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Pytest-cov - Code Coverage for Pytest

Pytest-cov is a plugin for pytest that provides code coverage reporting, helping you identify which parts of your code are being tested and which are not.

Overview

Pytest-cov helps improve test quality by:

  • Measuring how much of your code is executed during tests
  • Generating detailed reports on code coverage
  • Identifying untested code paths
  • Supporting branch coverage analysis
  • Integrating seamlessly with pytest
  • Providing various output formats (terminal, HTML, XML)

Installation

Pytest-cov is included as a development dependency:

# Install with other development dependencies
uv sync --dev

To install it directly:

uv pip install pytest-cov

How It's Used in This Project

In this project, Pytest-cov is used to:

  1. Measure test coverage across the codebase
  2. Generate coverage reports as part of the CI/CD pipeline
  3. Identify areas of the code that need more testing
  4. Enforce minimum coverage thresholds

Configuration in This Project

Pytest-cov is configured in the pyproject.toml file:

[tool.pytest.ini_options]
addopts = "--cov=src --cov-report=term --cov-report=html"

This configuration:

  • Measures coverage for code in the src directory
  • Outputs coverage reports to the terminal
  • Generates HTML reports for detailed analysis

Basic Usage

Running Tests with Coverage

To run tests with coverage:

# Run tests with coverage
uv run pytest --cov=src

# Run tests with coverage and generate HTML report
uv run pytest --cov=src --cov-report=html

# Run tests with coverage for specific modules
uv run pytest --cov=src.module1 --cov=src.module2

Common Command-Line Options

# Specify source directories to measure
uv run pytest --cov=src --cov=lib

# Generate different report formats
uv run pytest --cov=src --cov-report=term --cov-report=html --cov-report=xml

# Set minimum coverage threshold
uv run pytest --cov=src --cov-fail-under=90

# Show line numbers of missing coverage
uv run pytest --cov=src --cov-report=term-missing

Examples

Terminal Coverage Report

----------- coverage: platform linux, python 3.11.0-final-0 -----------
Name                    Stmts   Miss  Cover
-------------------------------------------
src/__init__.py             4      0   100%
src/module1.py             26      3    88%
src/module2.py             42      7    83%
src/utils.py               18      2    89%
-------------------------------------------
TOTAL                      90     12    87%

HTML Report

The HTML report provides a detailed view of coverage, including:

  • File-by-file breakdown
  • Line-by-line highlighting of covered and uncovered code
  • Branch coverage information
  • Summary statistics

Coverage Types

Pytest-cov supports different types of coverage measurement:

  1. Statement Coverage: Measures which statements in your code have been executed
  2. Branch Coverage: Measures which possible branches (if/else paths) have been taken
  3. Function Coverage: Measures which functions have been called
  4. Line Coverage: Measures which executable lines have been run

Best Practices

  1. Aim for high coverage: Strive for at least 80-90% code coverage.
  2. Focus on critical paths: Ensure business-critical code has near 100% coverage.
  3. Don't just chase numbers: High coverage doesn't guarantee good tests; focus on test quality too.
  4. Use branch coverage: Enable branch coverage to catch untested conditional paths.
  5. Set minimum thresholds: Use --cov-fail-under to enforce minimum coverage requirements.
  6. Review coverage reports regularly: Identify and address areas with low coverage.
  7. Include coverage in CI: Make coverage checks part of your continuous integration pipeline.

Resources