degal/utils.py
author Tero Marttila <terom@fixme.fi>
Sun, 14 Jun 2009 16:19:27 +0300
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"""
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    Miscellaneous utilities
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"""
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import functools
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class LazyProperty (object) :
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    """
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        Lazy-loaded cached properties.
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        This functions by overriding the class's property attribute with the actual value attribute in the instance's
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        dict, causing all future attribute lookups to return that directly.
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        >>> class Foo (object) :
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        ...     counter = 0
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        ...     @lazy_load
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        ...     def foo (self) :
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        ...             self.counter += 1
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        ...             return self.counter
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        ... 
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        >>> foo = Foo(); foo.foo, foo.foo, foo.counter
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        (1, 1, 1)
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    """
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    def __init__ (self, func) :
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        """
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            Initialize with no value
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        """
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        # the getter function
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        self.func = func
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        # the attribute name
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        self.name = func.__name__
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    def run (self, obj) :
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        """
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            Run the background func and return the to-be-cached value.
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            `obj` is the object instance
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        """
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        return self.func(obj)
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    def store (self, obj, value) :
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        """
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            Store the cached value, overring ourself
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        """
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#        print "[%x] %r.__dict__[%r] = %r" % (id(obj), obj, self.name, value)
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        obj.__dict__[self.name] = value
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    def __get__ (self, obj, owner) :
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        """
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            Generate the value, store it as the attribute and return it.
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        """
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        # compute value
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        value = self.run(obj)
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        # store it
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        self.store(obj, value)
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        # return it
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        return value
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class LazyIteratorProperty (LazyProperty) :
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    """
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        A lazy-loaded property that automatically converts an iterator/genexp into a non-mutable tuple.
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    """
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    def run (self, obj) :
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        return tuple(self.func(obj))
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lazy_load = LazyProperty
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lazy_load_iter = LazyIteratorProperty
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# testing
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if __name__ == '__main__' :
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    import doctest
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    doctest.testmod()
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