What's the logic behind this python global scoping magic?

Question

I was messing around with the scoping in python and found something that I think is rather strange:

g = 5

def foo(a):
    if a:
        global g
        g = 10
    else:
        g = 20


print("global g: ",g)

foo(False)
print("global g: ",g) # 20?! What?

foo(True)
print("global g: ",g)

My believe was that the second print should have been "5" since the global statement was never executed, but clearly, the output is 20(!).

What's the logic behind this?

Answer 1

The global keyword is used by the python compiler to mark a name in the function scope as global.

As soon as you use it anywhere in the function, that name is no longer a local name.

Note that if does not introduce a new scope, only functions and modules do (with classes, list, dict and set comprehensions being a special cases of function scopes).

A (hard to read and non-pythonic) work-around would be to use the globals() function:

def foo(a):
    if a:
        globals()['g'] = 10
    else:
        g = 20