Can not increment global variable from function in python

Question

From this code:

COUNT = 0

def increment():
    COUNT = COUNT + 1

increment()

I get the following error:

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    increment()
  File "test.py", line 4, in increment
    COUNT = COUNT+1
UnboundLocalError: local variable 'COUNT' referenced before assignment

Why? How can I increment the global variable COUNT from inside the function?

The use of `global` among beginners is usually a sign of bad design. — Rik Poggi, May 08 '12 at 21:51
without using `global` you can't modify the value of a global variable inside a function, you can only use it's value inside the function. But if you want to assign a new value to it then you've to use the `global` keyword first. — Ashwini Chaudhary, May 08 '12 at 21:53
This should answer your question: http://stackoverflow.com/questions/423379/using-global-variables-in-a-function-other-than-the-one-that-created-them Looks like Python won't change the global value unless you specify that it's what you want to do. — Phil Nicholson, May 08 '12 at 21:46

score 126 · Answer 1 · edited Jan 15 '23 at 06:06

126

Use a global statement, like so:

COUNT = 0

def increment():
    global COUNT
    COUNT = COUNT+1

increment()

Global variables can be accessed without using global, but the statement is required in order to change the value of the global variable.

edited Jan 15 '23 at 06:06

Karl Knechtel

62,466
11
102
153

answered May 08 '12 at 21:44

cobie

7,023
11
38
60

Thanks for the solution. This one is better as I have tried already using "count += 1" alone but wasn't working. This worked for me. Thanks. – PravinY Aug 16 '17 at 08:17
6

This no longer works. Old solution. – JumpingJacks May 15 '19 at 14:04

score 35 · Answer 2 · edited Jan 15 '23 at 06:07

This is because globals don't bleed into the scope of the function. Use the global statement to force this for assignment:

>>> COUNT = 0
>>> def increment():
...     global COUNT
...     COUNT += 1
... 
>>> increment()
>>> print(COUNT)
1

Note that using globals is a really bad idea - it makes code hard to read, and hard to use. Instead, return a value from your function (using return) and use that to do something. If the same data needs to be accessible from a range of functions, consider making a class.

It's also worth noting that CAPITALS is generally reserved for constants, so it's a bad idea to name global variables like this. For normal variables, lowercase_with_underscores is preferred.

Can not increment global variable from function in python

2 Answers2

Linked

Related