[Python] Python global variable changes depending on how script is run

I have a short example python script that I'm calling glbltest.py:

a = []

def fun():
global a
a = [20,30,40]

print("before ",a)
fun()
print("after ",a)


If I run it from the command line, I get what I expect:

$ python glbltest.py
before []
after [20, 30, 40]


I open a python shell and run it by importing, and I get basically the same thing:

>>> from glbltest import *
before []
after [20, 30, 4]


So far so good. Now I comment out those last three lines and do everything "by hand":

>>> from glbltest import *
>>> a
[]
>>> fun() # I run fun() myself
>>> a # I look at a again. Surely I will get the same result as before!
[] # No! I don't!


What is the difference between fun() being run "automatically" by the importing of the script, and me running fun() "by hand"?

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