Coverage thresholds in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a coverage threshold in testing?

A coverage threshold is a set minimum percentage of code that must be tested. If tests cover less than this, the test run fails.

beginner

How do you set a coverage threshold in pytest?

You set it using the --cov-fail-under option followed by a number, like --cov-fail-under=80 to require at least 80% coverage.

beginner

Why use coverage thresholds?

Coverage thresholds help keep tests thorough. They stop code from being accepted if tests don't cover enough, improving quality.

beginner

What happens if coverage is below the threshold in pytest?

The test run fails and pytest returns a non-zero exit code. This alerts you to add more tests.

intermediate

Can coverage thresholds be set for specific parts of code?

Yes, you can set thresholds for the whole project or for specific files or directories using configuration files like .coveragerc.

What does the pytest option --cov-fail-under=90 do?

A Fails tests if coverage is below 90%

B Sets coverage to exactly 90%

C Ignores coverage below 90%

D Runs tests only on 90% of code

The option --cov-fail-under=90 makes pytest fail the test run if coverage is less than 90%.

Why is setting a coverage threshold useful?

A To skip tests

B To speed up tests

C To reduce code size

D To ensure tests cover enough code

Coverage thresholds ensure tests cover enough code to catch bugs and maintain quality.

If coverage is 75% but threshold is 80%, what happens?

A Test run passes

B Test run fails

C Coverage is ignored

D Tests rerun automatically

If coverage is below the threshold, pytest fails the test run to signal insufficient testing.

Can coverage thresholds be set per file in pytest?

A Yes, using configuration files

B No, only global thresholds

C Only for test files

D Only for Python 2

Pytest allows setting coverage thresholds per file or directory via configuration files like .coveragerc.

What exit code does pytest return if coverage threshold is not met?

A Zero exit code

B Exit code 100

C Non-zero exit code

D No exit code

Pytest returns a non-zero exit code to indicate failure when coverage thresholds are not met.

Explain how to enforce a minimum coverage percentage using pytest.

Think about the command line option that sets the minimum coverage.

Describe why coverage thresholds improve software quality.

Consider the role of tests in catching problems.

Practice

1. What does setting a coverage threshold in pytest ensure?

easy

A. Only tests with coverage above 90% are executed

B. Tests run faster by skipping some checks

C. The test run fails if coverage is below a set percentage

D. Coverage reports are hidden from the user

Solution

1. Step 1: Understand coverage threshold purpose

   Coverage thresholds set a minimum coverage percentage to maintain test quality.

2. Step 2: Effect of coverage below threshold

   If coverage is below the threshold, pytest fails the test run to alert missing tests.

3. Final Answer:

   The test run fails if coverage is below a set percentage -> Option C

4. Quick Check:

   Coverage threshold = fail if below limit [OK]

Hint: Coverage threshold means minimum coverage to pass tests [OK]

Common Mistakes:

• Thinking threshold speeds up tests
• Believing threshold skips tests
• Assuming threshold hides reports

2. Which is the correct pytest command to set a coverage fail threshold at 80%?

easy

A. pytest --cov-fail-under=80

B. pytest --fail-under-cov=80

C. pytest --coverage-threshold=80

D. pytest --fail-coverage=80

Solution

1. Step 1: Recall pytest coverage threshold syntax

   The correct option uses --cov-fail-under to set minimum coverage.

2. Step 2: Match the correct command

   pytest --cov-fail-under=80 matches the exact pytest CLI option for coverage fail threshold.

3. Final Answer:

   pytest --cov-fail-under=80 -> Option A

4. Quick Check:

   Correct CLI option = --cov-fail-under [OK]

Hint: Use --cov-fail-under to set coverage threshold [OK]

Common Mistakes:

• Mixing option order or names
• Using non-existent flags
• Confusing coverage threshold with other options

3. Given this pytest command:
pytest --cov=myapp --cov-fail-under=90
and coverage report shows 85%, what will happen?

medium

A. Tests pass but with a warning

B. Tests fail due to coverage below 90%

C. Tests run without coverage check

D. Coverage report is not generated

Solution

1. Step 1: Understand the coverage threshold set

   The command sets a fail threshold at 90% coverage.

2. Step 2: Compare actual coverage with threshold

   Actual coverage is 85%, which is below 90%, so pytest fails the test run.

3. Final Answer:

   Tests fail due to coverage below 90% -> Option B

4. Quick Check:

   Coverage 85% < 90% threshold = fail [OK]

Hint: Coverage below threshold causes test failure [OK]

Common Mistakes:

• Assuming tests pass with warning
• Thinking coverage check is skipped
• Believing report is not generated

4. You set --cov-fail-under=75 but pytest does not fail even when coverage is 70%. What is the likely cause?

medium

A. You must use --fail-under-cov instead

B. Threshold value must be above 80%

C. Coverage is always ignored in pytest

D. Coverage plugin is not installed or enabled

Solution

1. Step 1: Check coverage plugin status

   If coverage plugin is missing or disabled, threshold has no effect.

2. Step 2: Understand threshold behavior

   Threshold works only if coverage plugin runs and measures coverage.

3. Final Answer:

   Coverage plugin is not installed or enabled -> Option D

4. Quick Check:

   Missing plugin = threshold ignored [OK]

Hint: Ensure coverage plugin is active for thresholds to work [OK]

Common Mistakes:

• Thinking threshold values have minimum limits
• Believing coverage is ignored by default
• Using wrong command option names

5. You want to enforce different coverage thresholds for branches and statements: 80% for branches and 90% for statements. Which pytest configuration snippet achieves this?

hard

A. [run]\nbranch = True\n[report]\nfail_under = 90\nfail_under_branch = 80

B. [run]\nbranch = True\n[report]\nfail_under = 80\nfail_under_branch = 90

C. [coverage]\nbranch = True\nthresholds = statements:90, branches:80

D. [coverage]\nfail_under_statements = 90\nfail_under_branches = 80

Solution

1. Step 1: Enable branch coverage in config

   Set branch = True under [run] to measure branch coverage.

2. Step 2: Set fail thresholds correctly

   Under [report], fail_under sets statement threshold, fail_under_branch sets branch threshold.

3. Final Answer:

   [run]\nbranch = True\n[report]\nfail_under = 90\nfail_under_branch = 80 -> Option A

4. Quick Check:

   Branch coverage enabled + correct thresholds = [run]\nbranch = True\n[report]\nfail_under = 90\nfail_under_branch = 80 [OK]

Hint: Use [run] branch=True and [report] fail_under_branch for branches [OK]

Common Mistakes:

• Swapping statement and branch thresholds
• Using wrong config section names
• Missing branch = True setting