Excluding code from coverage in PyTest - Build an Automation Script

Verify code exclusion from coverage report using pytest

Preconditions (2)

Step 1: Run pytest with coverage enabled on the module

Step 2: Check the coverage report generated

Step 3: Verify that the lines marked for exclusion are not counted in coverage

✅ Expected Result: Coverage report shows 100% coverage excluding the marked lines

Automation Requirements - pytest with coverage.py

Assertions Needed:

Coverage report excludes lines marked with # pragma: no cover

Coverage percentage matches expected value excluding excluded code

Best Practices:

Use # pragma: no cover comment to exclude lines

Run coverage with pytest-cov plugin

Generate coverage report in XML or terminal format for verification

Automated Solution

PyTest

import subprocess
import xml.etree.ElementTree as ET

def test_coverage_exclusion():
    # Run pytest with coverage and generate XML report
    result = subprocess.run([
        'pytest', '--cov=sample_module', '--cov-report=xml', '--maxfail=1', '--disable-warnings'
    ], capture_output=True, text=True)
    assert result.returncode == 0, f"Pytest failed:\n{result.stdout}\n{result.stderr}"

    # Parse coverage XML report
    tree = ET.parse('coverage.xml')
    root = tree.getroot()

    # Find coverage line-rate attribute
    line_rate = float(root.attrib.get('line-rate', '0'))

    # Assert coverage is 1.0 (100%) excluding excluded lines
    assert line_rate == 1.0, f"Expected 100% coverage excluding excluded lines, got {line_rate*100:.2f}%"

This test runs pytest with coverage enabled on a sample module that has some lines marked with # pragma: no cover to exclude them from coverage.

It generates an XML coverage report, then parses it to check the overall line coverage rate.

The assertion verifies that the coverage is 100%, meaning the excluded lines are not counted against coverage.

Using subprocess.run allows running pytest as a separate process and capturing output and exit code.

This approach ensures the coverage exclusion comment works as expected.

Common Mistakes - 3 Pitfalls

{'mistake': "Not using the '# pragma: no cover' comment to exclude code", 'why_bad': "Coverage tools won't know which lines to exclude, so coverage will be lower than expected.", 'correct_approach': "Add '# pragma: no cover' comment on lines or blocks to exclude them from coverage."}

{'mistake': 'Running pytest without coverage plugin or options', 'why_bad': 'No coverage data will be collected, so exclusion cannot be verified.', 'correct_approach': "Run pytest with '--cov' and '--cov-report' options to collect and generate coverage reports."}

Parsing coverage report incorrectly or not checking coverage percentage

Bonus Challenge

Now add a test that verifies exclusion of multiple code blocks marked with '# pragma: no cover' in different files

Show Hint

Practice

1. What is the purpose of adding # pragma: no cover in your Python test code when using pytest and coverage.py?

easy

A. To exclude specific lines from the coverage report

B. To mark lines that must always be covered by tests

C. To enable debugging mode in pytest

D. To automatically generate test cases for those lines

Solution

1. Step 1: Understand coverage exclusion

   The comment # pragma: no cover tells coverage.py to ignore that line when measuring test coverage.

2. Step 2: Purpose in test reports

   This helps keep coverage reports focused on meaningful code, excluding lines like debug prints or platform-specific code.

3. Final Answer:

   To exclude specific lines from the coverage report -> Option A

4. Quick Check:

   Exclude lines = D [OK]

Hint: Use '# pragma: no cover' to skip lines in coverage report [OK]

Common Mistakes:

• Thinking it marks lines to always test
• Confusing it with pytest debug flags
• Assuming it generates tests automatically

2. Which of the following is the correct way to exclude a single line from coverage in a Python file using pytest and coverage.py?

easy

A. print('Debug info') # exclude coverage

B. print('Debug info') # no cover pragma

C. print('Debug info') # pragma: no cover

D. print('Debug info') # skip coverage

Solution

1. Step 1: Identify correct pragma syntax

   The correct syntax to exclude a line is # pragma: no cover exactly as written.

2. Step 2: Match options to syntax

   Only print('Debug info') # pragma: no cover uses the exact correct comment syntax recognized by coverage.py.

3. Final Answer:

   print('Debug info') # pragma: no cover -> Option C

4. Quick Check:

   Exact pragma syntax = A [OK]

Hint: Remember exact comment: '# pragma: no cover' [OK]

Common Mistakes:

• Swapping words in the pragma comment
• Using incorrect comment keywords
• Missing 'pragma:' keyword

3. Consider this Python code snippet tested with pytest and coverage.py:

def func(x):
    if x > 0:
        return x
    else:
        return -x  # pragma: no cover

What will coverage.py report about the line with return -x if it is never executed?

medium

A. The line will be counted as uncovered and reduce coverage percentage

B. The line will be ignored and not affect coverage percentage

C. The test will fail due to missing coverage

D. Coverage.py will raise a syntax error

Solution

1. Step 1: Understand pragma effect on coverage

   The comment # pragma: no cover tells coverage.py to ignore that line regardless of execution.

2. Step 2: Effect on coverage report

   Since the line is ignored, not executing it does not reduce coverage percentage.

3. Final Answer:

   The line will be ignored and not affect coverage percentage -> Option B

4. Quick Check:

   Pragma ignores line in coverage = B [OK]

Hint: Lines with '# pragma: no cover' don't lower coverage [OK]

Common Mistakes:

• Assuming ignored lines count as uncovered
• Expecting test failure due to coverage
• Thinking pragma causes syntax errors

4. You wrote this code:

def example():
    print('Start')  # pragma no cover
    print('End')

But coverage.py still counts the first print line as uncovered. What is the likely problem?

medium

A. You must disable coverage for the whole file, not single lines

B. Coverage.py does not support excluding print statements

C. The comment must be placed on the line before the code

D. The pragma comment is missing the colon after 'pragma:'

Solution

1. Step 1: Check pragma syntax

   The correct syntax requires a colon: # pragma: no cover. Missing colon causes coverage.py to ignore the comment.

2. Step 2: Effect of incorrect syntax

   Without the colon, coverage.py treats the line normally and counts it as uncovered if not executed.

3. Final Answer:

   The pragma comment is missing the colon after 'pragma:' -> Option D

4. Quick Check:

   Colon required in pragma comment = A [OK]

Hint: Always include colon: '# pragma: no cover' [OK]

Common Mistakes:

• Omitting colon after 'pragma:'
• Placing comment on wrong line
• Thinking coverage can't exclude print statements

5. You want to exclude a block of code from coverage in a pytest project, but # pragma: no cover only works line-by-line. Which approach correctly excludes multiple lines without affecting other code?

hard

A. Add # pragma: no cover comment to each line in the block

B. Wrap the block in a function and exclude the whole function with a decorator

C. Use if False: around the block to skip it and exclude it from coverage

D. Add # pragma: no cover only on the first and last lines of the block

Solution

1. Step 1: Understand line-by-line exclusion

   The # pragma: no cover comment excludes coverage only for the line it is on, so each line must have it to be excluded.

2. Step 2: Evaluate other options

   Wrapping in a function or using if False: changes code behavior or testability; partial comments on first and last lines do not exclude intermediate lines.

3. Final Answer:

   Add '# pragma: no cover' comment to each line in the block -> Option A

4. Quick Check:

   Exclude multiple lines by commenting each line = C [OK]

Hint: Comment each line with '# pragma: no cover' to exclude block [OK]

Common Mistakes:

• Assuming one comment excludes multiple lines
• Using 'if False:' which affects runtime
• Trying to exclude with decorators without support