This work describes DATeS, a highly-extensible data assimilation package. DATeS seeks to provide a unified testing suite for data assimilation applications that allows researchers to easily compare different methodologies in different settings with minimal coding effort. The core of DATeS is written in Python. The main functionalities, such as model propagation, and assimilation, can however be written in low-level languages such as C or Fortran to attain high levels of computational efficiency.