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POCreating a singleton in Python
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copied!<p><em>This question is not for the discussion of whether or not the <a href="http://en.wikipedia.org/wiki/Singleton_pattern" rel="noreferrer">singleton design pattern</a> is desirable, is an anti-pattern, or for any religious wars, but to discuss how this pattern is best implemented in Python in such a way that is most pythonic. In this instance I define 'most pythonic' to mean that it follows the 'principle of least astonishment'</em>.</p> <p>I have multiple classes which would become singletons (my use-case is for a logger, but this is not important). I do not wish to clutter several classes with added gumph when I can simply inherit or decorate.</p> <p>Best methods:</p> <hr> <h2>Method 1: A decorator</h2> <pre><code>def singleton(class_): instances = {} def getinstance(*args, **kwargs): if class_ not in instances: instances[class_] = class_(*args, **kwargs) return instances[class_] return getinstance @singleton class MyClass(BaseClass): pass </code></pre> <p>Pros</p> <ul> <li>Decorators are additive in a way that is often more intuitive than multiple inheritance.</li> </ul> <p>Cons</p> <ul> <li>While objects created using MyClass() would be true singleton objects, MyClass itself is a a function, not a class, so you cannot call class methods from it. Also for <code>m = MyClass(); n = MyClass(); o = type(n)();</code> then <code>m == n && m != o && n != o</code></li> </ul> <hr> <h2>Method 2: A base class</h2> <pre><code>class Singleton(object): _instance = None def __new__(class_, *args, **kwargs): if not isinstance(class_._instance, class_): class_._instance = object.__new__(class_, *args, **kwargs) return class_._instance class MyClass(Singleton, BaseClass): pass </code></pre> <p>Pros</p> <ul> <li>It's a true class</li> </ul> <p>Cons</p> <ul> <li>Multiple inheritance - eugh! <code>__new__</code> could be overwritten during inheritance from a second base class? One has to think more than is necessary.</li> </ul> <hr> <h2>Method 3: A <a href="https://stackoverflow.com/questions/100003/what-is-a-metaclass-in-python">metaclass</a></h2> <pre><code>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] #Python2 class MyClass(BaseClass): __metaclass__ = Singleton #Python3 class MyClass(BaseClass, metaclass=Singleton): pass </code></pre> <p>Pros</p> <ul> <li>It's a true class</li> <li>Auto-magically covers inheritance</li> <li>Uses <code>__metaclass__</code> for its proper purpose (and made me aware of it)</li> </ul> <p>Cons</p> <ul> <li>Are there any?</li> </ul> <hr> <h2>Method 4: decorator returning a class with the same name</h2> <pre><code>def singleton(class_): class class_w(class_): _instance = None def __new__(class_, *args, **kwargs): if class_w._instance is None: class_w._instance = super(class_w, class_).__new__(class_, *args, **kwargs) class_w._instance._sealed = False return class_w._instance def __init__(self, *args, **kwargs): if self._sealed: return super(class_w, self).__init__(*args, **kwargs) self._sealed = True class_w.__name__ = class_.__name__ return class_w @singleton class MyClass(BaseClass): pass </code></pre> <p>Pros</p> <ul> <li>It's a true class</li> <li>Auto-magically covers inheritance</li> </ul> <p>Cons</p> <ul> <li>Is there not an overhead for creating each new class? Here we are creating two classes for each class we wish to make a singleton. While this is fine in my case, I worry that this might not scale. Of course there is a matter of debate as to whether it aught to be too easy to scale this pattern...</li> <li>What is the point of the <code>_sealed</code> attribute</li> <li>Can't call methods of the same name on base classes using <code>super()</code> because they will recurse. This means you can't customize <code>__new__</code> and can't subclass a class that needs you to call up to <code>__init__</code>.</li> </ul>

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