currently dicts have insert-or-update semantics, e.g.:
>>> dict([((), 1), ((), 2)])
{(): 2}
this is fine. however, in many cases insert or insert-or-ignore are more
useful: (hypothetical examples)
>>> InsertOrIgnoreDict([((), 1), ((), 2)])
{(): 1}
>>> InsertDict([((), 1), ((), 2)])
Traceback: [...]
ValueError: key exists
these are useful when dealing with custom config file formats, config
overrides, among other things.
additionally we could also get an UpdateDict, a *view* into a dict that
only allows updating existing entries:
>>> a = {'a': 1}
>>> b = {'a': 2, 'b': 3}
>>> UpdateDict(a).update(b)
>>> a
{'a': 2}
but I don't see an obvious usefulness to this one. it'd just be for
consistency with other SQL-esque dict wrappers (like the above
InsertOrIgnoreDict and InsertDict).
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