Question

My application has a structure similar to this one:

myapp.py
basemod.py
[pkg1]
    __init__.py
    mod1.py
[pkg2]
    __init__.py
    mod2.py

myapp.py:

import pkg1
import pkg2

if __name__ == '__main__':
    pkg1.main()
    pkg2.main()

basemod.py:

import pkg1


def get_msg():
    return pkg1.msg

pkg1/__init__.py:

import mod1

msg = None


def main():
    global msg
    mod1.set_bar()
    msg = mod1.bar

pkg1/mod1.py:

bar = None


def set_bar():
    global bar
    bar = 'Hello World'

pkg2/__init__.py:

import mod2


def main():
    mod2.print_foo()

pkg2/mod2.py:

import basemod

foo = basemod.get_msg()


def print_foo():
    print(foo)

If I run myapp.py I get:

None

While in my mind I’d expect:

Hello World

My goal is to keep the two packages completely independent from each other, and only communicating through basemod.py, which is a sort of API to pkg1.
I’m starting to think that I have not completely understood how imports among packages work, what am I doing wrong?

Thank you!

Answer 1

Took me a while to read through all that code, but it looks like your problem is in pkg2/mod2.py. The line foo = basemod.get_msg() is executed the first time that file is imported, and never again. So by the time you change the value of mod1.bar, this has already executed, and foo is None.

The solution should simply be to move that line into the print_foo function, so it is only executed when that function is called – which is after the code that sets the relevant value.