Small discussion is started at pull request [1]. There are doubts about the usefulness of incremental I/O API. SQLite is rarely used for storing blobs of the size of hundreds of megabytes. For smaller blobs it may be enough to read or write all data at once. There are also questions about the support of len(), since other file-like objects don't support len().

This discussion remembered me about mmap objects. mmap objects implement two protocols: file protocol and sequence protocol (including the support of len()). The BLOB object looks similar to the mmap object. sqlite3_blob_write() may only modify the contents of the BLOB; it is not possible to increase the size of a BLOB using this API. Maybe implement the sequence protocol in the BLOB object? Or implement only the sequence protocol and drop away the file protocol?

[1] https://github.com/python/cpython/pull/271