Excluding code from coverage in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of coverage report with excluded code

Given the following Python code with coverage exclusion comments, what will be the total coverage percentage reported by pytest-cov?

PyTest

def add(a, b):
    return a + b

# pragma: no cover
def unused_function():
    print("This function is not covered")

result = add(2, 3)
print(result)

A 50%

B 100%

C 0%

D 75%

💡 Hint

Code marked with # pragma: no cover is excluded from coverage.

The function unused_function is excluded from coverage by the # pragma: no cover comment. Only add is counted and fully covered, so coverage is 100%.

❓ assertion

intermediate

Correct assertion for coverage exclusion

Which assertion correctly verifies that a function marked with # pragma: no cover is excluded from coverage reports?

A assert coverage_report.total_coverage < 50

B assert 'unused_function' in coverage_report.executed_functions

C assert 'unused_function' not in coverage_report.executed_functions

D assert coverage_report.total_coverage == 0

💡 Hint

Functions excluded from coverage should not appear in executed functions list.

The function marked with # pragma: no cover is not executed or counted in coverage, so it should not appear in the list of executed functions.

🔧 Debug

advanced

Why is coverage including excluded code?

Given this code snippet, coverage reports include helper_function even though it has # pragma: no cover. What is the most likely reason?

def main():
    helper_function()

# pragma: no cover
def helper_function():
    print("Helper")

main()

A The comment is misplaced; it should be above the function

B Coverage tools ignore <code># pragma: no cover</code> inside functions

C The <code># pragma: no cover</code> comment must be on the same line as the function definition

D The function is called, so it cannot be excluded

💡 Hint

Placement of exclusion comments matters for coverage tools.

The # pragma: no cover comment must be placed on the function definition line or immediately after it. Placing it below or separated causes coverage tools to ignore it.

❓ framework

advanced

Configuring pytest-cov to exclude files

Which pytest-cov configuration snippet correctly excludes all files in the tests/helpers/ directory from coverage reports?

A

[coverage:report]
omit = tests/helpers/*

B

[coverage:run]
exclude = tests/helpers/*

C

[run]
exclude = tests/helpers/*

D

[run]
omit = tests/helpers/*

💡 Hint

Check pytest-cov documentation for the correct section and key to omit files.

The omit option under the [run] section tells coverage.py which files to exclude from coverage measurement.

🧠 Conceptual

expert

Impact of excluding code on test coverage metrics

Which statement best describes the impact of excluding code with # pragma: no cover on overall test coverage metrics?

A Excluding code increases coverage percentage by removing untested code from calculation

B Excluding code decreases coverage percentage because less code is counted

C Excluding code has no effect on coverage percentage

D Excluding code causes coverage tools to report errors

💡 Hint

Think about how coverage percentage is calculated.

Excluding code removes it from the total lines counted, so coverage percentage is calculated over fewer lines, often increasing the percentage.

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