
+        return self.empty
+
+    def is_boiled(self):
+        return self.boiled
+
+
+def chocolate_controller():
+    boiler = ChocolateBoiler()
+    boiler.fill()
+    boiler.boil()
+    boiler2 = ChocolateBoiler()
+    boiler.drain()
+    print(boiler2 is boiler)
+
+
+
+if __name__ == '__main__':
+    chocolate_controller()
+
+