Python @lazyprop decorator

Implementation of http://en.wikipedia.org/wiki/Lazy_initialization Based on http://stackoverflow.com/a/3013910 - extended to support invalidation / setting, fully unit tested

Basic usage - very_expensive_calculation() is only called the first time, the result is cached and returned for subsequent calls

class YourClass(object):
    @lazyprop
    def lazy(self):
        return very_expensive_calculation()

    def print_result(self):
        print "You can safely use lazy twice without any performance penalty: %s %s" % (self.lazy, self.lazy)

You can also invalidate the cache if needed

class YourClass(object):
    @lazyprop
    def lazy(self):
        return very_expensive_calculation()

    def change_params_for_expensive_calculation():
        ...
        # This will invalidate the lazy cache and force recalculation next time self.lazy is accessed
        del self.lazy 

Or even set it:

class YourClass(object):
    @lazyprop
    def lazy(self):
        return very_expensive_calculation()

    def do_something_else():
        ...
        # This will change the stored lazy cache value
        self.lazy = new_value()

See the unit tests for more examples