How to use a global variable in Python?

Question

I'm not sure why it's not working when I declare a global variable...

first_read = True

def main():

    if (first_read == True):
        print "hello world"
        first_read = False

    print 'outside of if statement'

if __name__ == '__main__':
    main()

My traceback shows the following error:

Traceback (most recent call last):
   File "true.py", line 12, in <module>
      main()   
   File "true.py", line 5, in main
     if (first_read == True): 
UnboundLocalError: local variable 'first_read' referenced before assignment

Possible duplicate of Tkinter Command Lambda Function Modify Variable — Perry
– Perry, Commented Aug 9, 2017 at 3:20

Tisp · Accepted Answer · 2017-08-09 03:22:13Z

3

You have to define variable as global:

first_read = True

def main():
    global first_read
    if (first_read == True):
       print "hello world"
       first_read = False

    print 'outside of if statement'

if __name__ == '__main__':
    main()

answered Aug 9, 2017 at 3:22

Tisp

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1 Comment

superloopnetworks Over a year ago

Just curious... I also have a global variable: 'ntw_device = []' declared.. But I didn't have to specify global ntw_device to use it... Why is that? Is it different for lists?

Chuong Huynh · Accepted Answer · 2017-08-09 03:22:47Z

2

In def main you should declare a global variable like this:

global first_read

this will use first_read as global variable in main function.

answered Aug 9, 2017 at 3:22

Chuong Huynh

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