ActiveState Code

Recipe 576563: Cached Property

A cached property is a read-only property that is calculated on demand and automatically cached. If the value has already been calculated, the cached value is returned.

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def cached_property(f):
    """returns a cached property that is calculated by function f"""
    def get(self):
        try:
            return self._property_cache[f]
        except AttributeError:
            self._property_cache = {}
            x = self._property_cache[f] = f(self)
            return x
        except KeyError:
            x = self._property_cache[f] = f(self)
            return x
        
    return property(get)

Discussion

Example:

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

def calc_air_speed_velocity(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/1000.0) # must sleep after flying
    t = time.time() - t0
    return distance/t

air_speed_velocity = cached_property(calc_air_speed_velocity)

s1 = Swallow(False) s2 = Swallow(True)

print s1.air_speed_velocity print s2.air_speed_velocity

print s1.air_speed_velocity # notice that the second two invocations do not take any time. print s2.air_speed_velocity

Comments

  1. 1. At 7:58 p.m. on 13 nov 2008, David Lambert said:

    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
  2. 2. At 12:07 p.m. on 6 may 2009, runsun pan said:

    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

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