uncertainties is a Python module, which allows calculations such as . (0.2 +/- 0.01) * 2 = 0.4 +/- 0.02 . to be performed transparently; much more complex mathematical expressions involving numbers with uncertainties can also be evaluated transparently. Correlations between expressions are correctly taken into account; x-x is thus exactly zero, for instance. The uncertainties produced by this module are what is predicted by error propagation theory.