I have the following project structure:

- project
  - src
    - module_a
      - foo.py
      - __init__.py
    - runner.py
    - constants.py
    - __init__.py

constants.py:

SOME_CONST = 5

foo.py:

from src.constants import SOME_CONST
class Foo:
    def some_func(self):
        ....

runner.py:

from src.module_a.foo import FOO

def main():
    Foo.some_func()

if __name__ == "__main__":
    main()

Trying to run python runner.py from the src folder gave me a ModuleNotFound about src when foo.py was trying to import from src.constants import SOME_CONST.

I understand that the issue was the python interpreter didn’t recognise the src folder as a module, even though it has an __init__.py file.
After creating a venv virtual environment the issue was resolved.

I have the following questions:

1. Why didn’t the python interpreter recognise the src folder as a module before creating a virtual environment?
2. Why did creating a virtual environment solve the issue?
3. How can I set up the project so that it will run without setting a virtual environment?

Thanks!