There is Compromise, a Python library supporting this

    >>> l = HalfIndexList('abcde')
    >>> l[0.5]
    'a'

and other similar crazy ideas, like SemiMutable data types, which only change value if you try to set them a few times.

https://github.com/travisjungroth/Compromise

Introduced in this short 2016 talk https://youtu.be/yC9m2GInXqU?t=337