My point is that SSL_write(3ssl) says "WARNING: When calling SSL_write() with num=0 bytes to be sent the behaviour is undefined."

Apparently on the particular openssl you're looking at, it gives SSL_ERROR_SYSCALL with error code == 0 and len == 0, but the openssl devs claim this is some arbitrary thing that shouldn't be depended on.

Just as a general principle it would be nice if performing ordinary operations on an SSLSocket from Python did not invoke undefined behavior :-)