Python "ModuleNotFoundError: No module named X" in CI

Python could not import a module. Either it was never installed for the interpreter running your code, or your own package layout puts it somewhere not on sys.path.

What this error means

The program or test run aborts with ModuleNotFoundError: No module named X. It can be a third-party package (not installed) or one of your own modules (not importable from where it runs).

Python traceback

Traceback (most recent call last):
  File "app/main.py", line 2, in <module>
    from app.utils import helper
ModuleNotFoundError: No module named 'app'

Common causes

Package not installed for this interpreter

A third-party import fails because it was installed into a different interpreter or venv than the one running the code.

Your own package is not on sys.path

For first-party imports like from app..., the project root is not on sys.path - usually because the package was not installed (pip install -e .) and CI runs from a different directory.

Missing init.py or wrong layout

A directory that should be an importable package may be missing an __init__.py, or the src-layout means the package only resolves after an editable install.

How to fix it

Install the dependency into the active interpreter

Terminal

python -m pip install <package>
python -c "import <package>; print(<package>.__file__)"

Install your project so first-party imports resolve

An editable install puts your package on sys.path regardless of working directory.