Global variable values in PyCharm (Python 3.6) console

Question

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, so I reverted to an if condition. I believe there's a way of doing this with a decorator, but that felt like gilding the Lilly.

Anyway, this is the context for my actual question: to help me work out what was going on 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, using the PyCharm console does the same thing:

>>> print(greeting)
Good Day
>>> dog = Animal(None)
>>> print(greeting)
Good Day

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? Or my lack of broader understanding of execution scope?

Thanks!!

Might have answered my own question from the top (excellent) answer to this: https://stackoverflow.com/questions/15959534/python-visibility-of-global-variables-in-imported-modules. It seems that when you "run" a module in PyCharm it's actually doing a "from import *", whereas "running" a module in IDLE is like executing the code line-by-line in the interpreter. This explains the behaviour of the global variables, which are actually copies of variables created at the point of execution. — enjayaitch, Jul 20 '18 at 11:48
edit to the above - probably inaccurate to say the "run" is equivalent to a `from import *`. If it were, it wouldn't execute the `if __name__ = '__main__'` block, which of course it does. But it's as if, once it's executed that and leaves the console running, subsequent interpreter commands do not reference the module global variable, but a copy of them taken at a point in time (as though it had executed `from import *`. — enjayaitch, Jul 20 '18 at 14:19

score 1 · Accepted Answer · answered Jul 20 '18 at 15:00

1

JetBrains have opened a bug report for me - confirmed the behaviour isn't as expected.

answered Jul 20 '18 at 15:00

enjayaitch

137
1
9

Global variable values in PyCharm (Python 3.6) console

1 Answers1