What is test coverage and how to measure it?

Question

Accepted Answer

**Test coverage** measures **how much code** is executed during tests.

```bash
# Install
pip install pytest-cov

# Run with coverage
pytest --cov=src --cov-report=html

# Terminal report
pytest --cov=src --cov-report=term-missing

# .coveragerc configuration
[run]
source = src
omit = */tests/*, */migrations/*

[report]
fail_under = 80
show_missing = true
exclude_lines =
    pragma: no cover
    def __repr__
    if __name__ == "__main__":
```

```python
# src/module.py
def calculate(a, b):
    return a + b  # Covered by test

def unused():
    pass  # Not covered

# tests/test_module.py
def test_calculate():
    assert calculate(2, 3) == 5
```

**Output:**
```
Name              Stmts   Miss  Cover
-------------------------------------
src/module.py          4      1    75%
-------------------------------------
TOTAL                  4      1    75%
```

Python: What is test coverage and how to measure it?