Can not increment global variable from function in python [duplicate]

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?

Answer 1

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.

Answer 2

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.