Debug Test Failures

Skill: Debug Test Failures

When to Use This Skill

This skill activates when you need to:

• Investigate why tests are failing
• Debug specific test failures
• Understand test error messages
• Fix flaky or intermittent test failures

Prerequisites

This assumes tests are configured and running. Use run-tests skill to execute tests first.

Step-by-Step Workflow

Step 1: Run Tests and Identify Failures

Run tests with verbose output:

# Python
pytest -v

# JavaScript
npm test -- --verbose

# Go
go test -v ./...

Identify the failing test(s):

• Note the test file and function name
• Read the error message carefully
• Check the assertion that failed

Step 2: Run Only the Failing Test

Python (pytest):

pytest tests/test_file.py::test_function_name -v

JavaScript (Jest):

npm test -- tests/example.test.js -t "test name pattern"

Go:

go test -run TestFunctionName -v

Step 3: Add Debug Output

Python - Add print statements:

def test_example():
    result = my_function(input_data)
    print(f"DEBUG: result = {result}")  # Debug output
    print(f"DEBUG: type = {type(result)}")
    assert result == expected

Run with output visible:

pytest -s tests/test_file.py::test_function_name

Python - Use pytest's debugging:

# Drop into debugger on failure
pytest --pdb

# Drop into debugger on first failure
pytest -x --pdb

JavaScript - Add console.log:

test('example', () => {
  const result = myFunction(inputData);
  console.log('DEBUG: result =', result);
  expect(result).toBe(expected);
});

Step 4: Common Failure Patterns

AssertionError / Unexpected Value

Cause: Expected value doesn't match actual value

Debug steps:

1. Print both expected and actual values
2. Check data types match (string vs int, etc.)
3. Check for whitespace, case sensitivity
4. Verify test data setup

Example fix:

# Before - fails due to extra whitespace
assert result == "hello"

# After - strip whitespace
assert result.strip() == "hello"

Import / Module Not Found

Cause: Module path issues or missing dependencies

Debug steps:

1. Check if module is installed: pip list | grep module
2. Verify PYTHONPATH or NODE_PATH
3. Install in development mode: pip install -e .
4. Check import statement spelling

Example fix:

# Add to tests/conftest.py
import sys
from pathlib import Path
sys.path.insert(0, str(Path(__file__).parent.parent / "src"))

Timeout Errors

Cause: Test taking too long or blocking operation

Debug steps:

1. Check for infinite loops
2. Verify async/await used correctly
3. Mock external API calls
4. Increase timeout temporarily to see if test passes

Example fix:

# Pytest - increase timeout
@pytest.mark.timeout(300)
def test_slow_operation():
    result = slow_function()
    assert result is not None

Fixture / Setup Errors

Cause: Test fixtures or setup not working

Debug steps:

1. Check fixture names match exactly
2. Verify fixture scope (function, class, module)
3. Print fixture values in test
4. Check conftest.py is in correct location

Example fix:

# Check fixture is working
def test_example(sample_data):
    print(f"Fixture value: {sample_data}")  # Debug
    assert sample_data is not None

Flaky / Intermittent Failures

Cause: Race conditions, timing issues, or external dependencies

Debug steps:

1. Run test multiple times: pytest --count=10
2. Check for shared state between tests
3. Verify test isolation (each test is independent)
4. Mock time-dependent or random operations
5. Check for test order dependencies

Example fix:

# Mock random values
from unittest.mock import patch

@patch('random.randint', return_value=5)
def test_with_random(mock_random):
    result = function_using_random()
    assert result == expected_value

Step 5: Use Debugging Tools

Python Debugger (pdb)

Drop into debugger on failure:

pytest --pdb

Add breakpoint in code:

def test_example():
    data = prepare_data()
    breakpoint()  # Python 3.7+
    # or: import pdb; pdb.set_trace()
    result = process(data)
    assert result == expected

pdb commands:

• l - list code
• n - next line
• s - step into function
• c - continue execution
• p variable - print variable
• pp variable - pretty print
• q - quit debugger

JavaScript Debugger

Node.js debugger:

node --inspect-brk node_modules/.bin/jest --runInBand

Add debugger statement:

test('example', () => {
  const data = prepareData();
  debugger;  // Pauses here in debugger
  const result = process(data);
  expect(result).toBe(expected);
});

VS Code Debugging

Python .vscode/launch.json:

{
  "name": "Python: Debug Tests",
  "type": "python",
  "request": "launch",
  "module": "pytest",
  "args": ["tests/test_file.py", "-v"]
}

JavaScript .vscode/launch.json:

{
  "name": "Jest: Debug",
  "type": "node",
  "request": "launch",
  "program": "${workspaceFolder}/node_modules/.bin/jest",
  "args": ["--runInBand", "--no-cache"],
  "console": "integratedTerminal"
}

Step 6: Fix and Verify

1. Make minimal changes to fix the issue
2. Run the failing test again
3. Run all related tests
4. Run full test suite to ensure no regressions

Common Issues and Solutions

Issue: Test passes locally but fails in CI

Solutions:

1. Check environment variables
2. Verify versions match (Python, Node, etc.)
3. Check for file system case sensitivity
4. Review CI logs for warnings
5. Test with same database/fixtures as CI

Issue: Can't reproduce failure locally

Solutions:

1. Clear caches: pytest --cache-clear or jest --clearCache
2. Run in clean environment (new virtualenv)
3. Check for environment-specific config
4. Run with same seed/random state
5. Check system resources (memory, disk space)

Issue: Error message unclear

Solutions:

1. Add custom error messages to assertions
2. Use pytest's -vv for extra verbose output
3. Check test framework documentation
4. Add logging before assertions
5. Simplify test to isolate issue

Success Criteria

• ✅ Identified root cause of test failure
• ✅ Fixed the issue with minimal changes
• ✅ Failing test now passes
• ✅ Related tests still pass
• ✅ Understanding why it failed to prevent future issues

Related Skills

• run-tests - Execute tests
• debug-code-profiling - Debug performance issues with profiling and critical path timing (for slow-code, not failing tests)
• pytest-setup - Configure test framework
• code-formatting - Fix linting/formatting issues

Documentation References

• pytest Debugging
• Jest Troubleshooting
• Python pdb Documentation

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