Well, the title already says it. Globals are module wide, not global global…. This means, that if you do
from module_b import doSomethingWithGlobalA global a a = 10 doSomethingWithGlobalA() # e.g. global a; a += 5; print(a) # a is still 10
The global would not change. This is because the global keyword only makes variables accessible to the module level – not beyond.
So if you instead of importing the module execute the file, it will run in the same context of the script calling it:
execfile('module_b.py')
global a
a = 10
doSomethingWithGlobalA() # e.g. global a; a += 5;
print(a) # a is now 15
This will execute the files instructions in the local context and will work as (naively) expected.
To avoid confusion: best is to never mess with globals anyways… Global state is in general considered bad practice (for several reasons).