When does socket.recv(recv_size) return?

Yes, your conclusion is correct. socket.recv is a blocking call.

socket.recv(1024) will read at most 1024 bytes, blocking if no data is waiting to be read. If you don't read all data, an other call to socket.recv won't block.

socket.recv will also end with an empty string if the connection is closed or there is an error.

If you want a non-blocking socket, you can use the select module (a bit more complicated than just using sockets) or you can use socket.setblocking.

I had issues with socket.setblocking in the past, but feel free to try it if you want.

It'll have the same behavior as the underlying recv libc call see the man page for an official description of behavior (or read a more general description of the sockets api).