Pint is a Python package to define, operate and manipulate physical
quantities: the product of a numerical value and a unit of measurement. It
allows arithmetic operations between them and conversions from and to
different units.

It is distributed with a comprehensive list of physical units, prefixes and
constants. Due to its modular design, you can extend (or even rewrite!) the
complete list without changing the source code. It supports a lot of numpy
mathematical operations without monkey patching or wrapping numpy.

WWW: http://pypi.python.org/pypi/Pint