pytest-cov for coverage - Test Execution Trace

Test Overview

This test runs a simple function and uses pytest-cov to check how much of the code is covered by the test. It verifies that the coverage report shows 100% coverage for the tested function.

Test Code - pytest

PyTest

import pytest

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

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

# To run coverage: pytest --cov=./ --cov-report=term-missing

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts by running pytest with coverage plugin enabled	Terminal ready to run pytest command	—	PASS
2	pytest discovers test_add function in test file	pytest test collection complete	—	PASS
3	pytest runs test_add which calls add(2, 3)	Function add executes and returns 5	assert add(2, 3) == 5	PASS
4	pytest-cov collects coverage data during test run	Coverage data recorded for add function	—	PASS
5	pytest finishes and prints coverage report showing 100% coverage for add function	Terminal displays coverage summary with no missing lines	Coverage report shows 100% coverage	PASS

Failure Scenario

Failing Condition: Test fails if add function returns wrong result or coverage is incomplete

Execution Trace Quiz - 3 Questions

What does pytest-cov do during the test run?

A It records which lines of code were executed

B It runs tests faster

C It changes the test results

D It skips tests without coverage

Key Result

Using pytest-cov helps you see exactly which parts of your code are tested, so you can improve your tests and avoid missing important cases.

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