Updating global variables from inside a function

The following code is written so that increase_x function can increase the value of x by 1.

def increase_x():
x = x + 1
print('The value of x is', x)

x = 10
increase_x()

However, this results in

UnboundLocalError: local variable 'x' referenced before assignment

Although we could access a global variable from the local scope, we need an additional statement in order to modify a global variable from code in a local scope.

With the example above, you can indicate that x in increase_x function is the global variable x by including the global statement global x. A global statement is typically used at the top of a function, by writing global followed with the name of the variable, as seen in the following.

Once a global statement is declared, it holds for the entire current code block in the local scope. Now, we have the desired result as follows.

The value of x is 11

In general, you should be careful when updating global state. The more global state you have, the harder your programs are to predict and debug.