Error Pattern Catalog

Error Pattern Catalog

Comprehensive catalog of common test error patterns organized by framework and language.

Table of Contents

1. Python (pytest, unittest)
2. JavaScript/TypeScript (Jest, Mocha, Vitest)
3. Java (JUnit, TestNG)
4. Go (testing package)
5. Cross-Framework Patterns

Python (pytest, unittest)

AssertionError

Pattern:

AssertionError: assert [actual] == [expected]

Variations:

• assert 5 == 10 - Simple value mismatch
• assert 'hello' in 'world' - Membership test failure
• assert len(items) == 3 - Length mismatch
• assert response.status_code == 200 - Attribute comparison

Common Causes:

• Logic errors in implementation
• Incorrect test expectations
• Off-by-one errors
• Wrong comparison operators

Quick Fix Pattern:

# Check if assertion logic is correct
assert actual == expected  # Is 'expected' value correct?

# Use pytest's detailed assertion rewriting
assert actual == expected, f"Expected {expected}, got {actual}"

# For floating point, use approx
from pytest import approx
assert 0.1 + 0.2 == approx(0.3)

AttributeError

Pattern:

AttributeError: 'NoneType' object has no attribute 'method_name'
AttributeError: module 'X' has no attribute 'Y'

Common Causes:

• Accessing attribute on None
• Typo in attribute name
• Wrong import (importing module instead of class)
• Missing initialization

Quick Fix Pattern:

# Add None check
if obj is not None:
    result = obj.method_name()

# Fix import
from module import ClassName  # Not just 'import module'

# Check initialization
def __init__(self):
    self.attribute = initial_value  # Don't forget to initialize

ImportError / ModuleNotFoundError

Pattern:

ModuleNotFoundError: No module named 'package_name'
ImportError: cannot import name 'ClassName' from 'module'

Common Causes:

• Package not installed
• Circular imports
• Wrong module path
• Missing __init__.py (Python 2.x)

Quick Fix Pattern:

# Install missing package
pip install package_name

# Check if in requirements.txt
grep package_name requirements.txt

# Verify package installation
pip list | grep package_name

FixtureNotFoundError (pytest)

Pattern:

fixture 'fixture_name' not found

Common Causes:

• Fixture not defined in test file or conftest.py
• Typo in fixture name
• Fixture in wrong scope
• Missing conftest.py import

Quick Fix Pattern:

# Define fixture in conftest.py
@pytest.fixture
def fixture_name():
    return value

# Or in test file
@pytest.fixture
def fixture_name():
    return value

# Check fixture scope
@pytest.fixture(scope="session")  # session, module, class, function

TypeError

Pattern:

TypeError: unsupported operand type(s) for +: 'int' and 'str'
TypeError: function() takes 2 positional arguments but 3 were given
TypeError: 'NoneType' object is not iterable

Common Causes:

• Type mismatch in operations
• Wrong number of arguments
• Trying to iterate over None
• Missing type conversion

Quick Fix Pattern:

# Fix type conversion
result = int(string_value) + 5

# Check argument count
def function(arg1, arg2):  # Match signature

# Handle None for iteration
for item in (items or []):  # Provide default empty list

JavaScript/TypeScript (Jest, Mocha, Vitest)

TypeError: Cannot read property 'X' of undefined

Pattern:

TypeError: Cannot read property 'name' of undefined
TypeError: Cannot read properties of null (reading 'id')

Common Causes:

• Accessing property on undefined/null
• Async data not loaded
• API returned null
• Object not initialized

Quick Fix Pattern:

// Option 1: Optional chaining
const name = user?.name;

// Option 2: Null check
if (user) {
    const name = user.name;
}

// Option 3: Default value
const name = user?.name ?? 'Default';

// Option 4: Ensure object exists
const user = await fetchUser() || {};

ReferenceError: X is not defined

Pattern:

ReferenceError: variableName is not defined
ReferenceError: functionName is not defined

