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03 Testing Fundamentals: Coverage Self-Study Guide

December 2025 (986 Words, 6 Minutes)

self-study coverage testing c0 c1 pytest-cov ci-cd chapter-03

Code Coverage Self-Study Guide

Welcome

This guide helps you master code coverage concepts and tools through structured self-study. The materials are designed for two 90-minute sessions, but you can work at your own pace.

What you’ll learn:

What code coverage measures and why it matters
The difference between Statement (C0) and Branch (C1) coverage
How to read and interpret coverage reports from pytest-cov
How to systematically design tests to improve coverage
The limitations of coverage (why 100% doesn’t mean bug-free)

Learning Path Overview

Note on time estimates: The durations listed below are for orientation only. There’s no need to skip an exercise just because the suggested time has passed—take the time you need to understand the material. Likewise, if you finish faster, feel free to move on without waiting.

Session 1: Theory & Analysis (~90 min)

Step	Exercise	Duration	Focus
1	Read this guide	10 min	Orientation
2	Coverage Concepts Quiz	25 min	Test understanding of C0, C1, subsumption
3	CFG Tracing Exercise	35 min	Trace test cases through Control Flow Graphs
4	Coverage Detective Part 1	20 min	Analyze coverage reports, identify gaps

Session 2: Practice & Application (~90 min)

Step	Exercise	Duration	Focus
5	Coverage Detective Part 2	25 min	Design tests to cover missing branches
6	GitHub Classroom Assignment	65 min	Hands-on: improve coverage on real code

How to Master Each Exercise

Before attempting each exercise, read the specified lecture sections. Click the links to jump directly to the relevant material.

Coverage Concepts Quiz

Read first:

From Chapter 03 (TDD and CI): TDD and CI:

Part 2: The Coverage Problem - Why we need coverage
Part 3: Setting Up Coverage Reporting - pytest-cov basics
Part 4: Integrating Coverage into CI - Thresholds and automation

From Chapter 03 (Testing Theory and Coverage): Testing Theory & Coverage:

Section 3: The Testing Theory - Program, Model, Test Suite, Coverage Criterion
Section 4: Code Coverage (C0 and C1) - Statement vs Branch coverage, Subsumption

Focus areas: Definitions, subsumption hierarchy, coverage limitations

CFG Tracing Exercise

Read first:

From Chapter 03 (Testing Theory and Coverage): Testing Theory & Coverage:

Section 4: Code Coverage (C0 and C1) - CFG examples, node/edge counting
Section 5: Systematic Test Design for Branch Coverage - Deriving tests from CFGs

Focus areas: Reading CFGs, tracing paths, calculating C0 and C1

Coverage Detective Exercise

Read first:

From Chapter 03 (TDD and CI): TDD and CI:

Part 3: Setting Up Coverage Reporting - Understanding --cov-report=term-missing

From Chapter 03 (Testing Theory and Coverage): Testing Theory & Coverage:

Section 5: Systematic Test Design - Designing tests for specific branches

Focus areas: Interpreting “Missing” lines, mapping lines to conditions, designing targeted tests

GitHub Classroom Assignment

Assignment link: Accept the Coverage Improvement Exercise

Read first:

From Chapter 03 (TDD and CI): TDD and CI:

Part 4: Integrating Coverage into CI - --cov-fail-under and GitHub Actions

Prerequisites for this exercise:

Python 3.12+ installed
uv package manager installed
Git configured and GitHub access

Focus areas: Running coverage locally, achieving threshold, CI feedback loop

Key Concepts Summary

Statement Coverage (C0)

The percentage of code statements executed by your tests.

def classify(x):
    if x > 0:           # Statement 1
        return "pos"    # Statement 2
    else:
        return "neg"    # Statement 3

Testing only classify(5) → C0 = 2/3 = 67%

Branch Coverage (C1)

The percentage of decision outcomes (True/False branches) executed.

Testing only classify(5) → C1 = 1/2 = 50% (only True branch taken)

Why C1 Subsumes C0

100% Branch Coverage guarantees 100% Statement Coverage because covering all branches requires executing all statements within those branches.

But the reverse is NOT true! High C0 doesn’t guarantee high C1.

Common Pitfalls

1. Chasing 100% Coverage

100% coverage does NOT mean bug-free code:

def test_bad():
    result = calculate_discount(100, True, "VIP")
    assert result is not None  # Weak assertion!

2. Ignoring Branch Coverage

You can have 100% C0 while missing entire branches:

def process(data, validate=True):
    if validate:
        check_data(data)  # Always tested with validate=True
    return transform(data)

3. Coverage Without Assertions

Running code isn’t testing it - you need meaningful assertions.

How to Get Feedback

During Quizzes and Exercises

Collapsible answers - Click “Show Answer” after attempting
Detailed explanations - Understand WHY answers are correct
Scoring guides - Self-assess your understanding

In GitHub Classroom

Immediate CI feedback - Push and see coverage results
Green check = 85%+ coverage achieved
Red X = Add more tests and push again

Quick Reference

pytest-cov Commands

# Basic coverage
uv run pytest --cov=src

# Show missing lines
uv run pytest --cov=src --cov-report=term-missing

# Fail if below threshold
uv run pytest --cov=src --cov-fail-under=80

# Branch coverage mode
uv run pytest --cov=src --cov-branch

Coverage Report Columns

Column	Meaning
Stmts	Total executable statements
Miss	Statements NOT executed
Cover	Percentage executed
Missing	Line numbers not covered

Key Formulas

Statement Coverage (C0):

\[C0 = \frac{\text{Executed Statements}}{\text{Total Statements}} \times 100\%\]

Branch Coverage (C1):

\[C1 = \frac{\text{Executed Branches}}{\text{Total Branches}} \times 100\%\]

Subsumption:

\(100\% \text{ C1} \Rightarrow 100\% \text{ C0}\) (but NOT vice versa)

Estimated Total Time

Component	Time
This guide	10 min
Coverage Concepts Quiz	25 min
CFG Tracing Exercise	35 min
Coverage Detective Exercise	45 min
GitHub Classroom Assignment	65 min
Total	~3 hours

Ready to Start?

Read the lecture sections linked above for your first exercise
Attempt the exercise without looking at answers
Check your understanding using the collapsible solutions
Move to the next exercise when ready

Begin with the Coverage Concepts Quiz after reading the recommended lecture sections.

Good luck!