What happens when a key is deleted? Will that leave an empty slot in
the entry array?
Yes. See the __delitem__() method in the pure python implemention
If so, will the array be compacted if the proportion
of entries grows beyond a certain limit?
Yes. Compaction happens during resizing() which triggers when
the dict reaches two-thirds full (including dummy entries).
See the __setitem__() method in the pure python implementation.
Adding a key would involve simply appending to the entry array (filling
the last empty slot), but if there could be empty slots in other
places, would it look for the first?
Yes. See the _next_open_index() helper method in the pure python implemention.
Actually, as I think about it, you could form a linked list (using the
'hash' field) of those empty slots that aren't part of the final
contiguous block and fill those preferentially.
That's the plan. See the comment above the keylist.index(UNUSED)
line in the _next_open_index() method in the pure python implementation.