Common Causes:

• Variable used before declaration
• Typo in variable name
• Missing import
• Scope issues

Quick Fix Pattern:

// Import missing module
import { functionName } from './module';

// Declare variable
const variableName = value;

// Check variable scope
function outer() {
    const variable = 'value';
    function inner() {
        console.log(variable);  // OK - closure
    }
}

Test timeout errors

Pattern:

Timeout - Async callback was not invoked within the 5000ms timeout
Error: Timeout of 2000ms exceeded

Common Causes:

• Async operation not completing
• Missing done() callback
• Missing await
• Infinite loop

Quick Fix Pattern:

// Jest: Increase timeout
jest.setTimeout(10000);

test('async test', async () => {
    await asyncOperation();  // Don't forget await
}, 10000);  // Or per-test timeout

// Mocha: Increase timeout
it('async test', async function() {
    this.timeout(5000);
    await asyncOperation();
});

// Ensure promise resolves
await expect(promise).resolves.toBeDefined();

Cannot find module

Pattern:

Cannot find module './moduleName'
Cannot find module 'package-name'

Common Causes:

• Module path incorrect
• Package not installed
• Missing file extension in import
• TypeScript path mapping issue

Quick Fix Pattern:

# Install package
npm install package-name

# Check import path
import { func } from './correct/path';  // Check relative path

# For TypeScript, check tsconfig.json paths
{
    "compilerOptions": {
        "baseUrl": ".",
        "paths": {
            "@/*": ["src/*"]
        }
    }
}

Async/Promise errors

Pattern:

UnhandledPromiseRejectionWarning
Error: Promise rejection was handled asynchronously

Common Causes:

• Promise rejection not caught
• Missing .catch()
• Missing try/catch in async function

Quick Fix Pattern:

// Option 1: Use try/catch
async function test() {
    try {
        await operation();
    } catch (error) {
        // Handle error
    }
}

// Option 2: Use .catch()
promise
    .then(result => {})
    .catch(error => {});

// Option 3: Test error with Jest
await expect(promise).rejects.toThrow();

Java (JUnit, TestNG)

AssertionFailedError

Pattern:

org.opentest4j.AssertionFailedError: expected: <200> but was: <404>
AssertionError: Expected 5 but was 3

Common Causes:

• Value mismatch
• Wrong expected value
• Logic error

Quick Fix Pattern:

// Use proper assertion
assertEquals(expected, actual);  // JUnit 4
assertThat(actual).isEqualTo(expected);  // AssertJ

// Add message for clarity
assertEquals("User should be created", 200, response.getStatus());

// For floating point
assertEquals(expected, actual, 0.001);  // delta

NullPointerException

Pattern:

java.lang.NullPointerException
java.lang.NullPointerException: Cannot invoke "Object.method()" because "object" is null

Common Causes:

• Accessing method/field on null reference
• Uninitialized field
• Method returning null unexpectedly

Quick Fix Pattern:

// Option 1: Null check
if (object != null) {
    object.method();
}

// Option 2: Optional (Java 8+)
Optional<String> optional = Optional.ofNullable(value);
optional.ifPresent(v -> process(v));

// Option 3: Objects.requireNonNull
Objects.requireNonNull(object, "Object must not be null");

// Option 4: Use @NonNull annotations
public void method(@NonNull Object param) { }

ClassNotFoundException / NoClassDefFoundError

Pattern:

java.lang.ClassNotFoundException: com.example.ClassName
java.lang.NoClassDefFoundError: com/example/ClassName

Common Causes:

• Class not in classpath
• Missing dependency
• Wrong package name
• Build issue

Quick Fix Pattern:

<!-- Maven: Add dependency -->
<dependency>
    <groupId>com.example</groupId>
    <artifactId>artifact</artifactId>
    <version>1.0.0</version>
    <scope>test</scope>
</dependency>

