Excluding code from coverage in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'excluding code from coverage' mean in testing?

It means telling the coverage tool to ignore certain parts of the code when measuring how much code is tested. This helps focus on important code and avoid noise from untestable or irrelevant parts.

beginner

How can you exclude a specific function from coverage in pytest?

You can add a comment like # pragma: no cover at the end of the function or line you want to exclude. Coverage tools will skip that code when reporting coverage.

beginner

Which pytest plugin helps measure code coverage?

The pytest-cov plugin integrates coverage measurement with pytest, making it easy to run tests and see coverage reports.

intermediate

Why might you want to exclude code from coverage reports?

You might exclude code that is hard to test, like debug code, platform-specific code, or code that runs only in rare cases. This keeps coverage numbers meaningful and avoids false negatives.

intermediate

How do you exclude entire files or directories from coverage in pytest?

You can configure the .coveragerc file with an omit section listing files or folders to skip. For example, omit = tests/* excludes all test files.

What comment tells coverage tools to ignore a line or function?

A # no test

B # pragma: no cover

C # skip test

D # ignore coverage

The standard comment to exclude code from coverage is '# pragma: no cover'.

Which pytest plugin is used to measure code coverage?

A pytest-coverage

B pytest-report

C pytest-cov

D pytest-check

The 'pytest-cov' plugin integrates coverage measurement with pytest.

Where do you configure files to exclude from coverage?

A .coveragerc

B setup.cfg

C pytest.ini

D tox.ini

The '.coveragerc' file is used to configure coverage options including files to omit.

Why exclude code from coverage reports?

A To increase coverage percentage artificially

B To reduce test time

C To hide bugs

D To keep coverage meaningful by ignoring untestable code

Excluding untestable or irrelevant code keeps coverage reports accurate and meaningful.

Which of these is a valid way to exclude a file from coverage?

A List the file in the 'omit' section of .coveragerc

B Add '# no cover' comment at the top

C Rename the file with .nocov extension

D Use pytest command line option '--exclude-file'

Files or directories can be excluded by listing them in the 'omit' section of the .coveragerc file.

Explain how to exclude a specific function from coverage in pytest and why you might do it.

Think about marking code lines and reasons for ignoring coverage.

Describe how to exclude entire files or folders from coverage reports using configuration.

Focus on configuration file and syntax.

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