I am new in python programming and I encounter some problem when I use import statement.
The following is my directory.
package/
    __init__.py
    main.py
    subpackage_a/
        __init__.py
        a.py
        subpackage_b/
            __init__.py
            b.py
       
In package/main.py I use import package.subpackage_a.a to import a.py.
In package/subpackage_a/a.py I try to add package/subpackage_a/subpackage_b to PYTHONPATH by modifying sys.path in a.py and use import b to import b.py.
However, when I run package/main.py, the ModuleNotFoundError appear.
Although changing PYTHONPATH arbitrarily is not a good habit, I still want to know why I can't successfully import package/subpackage_a/subpackage_b/b.py by modifying sys.path in package/subpackage_a/a.py when I run package/main.py. 
Thanks a lot.
 
    