// Gradle: Add dependency
testImplementation 'com.example:artifact:1.0.0'

ComparisonFailure

Pattern:

org.junit.ComparisonFailure: expected:<hello [world]> but was:<hello [there]>

Common Causes:

• String content mismatch
• Whitespace differences
• Line ending differences

Quick Fix Pattern:

// Normalize strings before comparison
String expected = "hello world".trim().replaceAll("\\s+", " ");
String actual = result.trim().replaceAll("\\s+", " ");
assertEquals(expected, actual);

// Or use contains/matches
assertTrue(result.contains("expected substring"));
assertTrue(result.matches("regex pattern"));

Go (testing package)

Test Panics

Pattern:

panic: runtime error: invalid memory address or nil pointer dereference
panic: index out of range [5] with length 3

Common Causes:

• Nil pointer dereference
• Index out of bounds
• Type assertion failure

Quick Fix Pattern:

// Check for nil
if ptr != nil {
    value := *ptr
}

// Check slice bounds
if i < len(slice) {
    value := slice[i]
}

// Safe type assertion
if val, ok := interfaceValue.(Type); ok {
    // Use val
}

// Recover from panic in test
defer func() {
    if r := recover(); r != nil {
        t.Errorf("Test panicked: %v", r)
    }
}()

Comparison Failures

Pattern:

got 5, want 10
got "hello", want "world"

Common Causes:

• Value mismatch
• Pointer vs value comparison
• Deep equality not used for structs/slices

Quick Fix Pattern:

// Basic comparison
if got != want {
    t.Errorf("got %v, want %v", got, want)
}

// Deep equality for complex types
if !reflect.DeepEqual(got, want) {
    t.Errorf("got %+v, want %+v", got, want)
}

// Use testify for better assertions
assert.Equal(t, want, got)
assert.ElementsMatch(t, wantSlice, gotSlice)

Race Conditions

Pattern:

WARNING: DATA RACE
Read at 0x00c000124080 by goroutine 8
Previous write at 0x00c000124080 by goroutine 7

Common Causes:

• Concurrent access to shared variable
• Missing mutex
• Channel misuse

Quick Fix Pattern:

// Use mutex
var mu sync.Mutex
mu.Lock()
sharedVar = value
mu.Unlock()

// Or RWMutex
var mu sync.RWMutex
mu.RLock()
val := sharedVar
mu.RUnlock()

// Or use channels
ch := make(chan int)
go func() {
    ch <- value
}()
result := <-ch

// Run tests with race detector
go test -race

Cross-Framework Patterns

Mock/Stub Not Called

Common across: pytest-mock, Jest, Mockito, gomock

Pattern:

• Python: AssertionError: Expected call not found
• JS: Expected mock function to have been called
• Java: Wanted but not invoked

Common Causes:

• Wrong code path executed
• Mock patched in wrong location
• Conditional logic preventing call
• Wrong mock instance

Quick Fix:

1. Verify code path reaches mock
2. Check mock is patched where used, not defined
3. Print/log to confirm execution
4. Relax assertion to check any call first

Async/Timing Issues

Common across: All frameworks

Symptoms:

• Intermittent failures
• Works locally, fails in CI
• Depends on system speed

Common Causes:

• Race conditions
• Insufficient wait times
• Order of execution assumptions

Quick Fix:

1. Add explicit waits/timeouts
2. Use retry logic for flaky operations
3. Avoid sleep() - use event/condition waits
4. Mock time-dependent code

Configuration/Setup Errors

Common across: All frameworks

Symptoms:

• Tests fail before running
• Framework can't find tests
• Import/module resolution fails

Files to check:

• Python: pytest.ini, setup.py, conftest.py
• JS: jest.config.js, package.json, tsconfig.json
• Java: pom.xml, build.gradle
• Go: go.mod

Quick Fix:

1. Verify test discovery patterns
2. Check dependency versions
3. Ensure test files match naming conventions
4. Validate configuration syntax