Coverage thresholds in PyTest - Test Execution Trace

Test Overview

This test runs pytest with coverage measurement and verifies that the code coverage meets the minimum threshold set in the configuration. It ensures that the tested code is sufficiently covered by tests.

Test Code - PyTest

PyTest

import subprocess
import sys

def test_coverage_threshold():
    # Run pytest with coverage and fail if coverage is below threshold
    result = subprocess.run([
        sys.executable, "-m", "pytest", "--cov=src", "--cov-fail-under=80"
    ], capture_output=True, text=True)
    # Check that pytest exited with code 0 (success)
    assert result.returncode == 0, f"Coverage threshold not met:\n{result.stdout}\n{result.stderr}"

Execution Trace - 4 Steps

Step	Action	System State	Assertion	Result
1	Test starts by running pytest with coverage options and threshold set to 80%	Terminal ready to execute pytest command	—	PASS
2	Subprocess runs pytest with --cov=src and --cov-fail-under=80	Pytest runs tests and measures coverage on 'src' folder	—	PASS
3	Pytest finishes and returns exit code	Coverage report generated, exit code indicates pass/fail	Check if return code is 0 (coverage >= 80%)	PASS
4	Assert that coverage threshold is met	Test framework receives assertion result	Assert result.returncode == 0	PASS

Failure Scenario

Failing Condition: Code coverage is below 80%, causing pytest to exit with non-zero code

Execution Trace Quiz - 3 Questions

What does the test check after running pytest with coverage?

A That all tests passed regardless of coverage

B That the coverage percentage is at least 80%

C That the source code has no syntax errors

D That pytest ran without any warnings

Key Result

Using coverage thresholds in automated tests helps ensure your code is well tested and prevents lowering test quality over time.

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