This primitive doesn't not well fit a case of compressed streams. A chunk of compressed data read from underlied file object can be uncompressed to unpredictable large data. We can't limit the size of buffer.

Another point is that buffer interface is not very appropriate for Python implementation. And we want left as much Python code in gzip, bz2, lzma and zipfile as possible. Copying from bytes into buffer and back will just waste resources.