module 'sample_project' has no attribute 'data'

Question

I thought I understand python import system, but it looks like I was wrong. For some reason, I cannot use modules from my package unless I explicitly import them in the __init__.py or in the script code. This is not what the documentation says and I have other projects (e.g., this one) that don't have this kind of issue.

To illustrate it, I created a super simple example. A project with a following structure:

sample_project
├── sample_project
│   ├── __init__.py
│   └── data.py
└── setup.py

setup.py looks like this

import setuptools

setuptools.setup(name='sample_project', packages=setuptools.find_packages()) 

and data.py like this

class Data:
    pass

__init__.py is empty.

After pip install -e . I can import sample_project, but sample_project.data throws

AttributeError: module 'sample_project' has no attribute 'data'

Note that data.py is not inside the submodule, so the issue is different from this one. This happens either if I am in the project directory or outside it.

I can fix it via import sample_project.data either in my script or in the __init__.py, but I don't understand why it does not work. It feels like the default __all__ does not work for some reason.

In the project I mentioned above I have multiple python files inside new_semantic_parsing directory. Some of them are imported in the __init__.py, some are not, but I don't have this problem with either of them.

E.g.

import new_semantic_parsing
new_semantic_parsing.utils

works fine even though utils is not explicitly imported in the __init__.py.

Clearly, there is an issue somewhere, but I can't see it.