Branch coverage in PyTest

Introduction

Branch coverage helps us check if every decision point in our code has been tested. It makes sure all possible paths are tried.

When you want to test all if-else conditions in your code.

When you want to find untested parts in decision-making code.

When you want to improve test quality by covering all branches.

When you want to avoid bugs hidden in rarely used code paths.

Syntax

PyTest

pytest --cov=your_module --cov-branch

This command runs tests and measures branch coverage.

Use --cov-branch to include branch coverage, not just line coverage.

Examples

Run tests on myapp and check branch coverage.

PyTest

pytest --cov=myapp --cov-branch

Run tests on calculator with branch coverage and verbose output.

PyTest

pytest --cov=calculator --cov-branch -v

Sample Program

This simple function returns the bigger of two numbers. The test checks both branches: when a is greater and when b is greater.

PyTest

def max_value(a, b):
    if a > b:
        return a
    else:
        return b


def test_max_value():
    assert max_value(5, 3) == 5
    assert max_value(2, 4) == 4

# Run with: pytest --cov=sample_module --cov-branch

Important Notes

Branch coverage is stricter than line coverage because it checks all decision outcomes.

Use branch coverage to find missing tests in if-else and switch cases.

High branch coverage means your tests explore all logical paths.

Summary

Branch coverage tests all possible paths in decision points.

Use pytest --cov --cov-branch to measure it.

It helps find untested branches and improve test quality.

Practice

1.

What does branch coverage measure in pytest testing?

easy

A. It checks if all decision paths in the code are tested.

B. It measures how many lines of code are executed.

C. It counts the number of test cases written.

D. It verifies the syntax correctness of the test code.

Solution

1. Step 1: Understand branch coverage concept

   Branch coverage ensures every possible path in decision points (like if-else) is tested.

2. Step 2: Compare with other coverage types

   Line coverage counts executed lines, but branch coverage focuses on decision paths.

3. Final Answer:

   It checks if all decision paths in the code are tested. -> Option A

4. Quick Check:

   Branch coverage = all decision paths tested [OK]

Hint: Branch coverage tests all if-else paths, not just lines [OK]

Common Mistakes:

• Confusing branch coverage with line coverage
• Thinking it counts test cases
• Assuming it checks syntax errors

2.

Which pytest command option enables branch coverage measurement?

easy

A. pytest --cov --cov-branch

B. pytest --branch-coverage

C. pytest --cov-branch

D. pytest --coverage-branch

Solution

1. Step 1: Recall pytest coverage options

   The option --cov enables coverage measurement, and --cov-branch adds branch coverage.

2. Step 2: Identify correct combined command

   Both options must be used together as pytest --cov --cov-branch to measure branch coverage.

3. Final Answer:

   pytest --cov --cov-branch -> Option A

4. Quick Check:

   Use --cov with --cov-branch for branch coverage [OK]

Hint: Use both --cov and --cov-branch together [OK]

Common Mistakes:

• Using only --cov-branch without --cov
• Typing incorrect option names
• Assuming branch coverage is default

3.

Given this code snippet tested with pytest and branch coverage enabled, what is the branch coverage result?

def check_num(x):
    if x > 0:
        return "Positive"
    else:
        return "Non-positive"

# Test cases:
assert check_num(5) == "Positive"

medium

A. 100% branch coverage

B. 75% branch coverage

C. 0% branch coverage

D. 50% branch coverage

Solution

1. Step 1: Identify branches in the code

   The function has two branches: one for x > 0 and one for else (x ≤ 0).

2. Step 2: Check which branches are tested

   The test calls check_num(5), which covers only the x > 0 branch, missing the else branch.

3. Final Answer:

   50% branch coverage -> Option D

4. Quick Check:

   One branch tested out of two = 50% [OK]

Hint: Test all if-else paths to get 100% branch coverage [OK]

Common Mistakes:

• Assuming one test covers all branches
• Confusing line coverage with branch coverage
• Ignoring else branch testing

4.

Identify the error in this pytest test that aims for full branch coverage:

def classify(n):
    if n > 10:
        return "High"
    elif n > 5:
        return "Medium"
    else:
        return "Low"

def test_classify():
    assert classify(12) == "High"
    assert classify(7) == "Medium"

medium

A. Syntax error in the test function.

B. Incorrect expected values in assertions.

C. Missing test for the else branch (n ≤ 5).

D. The function classify has no return statement.

Solution

1. Step 1: Analyze branches in classify()

   There are three branches: n > 10, n > 5, and else (n ≤ 5).

2. Step 2: Check test coverage of branches

   Tests cover n=12 (High) and n=7 (Medium), but no test covers n ≤ 5 (Low) branch.

3. Final Answer:

   Missing test for the else branch (n ≤ 5). -> Option C

4. Quick Check:

   All branches need tests for full coverage [OK]

Hint: Test all if, elif, else branches to avoid missing coverage [OK]

Common Mistakes:

• Assuming two tests cover all branches
• Looking for syntax errors where none exist
• Ignoring else branch testing

5.

You have this function. Which set of test inputs achieves 100% branch coverage?

def analyze_score(score):
    if score >= 90:
        return "A"
    elif score >= 75:
        return "B"
    else:
        return "C"

hard

A. [90, 85, 76]

B. [95, 80, 70]

C. [91, 74, 60]

D. [89, 84, 73]

Solution

1. Step 1: Identify all branches

   Branches: score ≥ 90 ("A"), score ≥ 75 ("B"), else "C".

2. Step 2: Check which inputs cover all branches

   Input 95 covers "A" branch; 80 covers score ≥ 75 ("B"); 70 covers else "C" branch. This covers all branches.

3. Final Answer:

   [95, 80, 70] -> Option B

4. Quick Check:

   Test inputs cover all if-elif-else paths [OK]

Hint: Pick inputs hitting each if, elif, else branch [OK]

Common Mistakes:

• Missing coverage for one of the branches
• Choosing inputs that skip else branch
• Assuming close values cover all branches