pytest-cov for coverage - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding pytest-cov output summary

You run pytest with pytest-cov on a project with 3 Python files. The coverage report shows 85% coverage overall. What does this mean?

A 85% of all lines in the 3 files were executed during tests

B 85% of the tests passed successfully

C 85% of the functions in the project have tests

D 85% of the test files were run by pytest

💡 Hint

Coverage measures how much code was run by tests, not test pass rate.

pytest-cov measures how many lines of code were executed during tests. 85% coverage means 85% of all lines in the files ran at least once.

❓ assertion

intermediate

Correct assertion to check coverage threshold

You want your pytest-cov to fail tests if coverage is below 90%. Which assertion in your pytest test is correct?

A assert coverage_percent > 90

B assert coverage_percent >= 90

C assert coverage_percent == 90

D assert coverage_percent <= 90

💡 Hint

You want to allow 90% or more coverage, not just exactly 90%.

The assertion should check coverage is at least 90%, so using >= 90 is correct.

🔧 Debug

advanced

Why does pytest-cov report 0% coverage?

You run pytest --cov=myapp tests/ but the coverage report shows 0% coverage. What is the most likely cause?

A The tests folder is empty

B pytest-cov is not installed correctly

C The <code>myapp</code> package is not imported or used in tests

D The coverage report file is corrupted

💡 Hint

Coverage measures executed code. If code is never run, coverage is zero.

If the tested package is not imported or run during tests, coverage will be zero because no code was executed.

❓ framework

advanced

Configuring pytest-cov to generate HTML report

Which pytest command correctly runs tests with coverage and generates an HTML coverage report in the htmlcov folder?

A pytest --cov=myapp --cov-report=html

B pytest --cov=myapp --html=htmlcov

C pytest --coverage=myapp --report=htmlcov

D pytest --cov-report=html --cov=myapp --output=htmlcov

💡 Hint

The option for HTML report is --cov-report=html.

The correct syntax is --cov-report=html to generate an HTML report in the default htmlcov folder.

🧠 Conceptual

expert

Interpreting branch coverage in pytest-cov

pytest-cov shows 75% branch coverage but 90% line coverage. What does this difference mean?

A Branch coverage counts test cases, line coverage counts lines

B Only 75% of lines were executed, but 90% of branches were tested

C Branch coverage is always lower than line coverage by definition

D Some decision points (if/else) were not fully tested, even though most lines ran

💡 Hint

Branch coverage checks if all possible paths in decisions ran.

Branch coverage measures if all possible paths in code decisions were tested. 75% means some if/else paths were missed, even if 90% of lines ran.

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