Kevin Smith wrote:
I have many cases in my code where I use a property for calculating a
value on-demand. Quite a few of these only need to be called once.
After that the value is always the same. In these properties, I set a
variable in the instance as a cached value and return that value on
subsequent calls. It would be nice if there was a descriptor that would
do this automatically. Actually, what would be really nice is if I
could replace the property altogether and put the calculated value in
its place after the first call, but the property itself prevents me from
doing that.
This should do it:
class CachingProperty(object):
def __init__(self, attr_name, calculate_function):
self._name = attr_name
self._calculate = calculate_function
def __get__(self, obj, type=None):
if obj is None:
return self
else:
value = self._calculate(obj)
setattr(obj, self._name, value)
return value
And example code:
>>> class Foo(object):
... def calculate_value(self):
... print 'Calculating...'
... return 42
... foo = CachingProperty('foo', calculate_value)
...
>>> bar = Foo()
>>> bar.__dict__
{}
>>> bar.foo
Calculating...
42
>>> bar.foo # Notice that the print statement doesn't run this time
42
>>> bar.__dict__
{'foo': 42}
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