Question

I’m digging around with python and networking.

while True:
   data = sock.recv(10240)

This is definitely listening. But it seems to need to be converted to a text string.

I’ve seen some people using struct.unpack(), but I’m not sure exactly how it works.
What’s the way to convert?

Answer 1

What you get back from recv is a bytes string:

Receive data from the socket. The return value is a bytes object representing the data received.

In Python 3.x, to convert a bytes string into a Unicode text str string, you have to know what character set the string is encoded with, so you can call decode. For example, if it’s UTF-8:

stringdata = data.decode('utf-8')

(In Python 2.x, bytes is the same thing as str, so you’ve already got a string. But if you want to get a Unicode text unicode string, it’s the same as in 3.x.)

The reason people often use struct is that the data isn’t just 8-bit or Unicode text, but some other format. For example, you might send each message as a “netstring“: a length (as a string of ASCII digits) followed by a : separator, then length bytes of UTF-8, then a ,—such as b"3:Abc,". (There are variants on the format, but this is the Bernstein standard netstring.)

The reason people use netstrings, or other similar techniques, is that you need some way to delimit messages when you’re using TCP. Each recv could give you half of what the other side passed with send, or it could give your 3 sends and part of the 4th. So, you have to accumulate a buffer of recv data, and then pull the messages out of it. And you need some way to tell when one message ends and the next begins. If you’re just sending plain text messages without any newlines, you can just use newlines as a delimiter. Otherwise, you’ll have to come up with something else—maybe netstrings, or using \0 as a delimiter, or using newlines as a delimiter but escaping actual newlines within the data, or using some self-delimited structured format like JSON.