This also happens for the writable() and seekable() methods. The problem is that those methods do not check whether the buffers have been closed in stringio.c. This is fixed in the attached patch for StringIO. BytesIO should be the same, but bytesio.c is structured differently, and I still have to understand the code. I will try to do this, and then add tests for this issue - which should go in one of the mixins of test_memoryio.py, I presume.