在多线程中附加字符串

Question

I'm doing multi-threading and trying to append ' ,F ' into the msg (string) ' pSDATA,0,4,2,2,0,4\\n '. Ultimately I would like to get ' pSDATA,0,4,2,2,0,4,F\\n ' However when I run the following code:

if(msg[0].lower() == 'p'):
    msg = msg[:-1] + ',F\n'
    pcQueue.put_nowait(msg[1:])
    print ("Message received: ") + msg[1:]

I got the following result:

,Fssage received: SDATA,0,4,2,2,0,4

I'm suspecting this is due to multi-threading. Any help would be greatly appreciated!

Answer 1

If you were trying to remove a trailing newline with msg[:-1] , be aware that the string could have ended with "\\r\\n" . \\r commonly has the effect of going back to the beginning of the line of output and continuing to write there. The right course of action depends on exactly why you wanted to remove the trailing newline, but if you wanted to remove exactly one from the end, optionally, supporting both \\r\\n and \\n , here's how you could do it:

def chomp(s):
    if s.endswith('\n'):
        if s.endswith('\r\n'):
            return s[:-2]
        return s[:-1]
    return s

⋮

if msg[0].lower() == 'p':
    msg = chomp(msg) + ',F\n'
    ⋮

Other things to choose from might include msg.rstrip() to remove any amount of all whitespace from the end, msg[:-2] if you know for sure it always ends with CR LF, ….

Also, if whatever protocol involved specifies a preference for CR LF, or even if it's just consistent with what you're passing along, you might want to be adding ,F\\r\\n instead of ,F\\n .

Answer 2

If you have multiple threads that print to the console at the same time, the printing isn't guaranteed to be atomic. This means that messages from different threads could indeed get interleaved on the console. If that's a problem, you should use synchronisation ( http://effbot.org/zone/thread-synchronization.htm ).

On the other hand, there's no reason to think that the messages getting put on the queue are corrupt in any way. If you think otherwise, you'll need to provide more details.