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"