flavio: A Python package for flavour physics phenomenology in the Standard Model and beyond. flavio is an open source tool for phenomenological analyses in flavour physics and other precision observables in the Standard Model and beyond. It consists of a library to compute predictions for a plethora of observables in quark and lepton flavour physics and electroweak precision tests, a database of experimental measurements of these observables, a statistics package that allows to construct Bayesian and frequentist likelihoods, and of convenient plotting and visualization routines. New physics effects are parameterised as Wilson coefficients of dimension-six operators in the weak effective theory below the electroweak scale or the Standard Model EFT above it. At present, observables implemented include numerous rare B decays (including angular observables of exclusive decays, lepton flavour and lepton universality violating B decays), meson-antimeson mixing observables in the Bd,s, K, and D systems, tree-level semi-leptonic B, K, and D decays (including possible lepton universality violation), rare K decays, lepton flavour violating τ and μ decays, Z pole electroweak precision observables, the neutron electric dipole moment, and anomalous magnetic moments of leptons. Not only central values but also theory uncertainties of all observables can be computed. Input parameters and their uncertainties can be easily modified by the user. Written in Python, the code does not require compilation and can be run in an interactive session. This document gives an overview of the features as of version 1.0 but does not represent a manual. The full documentation of the code can be found in its web site.