Using global variables between files?

The problem is you defined myList from main.py, but subfile.py needs to use it. Here is a clean way to solve this problem: move all globals to a file, I call this file settings.py. This file is responsible for defining globals and initializing them:

# settings.py

def init():
    global myList
    myList = []

Next, your subfile can import globals:

# subfile.py

import settings

def stuff():
    settings.myList.append('hey')

Note that subfile does not call init()— that task belongs to main.py:

# main.py

import settings
import subfile

settings.init()          # Call only once
subfile.stuff()         # Do stuff with global var
print settings.myList[0] # Check the result

This way, you achieve your objective while avoid initializing global variables more than once.

See Python's document on sharing global variables across modules:

The canonical way to share information across modules within a single program is to create a special module (often called config or cfg).

config.py:

x = 0   # Default value of the 'x' configuration setting

Import the config module in all modules of your application; the module then becomes available as a global name.

main.py:

import config
print (config.x)

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In general, don’t use from modulename import *. Doing so clutters the importer’s namespace, and makes it much harder for linters to detect undefined names.