全局变量在不同的函数中是不同的

Question

I know that using global variable in functions is wrong, but I have a code that uses these variables and I have an issue that I can't find how to solve.

I have a module with the following code:

messages = {}

def MessageReceived(message):
    global messages

    print messages
    msgNumber = message[0]
    if messages.has_key(msgNumber):
        messageData = messages.pop(msgNumber)
        ReleaseMessageNumber(msgNumber)
        if messageData["callback"] is not None:
            messageData["callback"](messageData["originalMessage"])

def SendMessage(message, callBack):
    global messages

    msgNumber = GetMessageNumber()

    messageToSend = msgNumber + message + "\0"

    messages[msgNumber] = { 'originalMessage' : message, 'time' : time.time(), 'callback' : callBack }
    print messages
    Send(messageToSend)

    return True

In general, the function "SendMessage" builds a message with a message number, sends the message an stores the message and a callback in a global dictionary.

The function "MessageReceived" gets the callback of the specific message from the global dictionary and calls it.

The problem is that it seems that 2 different dictionaries exists here.

When I send a message, I see that the dictionary grows (a new record is added to the dictionary every time I send a message), but the print at the "MessageReceived" function always shows an empty dictionary.

Can you help me with that? Where is the bug here?

Thanks!

Answer 1

Firstly, although it doesn't affect your problem there is no need to use the global keyword anywhere in this code. You never re-assign the global dicts, you only mutate them, so there is no need to declare them as global - Python finds them in the module scope anyway.

The problem though is that all module-level variables are per-process. If your receive function is always running in its own thread, it will never see any of the data from the view.

Since however you already know that using global variables is bad, I'm not sure why you are doing it. There are plenty of ways of passing data between processes; storing it in the database is probably the best if you're using Django.