The vast amounts of data to be collected by the Giant Radio Array for Neutrino Detection (GRAND) and its prototype — GRANDProto300 — require the use of a data format very efficient in terms of i/o speed and compression. At the same time, the data should be easily accessible, without the knowledge of the intricacies of the format, both for bulk processing and for detailed event-by-event analysis and reconstruction. We present the format and the structure prepared for GRAND data, the concept of the data-processing chain, and data-oriented and analysis-oriented interfaces written in Python.