Solve pytest import error (while python can)

After refactoring a formerly monolithic python script into several files, I started getting problems related to module imports. I shifted the source code into a package directory, and the test/ directory was now parallel to the package source directory. The full directory layout was like this:

<packagename>
|- <packagename>/
|  |- __init__.py
|  |- module_1.py
|  `- ...
|- test/
|  |- __init__.py
|  |- test_module1.py
|  `- ...
|- __init__.py
|- README.md
`- packagename.py

When running “pytest <packagename>”, I got:

ImportError while importing test module '...'

However, when running “packagename.py” (executable python script via shebang), everything worked fine. Both the test modules and the main script contained the same import statement:

from packagename import module_1

A google search turned up: https://stackoverflow.com/questions/41748464/pytest-cannot-import-module-while-python-can, which pointed me in the direction that the issue must be related to __init__.py

However, the solution was not to delete __init__.py in the test folder, but to delete __init__.py in the main package folder. I came to this solution based on the documentation of pytest: https://docs.pytest.org/en/latest/explanation/goodpractices.html#tests-as-part-of-application-code

The working layout is:

<packagename>
|- <packagename>/
|  |- __init__.py
|  |- module_1.py
|  `- ...
|- test/
|  |- __init__.py
|  |- test_module1.py
|  `- ...
|- README.md
`- packagename.py

Disclaimer:

I do not want to advertise this directory layout for python packages. In my use case, I have a python script inside a very large repository that is not distributed as a package outside of this repository; I just wanted to improve maintainability.
When you want to create a python package for distribution, you’re faced with many other considerations. There are two competing package structure styles: With or without the package sources inside a “src/” folder. The python packing guide advertises the “src/” folder layout. See https://packaging.python.org/en/latest/tutorials/packaging-projects/#a-simple-project for an example of “<packagename>/src/<packagename>” layout. If in doubt, go with one of the more popular cookiecutter templates.