Allow cache.get_or_set() to cache a None result

[Phill Tornroth]

get_or_set docstring says "If the key does not exist, add the key and set it to the default value." -- but that's not quite what it does. It will perform a set if the key doesn't exist, or if the cached value is None.

I think in order to be doing what it says on the tin it'd need to be:

if self.has_key(key, version=version):
        return self.get(key, version=version)
else:
    if callable(default):
        default = default()
        
    self.add(key, default, timeout=timeout, version=version)
    # Fetch the value again to avoid a race condition if another
    # caller added a value between the first get() and the add()
    # above.
    return self.get(key, default, version=version)

I'd find this useful in cases where None was an expensive result to arrive at. If there's spiritual alignment with the suggestion, I'm happy to prepare and submit a change with tests.

[Tim Graham]

I agree with your analysis. I read through previous related issues (#26792, #28601) and didn't see a good reason for the current behavior. I would question if if default is not None: should be there in your proposal (i.e. why shouldn't None by cached?).