very large dictionaries

[robin]

I need to do a search through about 50 million records, each of which
are less than 100 bytes wide. A database is actually too slow for
this, so I thought of optimising the data and putting it all in
memory.

There is a single key field, so a dictionary is an obvious choice for
a structure, since Python optimises these nicely.

But is there a better choice? Is it worth building some sort of tree?

-- robin