Python change variable in another module and save it

Question

I have a simple python package named my_package with three files:

init.py

from .a import func
from .settings import var

settings.py

var = 1

a.py

import .settings

def func():
    settings.var = 2

When I add this package to PATH and import it I run below script, couldn't get expected result:

import my_package

my_package.func()
print(my_package.var)

I am expecting to get '2' but it returns '1'. What am I doing wrong here?

Don't `from` import variables. (Really, you shouldn't be using global variables at all, let alone sharing them across modules, let alone doing so with `from` imports, which are essentially incompatible with all that.) — user2357112 supports Monica, Oct 23 '17 at 20:36
how can i import settings.var without using 'from' in __init__.py? — Elgin Cahangirov, Oct 23 '17 at 20:43

matusko · Answer 1 · 2017-10-24T09:10:08.230

-1

It happens because during import my_package a my_package.var is set to refer to int with value of 1. After running my_package.func() this reference is not touched, however my_package.settings.var is changed to point at new int object with value 2.

import my_package  # imports my_package.var with value 1 

my_package.func()  # sets my_package.settings.var to 2
print(my_package.var)  # prints 1
print(my_package.settings.var)  # prints 2

If you try to do the same with list, you can make it will work differently. By not creating new object at my_package.settings.list_var but rather modifying an entry in existing list.

# __init__.py
from .a import func
from .settings import list_var

_

# settings.py
list_var = [1]

_

# a.py
import .settings

def func():
    settings.list_var[0] = 2

Now running the similar code will actually change the list_var

import my_package  # imports my_package.var with value 1 

print(my_package.var[0])  # prints 1
my_package.func()  # sets list_var to [2]
print(my_package.var[0])  # prints 2

edited Oct 24 '17 at 09:10

answered Oct 23 '17 at 20:47

matusko

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what if 'var' is pandas dataframe? Should I insert that into list still? – Elgin Cahangirov Oct 23 '17 at 20:56
nope, pandas DataFrame is also non-primitive, so it is passed by ref, just like list – matusko Oct 23 '17 at 20:58
what is the most conventional method of using package-wide variables? – Elgin Cahangirov Oct 23 '17 at 21:01
https://stackoverflow.com/questions/1977362/how-to-create-module-wide-variables-in-python – matusko Oct 23 '17 at 21:07
Lists and ints are passed the exact same way. See https://nedbatchelder.com/text/names.html – user2357112 supports Monica Oct 24 '17 at 01:57

Python change variable in another module and save it

1 Answers1