submodule import problem

[snippsat]

I have read dozens of tutorials on how to make a package. I have read dozens of articles about import and __init__.py and PYTHONPATH and setup.py and every topic that looks like it may point toward and answer. I fear I am missing some vital piece Python understanding and this is so trivial that nobody talks about it.

Can talk a little about package an some links.

Files:

my_pack\
|-- __init__.py
|-- a.py
|-- b.py

__init__.py:

import my_pack.a
import my_pack.b

a.py:

def my_sum(x, y):    
    return x + y

b.py:

import my_pack.a

def average(x, y):
    return my_pack.a.my_sum(x, y) / 2

Now do import work this way import my_pack or from my_pack import a, b.
Use my_sum as sum word is used by Python.
Test:

E:\div_code
λ ptpython
>>> import my_pack

>>> my_pack.a.my_sum(4, 5)
9
>>> my_pack.b.average(4, 5)
4.5

>>> # Using from import
>>> from my_pack import a, b
>>> a.my_sum(9, 99)
108
>>> b.average(9, 99)
54.0

I like to keep imports as simple as possible for users of package eg not use . in import,but can use . internally in a package to eg lift sub-modules.

Think of when using one most used and popular packages in Python Requests.
The main import is simple import requests,
the call function/methods like this requests.get(), requests.post() ...ect.

So how dos Python find this package?
Python find bye looking in sys.path folders,so all module/package which is just files at botton is found by Python trough sys.path.

Some links here lift sub-modules up.
my_pack example

Here more about infrastructure around this,like if want to share at PyPi or make a wheel.
Packaging/Modules--Wheel--pip--setup.py--Freeze