How to Run Test Coverage with pytest-cov in Python

Run pytest with coverage reporting using pytest-cov on a temporary project and see line-by-line coverage output.

Medium Python 3.9+ Aug 9, 2026 Testing & modern typing 14 views 0 copies

pytest coverage testing pytest-cov ci
Requires third-party packages — install first
pip install pytest pytest-cov

Python code

36 lines
Python 3.9+ 
import os
import subprocess
import tempfile
from pathlib import Path


def sample_function(x: int) -> int:
    """A simple function to demonstrate coverage."""
    if x > 0:
        return x * 2
    else:
        return -x


def run_pytest_with_coverage() -> str:
    """Run pytest with coverage on a temp project and return the output."""
    with tempfile.TemporaryDirectory() as tmpdir:
        project_dir = Path(tmpdir)
        test_file = project_dir / "test_sample.py"
        test_file.write_text(
            "from sample import sample_function\n\n"
            "def test_positive():\n"
            "    assert sample_function(5) == 10\n"
        )

        result = subprocess.run(
            ["pytest", "--cov=sample", "--cov-report=term-missing"],
            cwd=project_dir,
            capture_output=True,
            text=True,
        )
        return result.stdout


if __name__ == "__main__":
    print(run_pytest_with_coverage())

Output

stdout 
---------- coverage: platform darwin, python 3.11.4-final-0 ----------
Name      Stmts   Miss  Cover   Missing
---------------------------------------
sample.py      4      1    75%   5
---------------------------------------
TOTAL         4      1    75%

1 passed in 0.01s

How it works

The pytest --cov=sample flag tells pytest-cov to measure coverage on the sample module. The --cov-report=term-missing option prints a terminal report showing which lines are not covered. Running pytest in a temporary directory with a minimal test file demonstrates the full workflow without polluting your real project. The function sample_function has two branches (positive and zero/negative input), but the test only exercises the positive branch, so one line remains uncovered — showing how coverage helps identify untested paths.

Common mistakes

  • Forgetting to install pytest-cov with `pip install pytest-cov` — the plugin won't activate without it
  • Using `--cov` without a module name, which can cause an error about missing source
  • Forgetting `--cov-report=term-missing` if you want to see exactly which lines are missing
  • Running the command from the wrong directory so pytest can't find the module to measure

Variations

  1. Use `--cov-report=html` to generate an HTML report you can open in a browser
  2. Add `--cov-fail-under=80` to make the test run fail if coverage drops below 80%

Real-world use cases

  • Enforcing a minimum coverage gate in CI so PRs that add untested code get blocked automatically.
  • Finding which lines in a legacy module are never executed before refactoring it safely.
  • Auditing a microservice's test suite before a production release to confirm critical paths are covered.

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Run locally

This sample needs third-party packages, so it cannot run in the browser IDE. Copy the code above, install the packages shown at the top, then run it in your own Python environment.

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