pytest-cov for coverage

Introduction

pytest-cov helps you see how much of your code is tested. It shows which parts are checked and which are not.

You want to check if your tests cover all important parts of your program.

Before sharing your code, to make sure tests are thorough.

When adding new features, to confirm tests cover them.

To find untested code that might cause bugs.

To improve test quality by focusing on uncovered code.

Syntax

PyTest

pytest --cov=your_package tests/

Replace your_package with the folder or module you want coverage for.

tests/ is the folder where your test files are located.

Examples

Runs tests in tests/ and measures coverage for the myapp package.

PyTest

pytest --cov=myapp tests/

Shows coverage report in the terminal and highlights missing lines.

PyTest

pytest --cov=myapp --cov-report=term-missing tests/

Generates an HTML report you can open in a browser to see coverage details.

PyTest

pytest --cov=myapp --cov-report=html tests/

Sample Program

This example has a simple module with two functions: add and subtract. The test only checks add. Running pytest with coverage will show subtract is not tested.

PyTest

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

# test_sample.py
import pytest
from sample import add

def test_add():
    assert add(2, 3) == 5

Important Notes

Install pytest-cov with pip install pytest-cov before using it.

Use --cov-report=term-missing to see exactly which lines are not covered.

HTML reports help visualize coverage in a friendly way.

Summary

pytest-cov shows how much of your code is tested.

Run tests with pytest --cov=your_package to get coverage info.

Use reports to find untested code and improve your tests.

Practice

1. What is the main purpose of using pytest-cov in your testing workflow?

easy

A. To generate test data automatically

B. To speed up test execution time

C. To automatically fix failing tests

D. To measure how much of your code is covered by tests

Solution

1. Step 1: Understand pytest-cov functionality

   pytest-cov is a plugin that measures code coverage during test runs.

2. Step 2: Identify the main goal of coverage tools

   Coverage tools show which parts of code are tested and which are not.

3. Final Answer:

   To measure how much of your code is covered by tests -> Option D

4. Quick Check:

   pytest-cov = code coverage measurement [OK]

Hint: Coverage means how much code tests run over [OK]

Common Mistakes:

• Confusing coverage with test speed
• Thinking pytest-cov fixes tests automatically
• Assuming pytest-cov generates test data

2. Which command correctly runs pytest with coverage for a package named myapp?

easy

A. pytest -cov myapp

B. pytest --cov=myapp

C. pytest --coverage myapp

D. pytest --cov tests/

Solution

1. Step 1: Recall correct pytest-cov syntax

   The correct option to enable coverage is --cov=package_name.

2. Step 2: Compare options

   pytest --cov=myapp uses --cov=myapp, which is the correct syntax. pytest --cov tests/ would incorrectly target the tests directory instead of the myapp package.

3. Final Answer:

   pytest --cov=myapp -> Option B

4. Quick Check:

   Use --cov=package [OK]

Hint: Use equal sign with --cov like --cov=package [OK]

Common Mistakes:

• Targeting tests directory instead of the package
• Using -cov instead of --cov
• Using --coverage instead of --cov

3. Given this command run:
pytest --cov=myapp tests/test_module.py
What output will you expect related to coverage?

medium

A. A report showing coverage percentage for myapp package

B. A list of all tests run without coverage info

C. An error saying coverage option is invalid

D. Coverage report for tests directory only

Solution

1. Step 1: Understand coverage target

   The --cov=myapp option measures coverage for the myapp package, not tests.

2. Step 2: Predict output

   Running tests in tests/test_module.py with coverage enabled will show coverage report for myapp code exercised by those tests.

3. Final Answer:

   A report showing coverage percentage for myapp package -> Option A

4. Quick Check:

   --cov=package shows coverage for that package [OK]

Hint: Coverage reports show tested code, not test files [OK]

Common Mistakes:

• Expecting coverage for test files instead of source code
• Thinking coverage option causes errors
• Confusing coverage report with test list output

4. You ran pytest --cov=myapp but no coverage report appeared. What is the most likely cause?

medium

A. You forgot to install pytest-cov plugin

B. You ran pytest without any test files

C. Your code has no functions to cover

D. You used wrong package name in --cov option

Solution

1. Step 1: Check plugin installation

   pytest-cov must be installed to enable coverage reporting.

2. Step 2: Understand symptoms

   If no coverage report appears, the plugin is likely missing or not activated.

3. Final Answer:

   You forgot to install pytest-cov plugin -> Option A

4. Quick Check:

   pytest-cov missing = no coverage report [OK]

Hint: Install pytest-cov to get coverage reports [OK]

Common Mistakes:

• Assuming coverage works without plugin installed
• Thinking coverage depends on code content
• Ignoring plugin installation errors

5. You want to generate an HTML coverage report for your package app using pytest-cov. Which command should you run?

hard

A. pytest --coverage=app --report=html

B. pytest --cov=app --html=coverage

C. pytest --cov=app --cov-report=html

D. pytest --cov app --html-report

Solution

1. Step 1: Recall pytest-cov report syntax

   The option --cov-report=html generates an HTML coverage report.

2. Step 2: Match correct command format

   pytest --cov=app --cov-report=html correctly uses --cov=app and --cov-report=html together.

3. Final Answer:

   pytest --cov=app --cov-report=html -> Option C

4. Quick Check:

   Use --cov-report=html for HTML coverage [OK]

Hint: Use --cov-report=html to get HTML coverage output [OK]

Common Mistakes:

• Using --html instead of --cov-report=html
• Omitting equal signs in options
• Confusing coverage options with test report options