About Python global variable

Question

    import sys
    from threading import Thread

    is_online=1

    class CommandListenerThread(Thread):
        global is_online
        def run(self):
          while is_online:
             next_command=sys.stdin.readlines();
             if next_command == 'exit':
                 is_online=0
             else:
                 print next_command



    listener=CommandListenerThread()
    listener.start()

When I run this python code,it shows an error: "UnboundLocalError: local variable 'is_online' referenced before assignment"

I tested another code which uses the same way to access the global variable inside a class,and it works fine. So,what is wrong with this specific code?

the code may look weird which using a thread to listen the command line,but it is just a part of my program which gets an error when I run the whole program.

thank you guys

Nir Alfasi · Accepted Answer · 2014-08-16 03:21:55Z

2

Move global is_online into run() to solve the error.

To address your other question (in a comment below), why not make it a static class variable ?

class CommandListenerThread(Thread):    
    is_online = 1    
    def run(self):
        print CommandListenerThread.is_online

In case that you have to use another code with a global is_online, you can take the DI (dependency injection) approach as follows:

import sys from threading import Thread

is_online = 2

class CommandListenerThread(Thread):

    def __init__(self, is_online):
        super(CommandListenerThread, self).__init__()       
        CommandListenerThread.is_online = is_online # now it's a static member
                                                    # if you want to make it an instance member use self.is_online

    def run(self):
        print CommandListenerThread.is_online        

listener=CommandListenerThread(is_online) # inject the value to the constructor 
listener.start()

edited Aug 16, 2014 at 3:21

answered Aug 16, 2014 at 3:00

Nir Alfasi

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6 Comments

user3788871 Over a year ago

Yeah,solved. So..Does it mean if I have n functions in a class which all need to access the global var x, I need to declare x as global inside each of these functions?

Nir Alfasi Over a year ago

Why do you need it to be global? can't it be a class variable ? see above

user3788871 Over a year ago

In my program,it has to be. The code above is just a testing code I created with the same pattern of a part of my original code where encounters a problem. In my program var "is_online" is shared by multiple classes.thanks

Nir Alfasi Over a year ago

@user3788871 I'll add another approach in a minute

user3788871 Over a year ago

What you mean in the second method is passing the global variable as a parameter to the class constructor. Are you sure that python passes reference to a function instead of a copy?

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