## 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.

```text
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`.

2. **Install pytest into that exact interpreter.** Run `python -m pip install pytest`.
   - Expected: `Successfully installed pytest-...` plus its dependencies.

3. **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.