reverse dict lookup & Relation class

[Aaron Brady]

Hi, this is a continuation of something that comes up now and again
about reverse lookups on dictionaries, as well as a follow-up to my
pursuit of a Relation class from earlier.

For a reverse lookup, you just need two lookups.
name= {}
phone= {}
name[ '555-963' ]= 'Joan'
phone[ 'Joan' ]= '555-963'

Though maybe the keys in 'name' should be names, instead of the values
in it.  The variable name doesn't clarify that.  (What is more natural
to you in reading code?  Is one obviously wrong?)

phone[ '555-963' ]= 'Joan'
name[ 'Joan' ]= '555-963'

To provide for non-unique fields, the structure becomes kind of
complicated.

phone[ '555-963' ]= 'Joan'
phone[ '555-964' ]= 'Joan'
name[ 'Joan' ]= [ '555-963', '555-964' ]

For uniform access, 'phone' can be a dict of strings to lists too.

phone[ '555-963' ]= [ 'Joan' ]
phone[ '555-964' ]= [ 'Joan' ]

To add a third field, the structure becomes again more complicated.
Either define a unique key:

phone[ '555-963' ]= [ object1 ]
phone[ '555-964' ]= [ object2 ]
name[ 'Joan' ]= [ object1, object2 ]
hourstocall= {}
hourstocall[ '9a-5p' ]= [ object1 ]
hourstocall[ '5p-11p' ]= [ object2 ]
records= {}
records[ object1 ]= ( 'Joan', '555-963', '9a-5p' )
records[ object2 ]= ( 'Joan', '555-964', '5p-11p' )

Or, and this is the interesting part, use the ('identificationally')
same tuples in both mappings, since the object is only mentioned, not
used.

phone[ '555-963' ]= [ ( 'Joan', '555-963', '9a-5p' ) ]
phone[ '555-964' ]= [ ( 'Joan', '555-964', '5p-11p' ) ]
name[ 'Joan' ]= [ ( 'Joan', '555-963', '9a-5p' ), ( 'Joan', '555-964',
'5p-11p' ) ]
hourstocall[ '9a-5p' ]= [ ( 'Joan', '555-963', '9a-5p' ) ]
hourstocall[ '5p-11p' ]= [ ( 'Joan', '555-964', '5p-11p' ) ]

What's the best way to construct this class?  Or, do you have an
argument that it could not be simpler than using a relational db?
Brainstorming, not flamestorming.