Pytest Coverage Measurement

Purpose

Code coverage measures how much of your code is tested. This skill provides strategies for meaningful coverage measurement and improving test quality.

When to Use This Skill

Use when measuring test coverage with "measure coverage", "track coverage", "identify untested code", or "set coverage thresholds".

Do NOT use for writing tests (use layer-specific testing skills), pytest configuration (use pytest-configuration), or fixing low coverage (identify gaps first, then use appropriate testing skill).

Quick Start

Generate coverage report:

# Generate HTML coverage report
pytest --cov=app --cov-report=html --cov-report=term-missing

# View HTML report
open htmlcov/index.html

# Fail if coverage below threshold
pytest --cov=app --cov-fail-under=80

Instructions

Step 1: Configure Coverage in pyproject.toml

[tool.pytest.ini_options]
addopts = [
    "--cov=app",                        # Source to measure
    "--cov-report=html",                # HTML report
    "--cov-report=term-missing",        # Terminal with missing lines
    "--cov-fail-under=80",              # Fail if < 80%
]

[tool.coverage.run]
source = ["app"]
branch = true  # Measure branch coverage (if/else paths)

omit = [
    "*/tests/*",
    "*/__pycache__/*",
    "*/venv/*",
    "*/.venv/*",
]

[tool.coverage.report]
precision = 2
show_missing = true
skip_covered = false  # Show all files, including 100% covered

# Lines to exclude from coverage
exclude_lines = [
    "pragma: no cover",                 # Manual exclusion
    "def __repr__",                     # Repr methods
    "raise NotImplementedError",        # Abstract methods
    "if TYPE_CHECKING:",                # Type checking only
    "if __name__ == .__main__.:",       # CLI entry points
    "@(abc\\.)?abstractmethod",         # Abstract methods
    "class .*\\bProtocol\\):",          # Protocols
]

[tool.coverage.html]
directory = "htmlcov"  # Output directory

Step 2: Understand Line vs Branch Coverage

from __future__ import annotations


# Line coverage: counts executed lines
# Branch coverage: counts each if/else path

def validate_order(order: Order) -> bool:
    """Example of branch coverage."""
    if not order.line_items:  # Branch 1: True
        return False  # Branch 2: False (2 paths)

    if order.total_price < 0:  # Branch 3: True
        return False  # Branch 4: False (2 more paths)

    return True  # Branch 5: Total 4 unique paths


# Test 1: Only tests the happy path
def test_valid_order():
    order = Order(line_items=[item], total_price=Money(100))
    assert validate_order(order) is True
    # Coverage: 5 lines, 2 branches (50% branch coverage)


# Test 2-5: Cover all paths for 100% branch coverage
def test_empty_items():
    order = Order(line_items=[], total_price=Money(100))
    assert validate_order(order) is False

def test_negative_total():
    order = Order(line_items=[item], total_price=Money(-100))
    assert validate_order(order) is False

def test_valid_order_all_paths():
    order = Order(line_items=[item], total_price=Money(100))
    assert validate_order(order) is True
    # Coverage: 5 lines, 4 branches (100% branch coverage)

Step 3: Set Coverage Targets by Layer

# Domain Layer: 95-100% coverage
# app/extraction/domain/
# app/storage/domain/
# app/reporting/domain/
#
# Pure business logic, no dependencies → easy to test exhaustively


# Application Layer: 85-95% coverage
# app/extraction/application/
# app/storage/application/
# app/reporting/application/
#
# Use cases, orchestration → test main paths, some error paths


# Adapter Layer: 75-85% coverage
# app/extraction/adapters/
# app/storage/adapters/
# app/reporting/adapters/
#
# External integrations → test critical paths, less error paths


# Infrastructure Layer: 60-75% coverage
# app/shared/
# Configuration, setup code → test critical paths only

Step 4: Run Coverage and Analyze Report

# Generate full report
pytest --cov=app --cov-report=html --cov-report=term-missing

# Output shows missing lines:
# Name                     Stmts  Miss  Cover   Missing
# ------------------------------------------------
# app/extraction/domain/entities.py    45     0   100%
# app/extraction/domain/value_objects  20     0   100%
# app/extraction/application/use_cases 60     5    92%   45-47, 89-91
# app/reporting/domain/entities.py     30     0   100%
# ------------------------------------------------
# TOTAL                              500    50    85%

Step 5: Identify Coverage Gaps

# Coverage by directory
pytest --cov=app/extraction --cov-report=term-missing

# Coverage for specific file
pytest --cov=app/extraction/domain --cov-report=term-missing

# View HTML report for interactive analysis
open htmlcov/app_extraction_domain_entities_py.html

# Check branch coverage specifically
pytest --cov=app --cov-report=term-missing:skip-covered

Step 6: Exclude Lines Appropriately

from typing import TYPE_CHECKING

if TYPE_CHECKING:  # pragma: no cover
    # Type checking only imports, never executed at runtime
    from myapp.domain.entities import Order


class BaseRepository(Protocol):
    """Protocol for repositories."""

    def save(self, entity: Entity) -> None:  # pragma: no cover
        """Abstract method, no implementation."""
        ...


def __repr__(self) -> str:  # pragma: no cover
    """Repr method, low value to test."""
    return f"Order(id={self.id})"


if __name__ == "__main__":  # pragma: no cover
    # CLI entry point, tested separately
    main()


