How can I change a global variable from within a function?

The error that you get when you try to run your code is:

UnboundLocalError: local variable 'a' referenced before assignment

… which, on the face of it, seems strange: after all, the first statement in the code above (a = 15) is an assignment. So, what’s going on?

Actually, there are two distinct things happening, and neither of them are obvious unless you already know about them.

First of all, you actually have two different variables:

• The a in your first line is a global variable (so called because it exists in the global scope, outside of any function definitions).

• The a in the other lines is a local variable, meaning that it only exists inside your test() function.

These two variables are completely unrelated to each other, even though they have the same name.

A variable is local to a function if there’s a statement assigning to it inside that function – for instance, your a = a +10 line.

Even so, the error still looks strange – after all, the very first thing you do inside test() is assign to a, so how can it be referenced beforehand?

The answer is that, in an assignment statement, Python evaluates everything on the right hand side of the = sign before assigning it to the name on the left hand side – so even though the assignment is written first in your code, a gets referenced first in that right hand side: a +10.

There are two ways you can get around this. The first is to tell Python that you really want the a inside test() to be the same a in the global scope:

def test():
    global a
    a = a + 10
    print(a)

This will work, but it’s a pretty bad way to write programs. Altering global variables inside functions gets hard to manage really quickly, because you usually have lots of functions, and none of them can ever be sure that another one isn’t messing with the global variable in some way they aren’t expecting.

A better way is to pass variables as arguments to functions, like this:

a = 15

def test(x):
    x = x + 10
    print(x)

test(a)

Notice that the name doesn’t have to be the same – your new definition of test() just says that it accepts a value, and then does something with it. You can pass in anything you like – it could be a, or the number 7, or something else. In fact, your code will always be easier to understand if you try to avoid having variables with the same name in different scopes.

If you play with the code above, you’ll notice something interesting:

>>> a = 15
>>> test(a)
25
>>> a
15

… a didn’t change! That’s because although you passed it into test() and it got assigned to x, it was then x that got changed, leaving the original a alone.

If you want to actually change a, you need to return your modified x from the function, and then reassign it back to a on the outside:

>>> a = 15
>>> 
>>> def test(x):
...     x = x + 10
...     print(x)
...     return x
... 
>>> a = test(a)
25
>>> a
25