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Design Pattern -- Inherit from an instance (Python recipe) by Daniel Cohn
ActiveState Code (http://code.activestate.com/recipes/577026/)

Take an instance (call it foo) and create a factory class (call it InstanceFactory) that produces foo's. Then inherit from InstanceFactory.

Python, 6 lines
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def MakeClassFromInstance(instance):
    from copy import deepcopy
    copy = deepcopy(instance.__dict__)
    InstanceFactory = type('InstanceFactory', (instance.__class__, ), {})
    InstanceFactory.__init__ = lambda self, *args, **kwargs: self.__dict__.update(copy)
    return InstanceFactory
Tags: design_pattern, factory_class, inheritance, instance

2 comments

Gabriel Genellina 14 years, 3 months ago  # | flag

Perhaps one should warn the users:

  • this only works with new-style classes

  • the original per-instance state becomes shared among all instances:

>>> class Foo(object):
...   def __init__(self):
...     self.items = []
...
>>> x = Foo()
>>> y = Foo()
>>> x.items.append(1)
>>> y.items
[]
>>> Foo2 = MakeClassFromInstance(x)
>>> z = Foo2()
>>> x.items.append(5)
>>> x.items
[1, 5]
>>> z.items
[1, 5]
Daniel Cohn (author) 14 years, 2 months ago  # | flag

Gabriel, thank you for that comment! While this recipe only applies to new style classes, the original per-instance state is No Longer shared among all instances. Below is a reproduction of your example using the updated recipe. Thanks again, and more discussion + use cases to follow.

>>> class Foo(object):
...     def __init__(self):
...         self.items = []
... 
>>> x = Foo()
>>> y = Foo()
>>> x.items.append(1)
>>> Foo2 = MakeClassFromInstance(x)
>>> z = Foo2()
>>> x.items.append(5)
>>> x.items
[1, 5]
>>> z.items
[1]
>>>
Created by Daniel Cohn on Tue, 2 Feb 2010 (MIT)
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