import package.b as b
print(b.average(2, 4))

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

import a
def average(x, y):
    return a.sum(x, y) / 2

Output:
program.py
package\
    __init__.py   <-- Empty
    a.py
    b.py

from . import a
def average(x, y):
    return a.sum(x, y) / 2

Error:
File "....\b.py" line 1 in <module>
    from . import a
ImportError: attempted relative import with no known parent package

Output:
python -m package.b

Quote: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.

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

import my_pack.a
import my_pack.b

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

import my_pack.a

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

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