Cached Property (Python recipe) by Ken Seehart

ActiveState Code (http://code.activestate.com/recipes/576563/)

It works as a decorator.

class Swallow:

    def __init__(self, laden):
        self.mass = 5.0
        self.laden = laden

    @cached_property
    def air_speed(self):
        coconut_mass = 16.0 
        t0 = time.time()
        mass = self.mass + (coconut_mass if self.laden else 0.0)
        distance = 43
        time.sleep(mass*distance/400.0) # must sleep after flying
        t = time.time() - t0
        return distance/t

This might be of interest:

Easy Property Creation in Python http://code.activestate.com/recipes/576742/

Only 7 lines of code, easy to understand, easy to use, easy to customize, save you a lot of typing

Thanks for writing this. Here's a slightly modified version using functools.wraps() which preserves the original property docstring and name:

import functools

def cached_property(fun):
    """A memoize decorator for class properties."""
    @functools.wraps(fun)
    def get(self):
        try:
            return self._cache[fun]
        except AttributeError:
            self._cache = {}
        except KeyError:
            pass
        ret = self._cache[fun] = fun(self)
        return ret
    return property(get)

Another modification/improvement: you can define member variable names, whose change should trigger the recalculation of the cached value.

def smart_cached_property(inputs=[]):
    def smart_cp(f):
        @functools.wraps(f)
        def get(self):
            input_values = dict((key,getattr(self,key)) for key in inputs )
            try:
                x = self._property_cache[f]
                if input_values == self._property_input_cache[f]:
                    return x
            except AttributeError:
                self._property_cache ={}
                self._property_input_cache = {}
            except KeyError:
                pass

            x = self._property_cache[f] = f(self)
            self._property_input_cache[f] = input_values

            return x
        return property(get)
    return smart_cp

Usage example:

@smart_cached_property(['mass','laden'])
def air_speed_velocity(self):
    ...
    ...