Question

Try this out:

A = 1
B = 2
C = A + B

def main():
    global C
    print A

The output of main() is 1.

Why is this? Why should main get to know about the other global variables used to evaluate C?

Answer 1

Global variables are always available to all local scopes in Python, including functions. In this case, within main() A, B, and C are all in scope.

The global keyword doesn’t do what it seems you think it does; rather, it permits a local scope to manipulate a global function (it makes global variables “writable”, so to speak). Consider these examples:

c = 4
print c
def foo():
    c = 3
    print c
foo()
print c

Here, the output will be

4
3
4

Now, consider this:

c = 4
print c
def foo():
    global c
    c = 3
    print c
foo()
print c

In this case, the output will be

4
3
3

In the first case, c = 3 merely shadows c until its scope is up (i.e. when the function definition ends). In the second case, we’re actually referring to a reference to a global c after we write global c, so changing the value of c will lead to a permanent change.