IntervalMultiDict about portion HOT 3 CLOSED

Here's an example of a potential API. Indexing accepts either an interval, a key, or a tuple (interval, key). Internally, the structure would be a dict of IntervalDict (performance might be an issue, but this new IntervalMultiDict would be mostly proposed for convenience).

>>> m = P.IntervalMultiDict()
>>> m[P.closed(8, 12), 'teacher'] = 't1'
>>> m[P.closed(8, 10), 'room'] = 'r1'
>>> m[P.closed(11, 12), 'room'] = 'r2'
>>> m['room']  # Providing a key returns a (new?) IntervalDict instance
IntervalDict({P.closed(8, 10): 'r1', P.closed(11, 12): 'r2'})
>>> m[P.closed(8, 10]  # Returns a (likely new) IntervalMultiDict
{'teacher': IntervalDict({P.closed(8,10): 't1'}), 'room': IntervalDict(....)}
>>> m[P.singleton(9)]  # If interval is a singleton, returns a classical dict
{'teacher': 't1', 'room': 'r1'}
>>> m[P.singleton(9), 'room']  # Singleton + key return a value
'r1'
>>> m[9, 'room']  # For convenience, if a tuple is passed and the interval is a singleton, its value can be provided directly
'r1'
>>> m[P.singleton(9), ['room', 'teacher']]  # A list of keys can be passed
{'room': 'r1', 'teacher': 't1'}
>>> m[P.closed(10, 12)] = { 'room': 'r3', 'course': 'c1'}  # Dict can be used for assignment
>>> m[P.singleton(11)]
{'teacher': 't1', 'room': 'r3', 'course': 'c1'}

Mutability would only we allowed through indexing (i.e., m[x] always return a new instance, either of an IntervalDict if x is a key or a list of keys, or an IntervalMultiDict if x is an Interval. Changes on these returned values has no impact on the initial structure. That means that m[p,k] = x mutates m while m[p][k] = x does not).

Since this API is quite distinct from the one of a traditional dictionary, perhaps this structure shouldn't be called "IntervalMultiDict". Using "Dict" in the name implicitly suggests we mimick the API of a dict (as was the case for IntervalDict) but this means we need to implement many more operations (such as keys(), values(), items() returning views), and this can be tricky at first

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While convenient, these two examples are "incorrect" in the sense that one would expect the output type to be consistent for all intervals, including singletons:

>>> m[P.singleton(9)]  # If interval is a singleton, returns a classical dict
{'teacher': 't1', 'room': 'r1'}
>>> m[P.singleton(9), 'room']  # Singleton + key return a value
'r1'

In the first example, P.singleton(9) cannot be replaced by 9 since there's no way to detect in m[x] whether x aims to refer to a value from the interval domain, or a key from the dictionary.

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I'll keep this for later...

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