regression tests (so-called test-driven development) that ensure that the software is installed and functioning correctly, yielding results that match those previously obtained from many different users and machines. This testing framework currently covers about 86% of the lines of MNE-Python code, which not only enhances the quality and stability of the software but also makes it easier to incorporate new contributions quickly without breaking existing code. Code quality is also improved by a peer review process among the developers. Any code contribution must be read by at least two people, the author and a reviewer, in order to mitigate the risk of errors. Moreover, the entire source code and full development history is made publicly available by a distributed version control system. This makes it possible to keep track of the development of the project and handle code changes in a way that minimizes the risk of rendering existing scripts and analysis pipelines inoperable. Finally, large parts of the source code are commented using inline documentation that allows for automatically building user manuals in PDF and HTML formats. The Ohloh.net1 source code analysis project attests that 35% of the source code consists of documentation and with this, MNE-Python scores in
