Implementation of get_deltanums() is wrong

[assarbad]

Presumably a relic from the evolution of the code, the implementation line should read:

return [d for d in self._deltas.keys()]

I noticed you're not using this function elsewhere in the module or the code, so that's the likely reason why this went unnoticed.

[assarbad]

In addition the KeyError line in get_delta() should be:

raise ValueError('Delta %s does not exist.' % deltanum)

(note the argument to the format string)