python bijection

[Joshua Bronson]

On Nov 20, 3:09 pm, Terry Reedy <tjreedy at udel.edu> wrote:
> Joshua Bronson wrote:
> > Anyone have any other feedback? For instance, is offering the __call__
> > syntax for the inverse mapping wonderful or terrible, or maybe both?
>
> Terrible ;-)
>
> Use standard subscripting with slices, and only that, to both get and set.
>
> Let m[4] == m[4:] == 'abc' # m[4:] is suggested alternative addition
> Then m[:'abc'] == 4
>
> m[4:] passes slice(4,None,None) to __getitem__
> m[:'abc'] passes slice(None,'abc',None)
>
> It just happens that dict items and slices use the same notation, but
> they do, so take advantage of that. In fact, to emphasize the symmetry
> of the bijective map, consider disallowing m[key] as ambiguous and
> require m[key:], along with m[:key] to access and set.
>
> Note that m[slice(1,2,3):] passes slice(slice(1, 2, 3), None, None), so
> this approach does not even prevent using slices as keys/values.
>
> In __setitem__, m[a:b] which passes slice(a,b,None) would have to be an
> error. In __getitem__, it could either be a error or return True/False
> depending on whether the pair is in the map. But this depends on whether
> __contains__ only tests keys or is modified to test pairs.
>
> Terry Jan Reedy

absolutely genius. implemented in the latest version:
http://bitbucket.org/jab/toys/src/tip/bijection.py

thank you for the terrific idea!