Truly Global Variable in python

Question

OK so the project I am working on requires* using a truly global variable (or five) and the other questions on here have to do with cross module variables (could be helpful later just not yet).

Here is my problem: I have a program that will be running an environmental simulation and I need to set some global variables to be used and modified inside some of the functions. How would one go about doing that?

*When I say require I mean that I haven't found a more efficient way to do this.

I'd probably use memcached or redis for something like this. — monkut
– monkut, Commented Nov 7, 2014 at 1:23
What does "truly global" mean if not "cross-module (like built-in) global"? You mean shared between processes? Shared between separate runs at different times? Shared between every copy of the program running on any computer in the world? Spherical, with a map projected on them? — abarnert
– abarnert, Commented Nov 7, 2014 at 1:39
As always it depends, but there are literally 3 cases in software development where global variable is the right answer. What exactly is preventing you from just passing the relevant data from one function to another? — Aida Paul
– Aida Paul, Commented Nov 7, 2014 at 1:43
What I am asking is if there is a way to make a variable accessible to all functions in one program. — unknown
– unknown, Commented Nov 7, 2014 at 1:44
Why do you believe that your program requires global variables? Sure, globals may be convenient, but they mess up modular design (and testing). You should probably be using a class to store the state parameters of your environmental simulation. — PM 2Ring
– PM 2Ring, Commented Nov 7, 2014 at 2:17

Robᵩ · Accepted Answer · 2014-11-07 02:49:28Z

4

Yes, there is a way to use and modify global variables inside functions that are spread across several modules. Put the variables in a module and import that module into your main program:

# globals.py
klaatu = "Once in Persia reigned a king ..."

# main_program.py
import globals

def modify_klaatu():
    print globals.klaatu
    globals.klaatu = "It was the best of times, it was the worst of times."

answered Nov 7, 2014 at 2:49

Robᵩ

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2 Comments

Mark Rucker Over a year ago

Do you know if this also works when multithreading?

Throw Away Account Over a year ago

This doesn't work if there is another module that also imports globals. That module will have a separate copy of globals, which won't be changed by the modify_klaatu function, which only modifies main_program.globals.klaatu.

Andy Jones · Accepted Answer · 2020-04-23 14:31:43Z

1

A somewhat dodgy but entirely dynamic variant of @rob's approach:

import types
import sys

sys.modules['myglobals'] = types.ModuleType('myglobals')
sys.modules['myglobals'].__dict__.update({'first': 'thing', 'second': 'thing'})

import myglobals
print(myglobals.first) # prints 'thing'

Or if you've only got one global, how about this extremely dodgy approach:

import sys
sys.modules['myglobal'] = 'thing'

import myglobal
print(myglobal) # prints 'thing'

answered Apr 23, 2020 at 14:31

Andy Jones

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