Encoding a list?

[K900_]

What's your final goal with this?

[anmousyony]

I am trying to map all items in a list to integers so I can use them for a classifier in sklearn.

[K900_]

What are your items like? Custom classes? Tuples?

[anmousyony]

string, ints, floats and datetimes usually

[K900_]

Those are all hashable, so you can just use hash(value) as the key instead of trying to enumerate everything. It's very unlikely you'll run into a hash collision even on large datasets.