Behaviour of global variables in "run with Python console"

I'm new to both Python and PyCharm, so please forgive ignorance.

I was trying to tech myself about the execution of functions when initialising classes - specifically, I want to re-use a database connection object if passed into a new instance, but create one if not.  I have a function get_cnx() that creates a connection.  I discovered that whether using a default argument in the __init__ statement to call get_cnx():

def __init__(self, db_cnx=get_cnx())

or whether using a keyword argument:

self.db_cnx = kwargs.get('db_cnx', get_cnx())

...the function is always executed regardless of the presence (or content) of the connection argument that's passed in.  Defeats the object of re-using a connection.

Anyway, in the course of working this out I created this simple test, as a module called "classes.py":

greeting = 'Good Day'


def my_func():
    global greeting
    greeting = 'Changed'
    return 'Hello'


class Animal:
    def __init__(self, greet):
        if not greet:
            self.greet = my_func()
        else:
            self.greet = greet


if __name__ == '__main__':
    cat = Animal(None)

If I run this module (with "Run with Python console" checked in the configuration), I see the global variable 'greeting' shown in blue as 'Changed', which is what I'd expect.

If I change the last bit to this:

if __name__ == '__main__':
    cat = Animal('Wotcha')

I see the global variable shown in blue as 'Good Day', which is also what I'd expect.

However, when I then type this into the console:

dog = Animal(None)

The global variable name turns red but still shows 'Good Day'.  Similarly if I type 'greeting' into the console it returns the same result.

Now, I loaded the module into IDLE and hit F5 (run module), and in the console, did this:

>>> greeting
'Good Day'
>>> dog = Animal(None)
>>> greeting
'Changed'

This is what I would have expected to see in the PyCharm console.  

Can someone explain what's going on?  Could it be a bug, or is it my lack of understanding of the way PyCharm deals with scope?

Thanks!!