  We present Lenstronomy, a multi-purpose open-source gravitational lens
modeling python package. Lenstronomy is able to reconstruct the lens mass and
surface brightness distributions of strong lensing systems using forward
modelling. Lenstronomy supports a wide range of analytic lens and light models
in arbitrary combination. The software is also able to reconstruct complex
extended sources (Birrer et. al 2015) as well as being able to model point
sources. We designed Lenstronomy to be stable, flexible and numerically
accurate, with a clear user interface that could be deployed across different
platforms. Throughout its development, we have actively used Lenstronomy to
make several measurements including deriving constraints on dark matter
properties in strong lenses, measuring the expansion history of the universe
with time-delay cosmography, measuring cosmic shear with Einstein rings and
decomposing quasar and host galaxy light. The software is distributed under the
MIT license. The documentation, starter guide, example notebooks, source code
and installation guidelines can be found at https://lenstronomy.readthedocs.io.
