On Wed, 23 Nov 2016 08:10 pm, Frank Millman wrote:
[...]
> The class has a getval() method to return the current value.
>
> Usually the value is stored in the instance, and can be returned
> immediately, but sometimes it has to be computed, incurring further
> database lookups.
This is called memoisation, or caching, and is a perfectly standard
programming technique. It's not without its traps though: there's a famous
quote that says there are only two hard problems in computing, naming
things and cache invalidation. But putting that aside:
def getval(self):
sentinel = object()
value = getattr(self, '_cached_value', sentinel)
if value is sentinel:
# compute the value and store it
value = ...
self._cached_value = value
return value
To invalidate the cache and force a recalculation:
del self._cached_value
Now it's easy to override:
class Subclass(ParentClass):
def getval(self):
value = super(Subclass, self).getval()
return value + 1
> This is what I have come up with.
>
> 1. Rename all instances of 'getval()' to '_getval()'.
>
> 2. Add a new method '_getval_with_comp()'.
>
> 3. When instantiating an object, check if it would need computation -
> if computation_required:
> self.getval = self._getval_with_comp
> else:
> self.getval = self._getval
So this check only happens once, on instantiation? And you're sure that once
the instance is created, there will never be any circumstances where you
want to re-calculate the value?
> 4. In _getval_with_comp, perform the computation, then add the following -
> self.getval = self._getval
> return self._getval()
So you have something like this?
def _getval(self):
return self._cached_value
def __getval_with_comp(self):
value = ... # long computation
self._cached_value = value
self.getval = self._getval
# return value
return self._getval()
# why call the method when you already know the answer?
How are subclasses supposed to override getval?
--
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.
--
https://mail.python.org/mailman/listinfo/python-list
