package script cant find sibling script when executed from outside

[Bock]

Hello together,
i just finished my first own package but facing a problem when importing and executing things from outside:
I made a very minimal example as demonstration:

Folder "mymodule" contains:

#__init__.py
# empty

#constants.py
my_const = 42

#function.py
def my_func():
       print(my_const)

On the same level as mymodule is :

# test.py
from mymodule.function import my_func
my_func()

Unfortunately i get the error message: "no module named 'constant' " which confuses me since i can import constant in function.py properly.

Question: Why is that? ( Maybe this code should that never be necessary - let me know anything you have in mind :)

Thank you very much,
Bock

PS: how can i make a "tab" in the python brackets of the thread editor?