官方文档对socket模式下的socket.send() 和 socket.sendall()解释如下：
sock.sendall(string[, flags])
Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Unlike send(), this method continues to send data from string until either all data has been sent or an error occure. None is returned on success. On error, an exception is raised, and there is no way to datermine how much data, if any, was successfully sent.
尝试发送string的所有数据， 成功则返回None， 失败则抛出异常。
socket.send(string [, flags])
Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Returns the number of bytes snet. Applications are responsible for checking that all data has been sent; if only some of the data was transmitted, the application needs to attempt delivery of the remaining data.
send()的返回值式发送的字节数量， 这个数量值可能小于要发送的string的字节数，也就是说可能无法发送string中所有的数据。如果有错误，则会抛出异常。
所以， 下面两段代码是等价的：
sock.sendall("Hello world
")
buffer = "Hello world
"
while buffer:
bytes = sock.send(buffer)
buffer = buffer[bytes:]