representing physical units

[Russ]

I know that python packages are available for representing physical
units, but I am getting frustrated trying to find them and determine
which is the best.

Rather than just using conventional dimensionless numbers with implicit
units, I would like to have a way to explicitly represent the units. If
incompatible units are added or subtracted, I would like to have an
error detected. Auto-conversions (e.g., from feet to miles) to enforce
consistency before adding or subtracting are not necessary. I would
even like to be able to multiply units. For example, I would like to
have meters returned when I multiply meters/second by seconds.

Where can I find a good package that does this? Thanks.