Using global variables between files

Question

I m bit confused about how the global variables work  I have a large project  with around 50 files  and I need to define global variables for all those files   What I did was define them in my projects main py file  as following        myproject main py    Define global myList global myList myList         Imports import subfile    Do something subfile stuff   print myList 0     I m trying to use myList in subfile py  as following       myproject subfile py    Save  hey  into myList def stuff        globals    myList   append  hey     An other way I tried  but didn t work either       myproject main py    Import globfile     import globfile    Save myList into globfile globfile myList         Import subfile import subfile    Do something subfile stuff   print globfile myList 0     And inside subfile py I had this        myproject subfile py    Import globfile import globfile    Save  hey  into myList def stuff        globfile myList append  hey     But again  it didn t work  How should I implement this  I understand that it cannot work like that  when the two files don t really know each other  well subfile doesn t know main   but I can t think of how to do it  without using io writing or pickle  which I don t want to do

User · Answer

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")

User · Answer

I just came across this post and thought of posting my solution  just in case of anyone being in the same situation as me  where there are quite some files in the developed program  and you don t have the time to think through the whole import sequence of your modules  if you didn t think of that properly right from the start  such as I did   In such cases  in the script where you initiate your global s   simply code a class which says like  class My Globals    def   init   self       self global1    quot initial value 1 quot      self global2    quot initial value 2 quot           and then use  instead of the line in the script where you initiated your globals  instead of global1    quot initial value 1 quot   use globals   My Globals    I was then able to retrieve   change the values of any of these globals via globals desired global  in any script  and these changes were automatically also applied to all the other scripts using them  All worked now  by using the exact same import statements which previously failed  due to the problems mentioned in this post   discussion here  I simply thought of global object s properties being changing dynamically without the need of considering   changing any import logic  in comparison to simple importing of global variables  and that definitely was the quickest and easiest  for later access  approach to solve this kind of problem for me

User · Answer

Your 2nd attempt will work perfectly  and is actually a really good way to handle variable names that you want to have available globally   But you have a name error in the last line   Here is how it should be        myproject main py    Import globfile     import globfile    Save myList into globfile globfile myList         Import subfile import subfile    Do something subfile stuff   print globfile myList 0     See the last line   myList is an attr of globfile  not subfile   This will work as you want   Mike

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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        or          from config import x print  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

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