Coverage reporting in Flask - Step-by-Step Execution

Concept Flow - Coverage reporting

Write Flask app code

↓

Write test functions

↓

Run tests with coverage tool

↓

Coverage tool tracks executed lines

↓

Generate coverage report

↓

Review report to find untested code

↓

Improve tests and repeat

This flow shows how coverage reporting tracks which parts of your Flask app code run during tests, helping you find untested code.

Execution Sample

Flask

from flask import Flask
app = Flask(__name__)

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

# Run tests with coverage

A simple Flask app with one route; coverage tracks which lines run when tests call this route.

Execution Table

Step	Action	Code Line	Coverage Status	Notes
1	Start coverage tool	N/A	No lines executed yet	Coverage tool initialized
2	Run test calling '/' route	def home():	Executed	Function home is called
3	Return response	return 'Hello, world!'	Executed	Return statement runs
4	Test ends	N/A	No new lines executed	Test finished
5	Generate coverage report	All lines	home() lines: covered; others: not covered	Report shows tested and untested lines

💡 Tests complete; coverage report generated showing which lines ran

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
coverage_status	Not started	Tracking started	Lines in home() marked executed	Report generated with coverage info

Key Moments - 2 Insights

Why does coverage report show some lines as not executed even though the app runs?

Because coverage only tracks lines run during tests; if tests don't call certain routes or functions, those lines remain unexecuted as shown in the execution_table step 5.

Does coverage track code outside of test runs?

No, coverage only tracks code executed during the test run session, as seen in execution_table steps 2-4.

Visual Quiz - 3 Questions

Look at the execution table, which step marks the function 'home' as executed?

A Step 2

B Step 1

C Step 5

D Step 4

💡 Hint

Check the 'Coverage Status' column in step 2 where 'def home()' is marked executed.

At which step does the coverage tool generate the report?

A Step 3

B Step 5

C Step 4

D Step 2

💡 Hint

Look for the step mentioning 'Generate coverage report' in the 'Action' column.

If a test does not call the '/' route, how would the coverage status change in the report?

A Lines in home() would be marked as executed

B Coverage tool would fail to run

C Lines in home() would be marked as not executed

D All lines would be marked executed

💡 Hint

Refer to the 'Coverage Status' in step 5 showing lines not executed if tests skip them.

Concept Snapshot

Coverage reporting in Flask:
- Write your Flask app and tests.
- Run tests with coverage tool (e.g., coverage.py).
- Coverage tracks which lines run during tests.
- Generate report to see tested vs untested code.
- Use report to improve test coverage.

Full Transcript

Coverage reporting in Flask helps you see which parts of your app code run during tests. You write your Flask app and test functions. When you run tests with a coverage tool, it tracks each line executed. After tests finish, the tool generates a report showing which lines were tested and which were not. This helps you find untested code so you can add more tests. The process starts with writing code, running tests with coverage, tracking executed lines, generating the report, and then improving tests based on the report.