Coverage reporting in Flask

Introduction

Coverage reporting helps you see which parts of your Flask app's code are tested. It shows what is covered by tests and what is not.

You want to check if your Flask app has enough tests.

You want to find untested parts of your code to improve quality.

You want to measure test progress during development.

You want to ensure critical routes and functions are tested before release.

Syntax

Flask

coverage run -m pytest
coverage report
coverage html

coverage run -m pytest runs tests while tracking coverage.

coverage report shows coverage summary in the terminal.

coverage html creates a detailed HTML report you can open in a browser.

Examples

Run your Flask tests with coverage tracking.

Flask

coverage run -m pytest

See a summary of which files and lines were tested.

Flask

coverage report

Generate a detailed HTML report to view coverage visually.

Flask

coverage html

Sample Program

This Flask app has two routes: '/' and '/about'. The test only checks the '/' route. Running coverage will show that '/about' is not tested.

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello, world!'

@app.route('/about')
def about():
    return 'About page'

# test_app.py
import pytest

from app import app

@pytest.fixture
def client():
    with app.test_client() as client:
        yield client

def test_home(client):
    response = client.get('/')
    assert response.data == b'Hello, world!'

# Run coverage commands in terminal:
# coverage run -m pytest
# coverage report
# coverage html

Important Notes

Make sure to install coverage with pip install coverage.

Run coverage commands from your project root where your Flask app and tests are.

Use the HTML report to easily see which lines are missing tests.

Summary

Coverage reporting shows which parts of your Flask app are tested.

Use coverage run -m pytest to run tests with coverage.

Check results with coverage report or coverage html.

Practice

1. What is the main purpose of coverage reporting in a Flask application?

easy

A. To improve the speed of the Flask app

B. To deploy the Flask app to a server

C. To show which parts of the app are tested by your tests

D. To create a user interface for the app

Solution

1. Step 1: Understand coverage reporting

   Coverage reporting tracks which parts of your code are executed during tests.

2. Step 2: Identify its purpose in Flask

   It helps you see which parts of your Flask app have tests and which do not.

3. Final Answer:

   To show which parts of the app are tested by your tests -> Option C

4. Quick Check:

   Coverage reporting = shows tested parts [OK]

Hint: Coverage shows tested code parts, not speed or deployment [OK]

Common Mistakes:

• Confusing coverage with app speed optimization
• Thinking coverage deploys the app
• Assuming coverage creates UI

2. Which command correctly runs tests with coverage in a Flask project?

easy

A. pytest coverage run

B. coverage run -m pytest

C. flask coverage run

D. coverage test run

Solution

1. Step 1: Recall the coverage run syntax

   The correct command to run tests with coverage is 'coverage run -m pytest'.

2. Step 2: Check other options

   Other options mix commands incorrectly or use invalid syntax.

3. Final Answer:

   coverage run -m pytest -> Option B

4. Quick Check:

   Correct coverage test run command = coverage run -m pytest [OK]

Hint: Use 'coverage run -m pytest' to run tests with coverage [OK]

Common Mistakes:

• Swapping order of coverage and pytest
• Using 'flask' command with coverage incorrectly
• Typing 'coverage test run' which is invalid

3. Given you ran coverage run -m pytest successfully, which command shows a text summary of coverage results?

medium

A. coverage report

B. pytest --coverage

C. coverage html

D. flask coverage report

Solution

1. Step 1: Identify commands for coverage output

   'coverage report' shows a text summary in the terminal, while 'coverage html' creates a web page report.

2. Step 2: Eliminate incorrect options

   'pytest --coverage' is not a standard command, and 'flask coverage report' is invalid.

3. Final Answer:

   coverage report -> Option A

4. Quick Check:

   Text summary command = coverage report [OK]

Hint: Use 'coverage report' for text summary, 'coverage html' for web report [OK]

Common Mistakes:

• Confusing 'coverage html' with text output
• Using invalid commands like 'pytest --coverage'
• Trying 'flask coverage report' which doesn't exist

4. You ran coverage run -m pytest but coverage report shows 0% coverage. What is the likely cause?

medium

A. Tests did not execute any Flask app code

B. You forgot to install Flask

C. Coverage report command is misspelled

D. You ran coverage without pytest

Solution

1. Step 1: Understand 0% coverage meaning

   0% coverage means no code was executed during tests, so coverage tool found no lines run.

2. Step 2: Analyze possible causes

   If tests ran but did not call Flask app code, coverage stays zero. Missing Flask or misspelling commands would cause errors, not zero coverage.

3. Final Answer:

   Tests did not execute any Flask app code -> Option A

4. Quick Check:

   0% coverage = no app code run in tests [OK]

Hint: 0% means tests ran but didn't touch app code [OK]

Common Mistakes:

• Assuming 0% means Flask is not installed
• Thinking coverage report command typo causes 0%
• Believing running coverage without pytest causes 0%

5. You want to generate a detailed HTML coverage report for your Flask app after running tests. Which sequence of commands is correct?

hard

A. flask coverage run; coverage html; open html/index.html

B. pytest run coverage; coverage report; open index.html

C. coverage test run; coverage html; open coverage/index.html

D. coverage run -m pytest; coverage html; open htmlcov/index.html

Solution

1. Step 1: Run tests with coverage

   Use 'coverage run -m pytest' to run tests and collect coverage data.

2. Step 2: Generate HTML report and open it

   Run 'coverage html' to create the report in 'htmlcov' folder, then open 'htmlcov/index.html' to view it.

3. Step 3: Verify other options

   Other options use invalid commands or wrong paths for report files.

4. Final Answer:

   coverage run -m pytest; coverage html; open htmlcov/index.html -> Option D

5. Quick Check:

   Run tests, generate HTML, open htmlcov/index.html [OK]

Hint: Run tests, then 'coverage html', then open htmlcov/index.html [OK]

Common Mistakes:

• Using wrong command order or invalid commands
• Opening wrong file path for HTML report
• Confusing 'pytest run coverage' with correct syntax