"If you give something an _, then it is not considered part of the public API and it (the internal API, not the value) is subject to change, which means you should *not* suggest that users change it.  If they find it and want to change it anyway, that's their lookout.  That's the consenting adults part :)"

This sums it up well - it should be considered a detail of implementation, thus the _. When I said I wanted to allow the value to be modified, I was thinking of the 'consenting adult' type of usage, not everyday usage. Of course there is no reasonable way to prevent the value being modified anyway.

So - _MAXCACHE can stay where it is, but should not be documented or be included in __all__. Does that seem reasonable?