Nested Package: AttributeError: module 'app' has no attribute

I'm just learning python, coming from a C# & Java background, and I'm pretty confused by the import system. Just trying to run a simple test for learning purposes, but getting an error AttributeError: module 'app' has no attribute 'example'

See code below, can someone explain why the error is thrown? I only seem to encounter this issue when there's a package inside a package, as seen in the instance of package "example" contained inside package "app"

run.py

import app

app/__init__.py

import app.example

app/example/__init__.py

import app.example.a

app/example/a.py

import app.example.b
print("TESTING: " + str(app.example.b)) #error is thrown on this line

app/example/b.py

print("LOADED B")

Error Thrown:

"C:\Program Files\Python 3.5\npwc-services\Scripts\python.exe" C:/Users/xxxxxxxx/PycharmProjects/untitled1/run.py
Traceback (most recent call last):
LOADED B
  File "C:/Users/xxxxxxxx/PycharmProjects/untitled1/run.py", line 1, in <module>
    import app
  File "C:\Users\xxxxxxxx\PycharmProjects\untitled1\app\__init__.py", line 1, in <module>
    import app.example
  File "C:\Users\xxxxxxxx\PycharmProjects\untitled1\app\example\__init__.py", line 1, in <module>
    import app.example.a
  File "C:\Users\xxxxxxxx\PycharmProjects\untitled1\app\example\a.py", line 2, in <module>
    print("TESTING: " + str(app.example.b))
AttributeError: module 'app' has no attribute 'example'

Process finished with exit code 1

Directory Structure: