How does 'global' behave under an if statement?

Question

In my program, I wanted a variable global only under some circumstances. Say it looks like this:

a = 0
def aa(p):
    if p:
        global a
    a = 1
    print("inside the function " + str(a))


print(a)
aa(False)
print("outside the function " + str(a))

I was expecting the result to be:

0
inside the function 1
outside the function 0

However it turned out to be:

0
inside the function 1
outside the function 1

So, I was thinking, "Okay maybe the Python compiler makes the variable global whenever it sees the 'global' keyword no matter where it is located". Is this how Python works with global vars? Am I misunderstanding?

Answer 1

Yes, you're understanding things correctly.

The global statement isn't something that's evaluated at runtime. It's really a directive to the parser that essentially tells it to treat all listed identifiers (a here) as referring to the global scope. From the docs on the global statement:

The global statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals.

It then continues to state how global is really is a directive:

Programmer’s note: global is a directive to the parser.

Using it conditionally doesn't make any difference: its presence has already been detected in the parsing stage and, as a result, the byte-code generated for grabbing the names has already been set to look in the global scope (with LOAD/STORE GLOBAL).

This is why, if you dis.dis a function containing a global statement, you won't see any relevant byte-code for global. Using a silly function:

from dis import dis
def foo():
    "I'm silly"
    global a  

dis(foo)
  2           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

Nothing is generated for global a because the information it provides has already been used!