Using global variables between files?

Question

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)

---

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.

You can think of Python global variables as "module" variables - and as such they are much more useful than the traditional "global variables" from C.

A global variable is actually defined in a module's __dict__ and can be accessed from outside that module as a module attribute.

So, in your example:

# ../myproject/main.py

# Define global myList
# global myList  - there is no "global" declaration at module level. Just inside
# function and methods
myList = []

# Imports
import subfile

# Do something
subfile.stuff()
print(myList[0])

And:

# ../myproject/subfile.py

# Save "hey" into myList
def stuff():
     # You have to make the module main available for the 
     # code here.
     # Placing the import inside the function body will
     # usually avoid import cycles - 
     # unless you happen to call this function from 
     # either main or subfile's body (i.e. not from inside a function or method)
     import main
     main.mylist.append("hey")

Using from your_file import * should fix your problems. It defines everything so that it is globally available (with the exception of local variables in the imports of course).

for example:

##test.py:

from pytest import *

print hello_world

and:

##pytest.py

hello_world="hello world!"

Hai Vu answer works great, just one comment:

In case you are using the global in other module and you want to set the global dynamically, pay attention to import the other modules after you set the global variables, for example:

# settings.py
def init(arg):
    global myList
    myList = []
    mylist.append(arg)


# subfile.py
import settings

def print():
    settings.myList[0]


# main.py
import settings
settings.init("1st")     # global init before used in other imported modules
                         # Or else they will be undefined

import subfile    
subfile.print()          # global usage