Singleton with Metaclass

Singletons get a bad wrap but they can be effective when used properly. I came across this pattern for implementing them in Python a couple of years back, it worked well in 2.7 but may be out of date for 3.

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class Singleton(type):
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  def __init__(cls, name, bases, dict):
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    super(Singleton, cls).__init__(name, bases, dict)
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    cls._instance = None
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  def __call__(cls, *args, **kw):
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    if cls._instance is None:
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      cls._instance = super(Singleton, cls).__call__(*args, **kw)
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    return cls._instance

This can now be implemented in any class you wish to make a singleton like so:

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class SingleInstance():
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  __metaclass__ = Singleton
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  def __init__(self):
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    pass
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  def __str__(self):
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    return 'SingleInstance'