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Caching Singleton class (subclassable) (Python recipe) by Eugene Mirotin
ActiveState Code (http://code.activestate.com/recipes/576573/)

Allows decorating and caching expensive data consuming tasks

Python, 31 lines
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class CachedSingleton(object):
    """Provides caching singleton storage for data access decoration.
    Usage:
        class CachedSingletonClass(CachedSingleton):
            def _getitem(self, name):
                # implement data getting routine, such as db access
        
        CachedSingletonClass().attribute1 # returns value as if _getitem('attribute1') was called  
        CachedSingletonClass().attribute2 # returns value as if _getitem('attribute2') was called  
        CachedSingletonClass().__doc__ # returns real docstring  
    """    
    __instance = None
    
    def __new__(classtype, *args, **kwargs):
        if classtype != type(classtype.__instance):
            classtype.__instance = object.__new__(classtype, *args, **kwargs)
            classtype.__instance.cache = {}    
        
        return classtype.__instance
       
    # to be implemented by contract in the descendant classes
    def _getitem(self, name):
        return None
        
    def __getattribute__(self, name):
        try:
            return object.__getattribute__(self, name)
        except:
            if not self.cache.has_key(name):
                self.cache[name] = self._getitem(name)
            return self.cache[name]
Tags: data_access, data_caching, singleton

3 comments

Alexander Artemenko 15 years, 4 months ago  # | flag

I think, that this is the shorter and faster implementation:

class CachedSingleton(object):
    """Provides caching singleton storage for data access decoration.
    Usage:
        class CachedSingletonClass(CachedSingleton):
            def _getitem(self, name):
                # implement data getting routine, such as db access

        CachedSingletonClass().attribute1 # returns value as if _getitem('attribute1') was called··
        CachedSingletonClass().attribute2 # returns value as if _getitem('attribute2') was called··
        CachedSingletonClass().__doc__ # returns real docstring··
    """
    __instance = {}

    def __init__(self):
        self.__dict__ = self.__instance

    def _getitem(self, name):
        'to be implemented by contract in the descendant classes'
        return None

    def __getattr__(self, name):
        value = self._getitem(name)
        setattr(self, name, value)
        return value
David Lambert 15 years, 4 months ago  # | flag

Rather than returning None, I'd have CachedSingleton._getitem

    raise Exception(NotImplemented)
Eugene Mirotin (author) 15 years, 4 months ago  # | flag

Alexander, your solution does not solve 2 problems: 1) caching, as you call the _getitem every time 2) accessing existing attributes of the object (like __doc__) instead of calling _getitem for them

David, thanks, raise is much more pythonic in this case

Created by Eugene Mirotin on Wed, 26 Nov 2008 (MIT)
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