ModuleNotFoundError: No module named 'pytest'

TL;DR

Run python -m pip install pytest, then verify with pytest --version. This error means pytest is not installed in the Python environment that is actually running your code, which is usually a different interpreter or virtualenv than the one you installed into.

ModuleNotFoundError: No module named 'pytest'

Fix it

1. Confirm which Python is running. Run which python (or where python on Windows) and python --version. Then run python -m pip show pytest. If pip says it is not installed, that is your answer.

• Expected: pip show pytest prints version info, or WARNING: Package(s) not found: pytest.

1. Install pytest into that exact interpreter. Run python -m pip install pytest.

• Expected: Successfully installed pytest-... plus its dependencies.

1. Verify. Run pytest --version.

• Expected: something like pytest 8.3.4.

When this applies

• You just created a fresh virtualenv or container and forgot to install test deps.
• Your IDE runs a different interpreter than your terminal.
• CI installs dependencies in one step but runs pytest in another without them.

When it does not apply

• If pytest --version works but a plugin fails to load, the problem is the plugin, not pytest itself.
• If the error names a different module (like _pytest.scope), see the version-conflict skills instead.

Tool compatibility

pytest 7.x and 8.x, Python 3.8+. Install via pip: pip install pytest.

Why it happens

Python resolves imports against sys.path of the running interpreter. pip install pytest may have targeted a different interpreter (system python vs venv vs conda), so the running one never sees it. Using python -m pip guarantees the installer and the runner are the same binary.

Edge cases

• On Debian/Ubuntu, pip install pytest may refuse and tell you to use a venv (PEP 668). Create one: python3 -m venv .venv && source .venv/bin/activate.
• If you use conda, prefer conda install pytest inside the active env, or python -m pip install pytest with the env's python.