  We present a new open-source Python package, krotov, implementing the quantum
optimal control method of that name. It allows to determine time-dependent
external fields for a wide range of quantum control problems, including
state-to-state transfer, quantum gate implementation and optimization towards
an arbitrary perfect entangler. Krotov's method compares to other
gradient-based optimization methods such as gradient-ascent and guarantees
monotonic convergence for approximately time-continuous control fields. The
user-friendly interface allows for combination with other Python packages, and
thus high-level customization. The package is being developed at
https://github.com/qucontrol/krotov