@abstractmethod
def abstract_method(self) -> None:  # pragma: no cover
    """Abstract method, no implementation."""
    pass

Step 7: Track Coverage Trends

# Save coverage data to JSON for tracking
pytest --cov=app --cov-report=json

# Then analyze coverage.json to track improvements over time
import json

with open("coverage.json") as f:
    data = json.load(f)

total_coverage = data["totals"]["percent_covered"]
print(f"Total coverage: {total_coverage}%")

# Track per module
for module, coverage in data["files"].items():
    print(f"{module}: {coverage['summary']['percent_covered']}%")

Step 8: Create Coverage Badges and Reports

# Generate coverage report that CI/CD can use
pytest --cov=app --cov-report=xml --cov-report=term

# Upload to coverage tracking services:
# - codecov.io
# - coveralls.io
# - codeclimate.com

Step 9: Enforce Coverage in CI/CD

# GitHub Actions example
name: Tests

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      - name: Install uv
        run: curl -LsSf https://astral.sh/uv/install.sh | sh

      - name: Install dependencies
        run: uv sync

      - name: Run tests with coverage
        run: uv run pytest --cov=app --cov-report=xml --cov-fail-under=80

      - name: Upload coverage to codecov
        uses: codecov/codecov-action@v3
        with:
          files: ./coverage.xml
          fail_ci_if_error: true

Step 10: Coverage-Driven Test Development

# Process:
# 1. Run coverage before writing tests
# 2. Identify untested lines
# 3. Write tests to cover them
# 4. Re-run coverage to verify

from app.extraction.domain.value_objects import ProductTitle

# Before tests:
# ProductTitle: 20 lines, 0% covered

# Run tests:
# pytest --cov=app/extraction/domain --cov-report=term-missing

# See missing lines in output:
# ProductTitle: 20 lines, 5 missing → 75% covered

# Write tests for missing lines:
# - test_valid_title
# - test_title_too_long
# - test_immutability
# - test_equality
# - test_hashing

# After tests:
# ProductTitle: 20 lines, 0 missing → 100% covered

Examples

Example 1: Complete Coverage Configuration

[tool.pytest.ini_options]
addopts = [
    "--strict-markers",
    "--cov=app",
    "--cov-report=html",
    "--cov-report=term-missing",
    "--cov-report=xml",
    "--cov-fail-under=80",
]

[tool.coverage.run]
source = ["app"]
branch = true
omit = [
    "*/tests/*",
    "*/__pycache__/*",
    "*/venv/*",
    "*/.venv/*",
]

[tool.coverage.report]
precision = 2
show_missing = true
skip_covered = false

exclude_lines = [
    "pragma: no cover",
    "def __repr__",
    "raise NotImplementedError",
    "if TYPE_CHECKING:",
    "if __name__ == .__main__.:",
    "@(abc\\.)?abstractmethod",
    "class .*\\bProtocol\\):",
]

[[tool.coverage.paths]]
source = ["app"]
tests = ["tests"]

[tool.coverage.html]
directory = "htmlcov"

[tool.coverage.xml]
output = "coverage.xml"

Example 2: Layer-Specific Coverage Tracking

# Create script to track coverage by layer
import subprocess
import json
from pathlib import Path

def get_coverage_by_module():
    """Get coverage report for each module."""
    result = subprocess.run(
        ["pytest", "--cov=app", "--cov-report=json"],
        capture_output=True,
        text=True,
    )

    with open("coverage.json") as f:
        data = json.load(f)

    # Organize by layer
    layers = {
        "domain": [],
        "application": [],
        "adapters": [],
        "infrastructure": [],
    }

    for module, coverage_data in data["files"].items():
        percent = coverage_data["summary"]["percent_covered"]

        if "domain" in module:
            layers["domain"].append((module, percent))
        elif "application" in module:
            layers["application"].append((module, percent))
        elif "adapters" in module:
            layers["adapters"].append((module, percent))
        else:
            layers["infrastructure"].append((module, percent))

    # Print summary
    for layer_name, modules in layers.items():
        if modules:
            avg = sum(p for _, p in modules) / len(modules)
            print(f"{layer_name}: {avg:.1f}%")
            for module, percent in modules:
                print(f"  {module}: {percent:.1f}%")

if __name__ == "__main__":
    get_coverage_by_module()

Example 3: Identifying Coverage Gaps

# Find uncovered code in specific module
pytest --cov=app/extraction/domain --cov-report=term-missing app/extraction/domain

# View which lines need tests
# app/extraction/domain/entities.py:45 if order.total < 0:
# app/extraction/domain/entities.py:46     raise ValueError()
# app/extraction/domain/entities.py:47
# app/extraction/domain/entities.py:89 except InvalidOrderException:
# app/extraction/domain/entities.py:90     logger.error()
# app/extraction/domain/entities.py:91

# Write tests to cover those lines
def test_negative_total_raises_error():
    """Test line 45-46."""
    with pytest.raises(ValueError):
        Order(..., total_price=Money(-100))

def test_invalid_order_caught():
    """Test line 89-91."""
    # Test code that triggers the exception handler

Requirements

Python 3.11+
pytest >= 7.0
pytest-cov >= 4.0

pytest-coverage-measurement