Sorry for the late reply. Adding a new "*.npy" format feature to allow writing to the file in chunks is nice but seems a bit limited. As I understand the proposal, reading the file back can only be done in the chunks that were originally written. I think other libraries like zar or h5py have solved this problem in a more flexible way. Is there a reason you cannot use a third-party library to solve this? I would think if you have an array too large to write in one chunk you will need third-party support to process it anyway.