A list where most (>95%) values will be None (or default)
Inspired by http://stackoverflow.com/q/17522753/78845
A “sparse list” is a list where most (say, more than 95% of) values will be None (or some other default) and for reasons of memory efficiency you don’t wish to store these (cf. http://en.wikipedia.org/wiki/Sparse_array ).
This implementation has a similar interface to Python’s built-in list but stores the data in a dictionary to conserve memory.