Python weird variable scope issue

Question

I have my code as below.

def test():
   print num1
   print num
   num += 10

if __name__ == '__main__':
   num = 0
   num1 = 3
   test()

When executing the above python code I get the following output.

3 
Traceback (most recent call last):
File "<stdin>", line 3, in <module>
File "<stdin>", line 2, in test
UnboundLocalError: local variable 'num' referenced before assignment

I do not know why particularly num is not available in test method. It is very weird for me and i did not face this yet before.

Note: I am using python 2.7.

Assuming you indent properly, this code works. Please check again that you show the exact code that throws this error, you may have a typo somewhere. — , Nov 18 '13 at 12:08
@delnan I used the num variable in a place as in my last edit. Hope it makes the error in the code. Can you look the above code? — кคгtђเςк ĞŔĶ, Nov 18 '13 at 12:53
possible duplicate of [UnboundLocalError in Python](http://stackoverflow.com/questions/9264763/unboundlocalerror-in-python) — , Nov 18 '13 at 12:54
See, now it's easy. If you'd had included the right code (or even just the exception message) from the start, you would have gotten your answer an hour sooner ;-) — , Nov 18 '13 at 12:56
Cool, your edit brought me a downvote because my answer is no longer valid. — Sebastian Mach, Nov 19 '13 at 11:18
@phresnel I downvoted your answer, *before* the edit, because I didn't think it was useful for the question as of back then. — , Nov 19 '13 at 16:20
@delnan: I think you mean my wrong wrong answer. I provided another one which pointed out the incorrect indendation, it was roughly "After correcting the indendation, it works (http://ideone.com/wNRgCF ):". The downvotes on my first answer, which you commented on, were absolutely justified. — Sebastian Mach, Nov 19 '13 at 23:04
@phresnel No, I downvoted your other answer as well. It should have been a comment, and in fact someone had already said the same thing in the comments. — , Nov 20 '13 at 14:09
@delnan: _Imho_, under the assumption the OP pasted his code 1:1, my newer answer was valid. However, one (cannot|can forever) discuss opinion of course ;) — Sebastian Mach, Nov 20 '13 at 15:14

tawmas · Answer 1 · 2014-02-06T18:44:40.850

1

Since you're assigning to num inside the test function, python considers it to be a local variable. This is why it complains that you are referencing to a local variable before assigning it.

You can fix this by explicitly declaring num to be global:

def test():
    global num
    print num1
    print num
    num += 10

if __name__ == '__main__':
    num = 0
    num1 = 3
    test()

edited Feb 06 '14 at 18:44

answered Nov 18 '13 at 12:56

tawmas

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score 1 · Accepted Answer · answered Nov 18 '13 at 12:58

num appears in an assignment statement inside the definition of the test. This makes num a local variable. Since scope determination is made at compile time, num is a local variable even in the print statement that precedes the assignment. At that time, num has no value, resulting in the error. You need to declare num as global if you want to access the global value. num1 does not have this problem, since you never try to assign to it.

def test():
    global num
    print num1
    print num
    num += 10

Python weird variable scope issue

2 Answers2