Run a program containing submodules

Assume we have the following layout:
– module_with_sub_module: the directory that contains the application(It is not the base module directory but a directory that holds it ).
– foo/app: a submodule
– foo/business: a submodule
– foo/core: a submodule



And here the main program:

from foo.business.FooClass import FooClass
 
 
def main():
    foo = FooClass()
    input = 'lion'
    output: str = foo.do_foo(input)
    print(f'input={input},output={output}')
 
 
if __name__ == '__main__':
    main()

Type of errors encountered

Error when we execute the application from the root module:
Execute the application in this ways trigger a ModuleNotFoundError:
module_with_sub_module$ python3.9 foo/app/Main.py

Traceback (most recent call last):
  File "/home/david/python-workspace/python_blog_examples/module_with_sub_module/foo/app/Main.py", line 8, in <module>
    from foo.business.FooClass import FooClass

If we print the value of sys.path, we could see that the base module is not inside it, only the module of the executed program/file is in it: /home/david/python-workspace/python_blog_examples/module_with_sub_module/foo/app

sys.path=['/home/david/python-workspace/python_blog_examples/module_with_sub_module/foo/app',
'/usr/lib/python39.zip', '/usr/lib/python3.9', '/usr/lib/python3.9/lib-dynload', '/home/david/.local/lib/python3.9/site-packages', '/usr/local/lib/python3.9/dist-packages', '/usr/lib/python3/dist-packages']

Solution:
Add the working directory, that is also here the project root directory in sys.path.
You have mainly 2 ways to do that:
– Updating sys.path before importing any submodules in the main program:

sys.path.append(os.getcwd())

If the error persists, you can print the value of sys.path.
So the main program could look like finally :

import os
import sys
 
# PYTHONPATH
sys.path.append(os.getcwd())
print(f'after sys.path={sys.path}')
 
from foo.business.FooClass import FooClass
 
 
def main():
    foo = FooClass()
    input = 'lion'
    output: str = foo.do_foo(input)
    print(f'input={input},output={output}')
 
 
if __name__ == '__main__':
    main()

– Updating the PYTHONPATH env variable before launching the program:
export PYTHONPATH=.:${PYTHONPATH}

Error because of missing __init__.py files:
You must add __init__.py in the submodules where you have the import errors.