leo-claude-mktplace/plugins/saas-test-pilot/skills/mock-patterns.md

description

description
Mocking, stubbing, and dependency injection strategies for tests

Mock Patterns Skill

Overview

Mocking strategies and best practices for isolating code under test from external dependencies.

When to Mock

Situation	Mock?	Reason
External API calls	Yes	Unreliable, slow, costs money
Database queries	Depends	Mock for unit, real for integration
File system	Depends	Mock for unit, tmpdir for integration
Time/date functions	Yes	Deterministic tests
Random/UUID generation	Yes	Reproducible tests
Pure utility functions	No	Fast, deterministic, no side effects
Internal business logic	No	Test the real thing

Python Mocking

unittest.mock / pytest-mock

patch("module.path.to.dependency")     # Replaces at import location
patch.object(MyClass, "method")        # Replaces on specific class
MagicMock(return_value=expected)       # Creates callable mock
MagicMock(side_effect=Exception("e"))  # Raises on call

Critical rule: Patch where the dependency is USED, not where it is DEFINED.

• If views.py imports from services import send_email, patch views.send_email, NOT services.send_email.

pytest-mock (preferred)

Use the mocker fixture for cleaner syntax:

• mocker.patch("module.function") — auto-cleanup after test
• mocker.spy(obj, "method") — record calls without replacing

JavaScript Mocking

Jest

jest.mock("./module")              // Auto-mock entire module
jest.spyOn(object, "method")       // Spy without replacing
jest.fn().mockReturnValue(value)   // Create mock function

Vitest

vi.mock("./module")                // Same API as Jest
vi.spyOn(object, "method")
vi.fn().mockReturnValue(value)

Mock vs Stub vs Spy

Type	Behavior	Use When
Mock	Replace entirely, return fake data	Isolating from external service
Stub	Provide canned responses	Controlling specific return values
Spy	Record calls, keep real behavior	Verifying interactions without changing behavior

Dependency Injection Patterns

Prefer DI over mocking when possible:

• Constructor injection: pass dependencies as constructor args
• Function parameters: accept collaborators as arguments with defaults
• Context managers: swap implementations via context

DI makes tests simpler and avoids brittle mock paths.

Anti-Patterns

• Mocking too deep (mock chains: mock.return_value.method.return_value)
• Asserting on mock call counts instead of outcomes
• Mocking the system under test
• Not resetting mocks between tests (use autouse fixtures or afterEach)