What is the correct folder structure to use for a Python project using pytest?

Question

I try to organized my Python projects using a folder structure. When I need to make tests I use something like the following.

.
|-- src
|   |-- b.py
|   `-- main.py
`-- tests
    `-- test_main.py

There is just one big problem with this approach. Pytest won't run if main.py is importing b.py.

So far I've tried placing empty __init__.py files inside the src and tests folders, both independently and together, but any of those seems to work.

It seems to me this is a pretty standard project, but I haven't been able to find a solution online. Should I use a different folder structure? Is there any recommended way to use pytest with this kind of projects?

---

This are the contents of the files:

# b.py
def triplicate(x):
        return x * 3

# main.py
from b import triplicate

def duplicate(x):
        return x * 2

# test_main.py
from src.main import duplicate


def test_duplicate():
        assert duplicate(2) == 4

And this is the error I get when running pytest:

==================================================================================================== ERRORS ====================================================================================================
_____________________________________________________________________________________ ERROR collecting tests/test_main.py ______________________________________________________________________________________
ImportError while importing test module 'C:\Users\edwar\test_pytest\tests\test_main.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
c:\python39\lib\importlib\__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests\test_main.py:1: in <module>
    from src.main import duplicate
src\main.py:1: in <module>
    from b import triplicate
E   ModuleNotFoundError: No module named 'b'
=========================================================================================== short test summary info ============================================================================================
ERROR tests/test_main.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
=============================================================================================== 1 error in 0.15s ===============================================================================================

Answer 1

Python uses the 'environment variable' PYTHONPATH to look for sources to import code from. By default, the directory you execute a python program is automatically included, but you want to include something like this when you test:

PYTHONPATH=$PYTHONPATH,../src python test_main.py

This is if you're executing a test from the source directory. Tools like IntelliJ (PyCharm) will let you add this as a value in your test invocation. Alternatively you can use export PYTHONPATH=.... (Note this is for a *nix environment, your mileage on windows may vary.)

The upshot is that every directory in PYTHONPATH will be loaded and Python will attempt to use it as a 'root' for modules you try to import. Your basic directory structure is the most idiomatic.

• See this answer for more on configuring PYTHONPATH correctly.
• See this doc for more about how the PYTHONPATH is modified and used 'under the hood'.
• See this answer for options to include the src directory when running pytest tests.
• See this blog post about using autoenv (a Python library) to enable the usage of .env files to manage this for you (at least within a virtualenv setup - a good idea generally).
• setup.py is also idiomatic for including many modules, and may provide a more convenient path for the situation you're handling.