Coverage reporting in Flask - Mini Project: Build & Apply

Coverage Reporting in Flask

📖 Scenario: You are building a small Flask web application and want to check how much of your code is tested by your tests. This helps you find parts of your code that need more testing.

🎯 Goal: Create a Flask app, set up coverage configuration, run tests with coverage, and generate a coverage report.

📋 What You'll Learn

Create a simple Flask app with one route

Add a configuration variable to enable testing mode

Write a test function to test the route

Run coverage to measure test coverage and generate a report

💡 Why This Matters

🌍 Real World

Coverage reporting helps developers see which parts of their web app are tested and which are not, improving code quality.

💼 Career

Understanding coverage reporting is important for software developers and testers to ensure reliable and maintainable code.

1

Create a simple Flask app

Create a Flask app by writing from flask import Flask and then create an app variable with app = Flask(__name__). Add a route / that returns the string 'Hello, Coverage!'.

Flask

from flask import Flask

# Create the Flask app
# Your code here

Hint

Use Flask(__name__) to create the app and @app.route('/') to define the home page.

2

Add testing configuration

Add a configuration variable app.config['TESTING'] = True to enable testing mode in the Flask app.

Flask

from flask import Flask

app = Flask(__name__)

# Enable testing mode
# Your code here

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

Hint

Set app.config['TESTING'] = True right after creating the app.

3

Write a test function for the route

Write a test function called test_home that uses the Flask test client to send a GET request to '/' and asserts the response data equals b'Hello, Coverage!'.

Flask

from flask import Flask

app = Flask(__name__)
app.config['TESTING'] = True

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

# Write test function here
# Your code here

Hint

Use app.test_client() to create a client and send a GET request to '/'.

4

Run coverage and generate report

Add the code to run coverage by importing coverage, starting coverage with cov = coverage.Coverage() and cov.start(), then run the test function test_home(), stop coverage with cov.stop(), save coverage data with cov.save(), and finally generate a report with cov.report().

Flask

from flask import Flask
import coverage

app = Flask(__name__)
app.config['TESTING'] = True

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

def test_home():
    with app.test_client() as client:
        response = client.get('/')
        assert response.data == b'Hello, Coverage!'

# Start coverage, run test, stop coverage, save and report
# Your code here

Hint

Use the coverage module to measure coverage around the test function call.

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