test.py

setup = '''
class prop:
    def __init__(self, getter):
        self.getter = getter

    def __get__(self, instance, owner=None):
        if instance is None:
            return self
        
        value = instance.__dict__[self.getter.__name__] = self.getter(instance)
        # Uncomment this to see that prop doesn't call more than once, but prap does.
        # print(value)
        return value


class prap(prop):

    def __set__(self, instance, value):
        raise AttributeError()


class X:
    @prop
    def val1(self):
        return True

    @prap
    def val2(self):
        return False

x = X()
'''

import timeit

print(timeit.timeit('x.val1', setup))
print(timeit.timeit('x.val2', setup))