package script cant find sibling script when executed from outside

[snippsat]

As you see all files as you describe.

C:\Python310\mymodule
λ ls
__init__.py  constants.py  function.py  test.py 

my_func() will never work have to change to this.

#function.py
from mymodule.constants import my_const

def my_func():
    print(my_const)

Now can run test.py,and it will work.

C:\Python310\mymodule
λ python test.py
42

A little more about how a package works.

C:\Python310\mymodule
λ ptpython
>>> import mymodule
>>>
>>> dir(mymodule)
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__'

As see here so is nothing added to namespace if just import like this.
I like to modify so can just to simple import like this and files are added.
In __init__.py

import mymodule.constants
import mymodule.function

Test again.

C:\Python310\mymodule
λ ptpython
>>> import mymodule
>>>
>>> dir(mymodule)
['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__', 'constants', 'function', 'mymodule']

>>> mymodule.constants.my_const
42

>>> mymodule.function.my_func()
42

Or can import like this without modify __init__.py.
This will bring constants and function into the namespace of package.

C:\Python310\mymodule
λ ptpython
>>> from mymodule import constants, function
>>> constants.my_const
42
>>> function.my_func()
42

---

Some more examples you can look in link in this post.