Fake-Driven Testing Architecture for Python

Use this skill when: Writing tests, fixing bugs, adding features, or modifying gateway layers in Python projects.

Prerequisites: For Python code standards, load the dignified-python-313 skill first. This skill focuses on testing architecture, not Python syntax.

Overview

This skill provides a defense-in-depth testing strategy with five layers for Python applications:

┌─────────────────────────────────────────┐
│  Layer 5: Business Logic Integration Tests (5%)  │  ← Smoke tests over real system
├─────────────────────────────────────────┤
│  Layer 4: Business Logic Tests (70%)   │  ← Tests over fakes (MOST TESTS)
├─────────────────────────────────────────┤
│  Layer 3: Pure Unit Tests (10%)        │  ← Zero dependencies, isolated testing
├─────────────────────────────────────────┤
│  Layer 2: Integration Sanity Tests (10%)│  ← Fast validation with mocking
├─────────────────────────────────────────┤
│  Layer 1: Fake Infrastructure Tests (5%)│  ← Verify test doubles work
└─────────────────────────────────────────┘

Philosophy: Test business logic extensively over fast in-memory fakes. Use real implementations sparingly for integration validation.

Terminology note: The "gateway layer" (also called adapters/providers) refers to thin wrappers around heavyweight external APIs (databases, filesystems, HTTP APIs, message queues, etc.). The pattern matters more than the name.

Quick Decision: What Should I Read?

Adding a feature or fixing a bug? → Read quick-reference.md first, then workflows.md#adding-a-new-feature

Need to understand where to put a test? → Read testing-strategy.md

Working with Python-specific patterns? → Read python-specific.md

Adding/changing a gateway interface? → Read gateway-architecture.md, then workflows.md#adding-a-gateway-method

Creating a backend (higher-level abstraction over gateways)? → Read gateway-architecture.md#gateways-vs-backends - backends compose gateways and do NOT have fakes

Need to implement a specific pattern (CliRunner, builders, etc.)? → Read patterns.md

Not sure if I'm doing it right? → Read anti-patterns.md

Just need a quick lookup? → Read quick-reference.md

When to Read Each Reference Document

📖 gateway-architecture.md

Read when:

• •Adding or changing gateway/ABC interfaces
• •Understanding the ABC/Real/Fake/DryRun pattern
• •Need examples of gateway implementations
• •Want to understand what gateways are (and why they're thin)
• •Creating a backend (higher-level abstraction that composes gateways)

Contents:

• •What are gateway classes? (naming: gateways/adapters/providers)
• •The four implementations (ABC, Real, Fake, DryRun)
• •Code examples for each
• •When to add/change gateway methods
• •Design principles (keep gateways thin)
• •Common gateway types (Database, API, FileSystem, MessageQueue)
• •Gateways vs Backends - critical distinction for DI boundaries

📖 testing-strategy.md

Read when:

• •Deciding where to put a test
• •Understanding the five testing layers
• •Need test distribution guidance (5/70/10/10/5 rule)
• •Want to know which layer tests what

Contents:

• •Layer 1: Unit tests of fakes (verify test infrastructure)
• •Layer 2: Integration sanity tests with mocking (quick validation)
• •Layer 3: Pure unit tests (zero dependencies, isolated testing)
• •Layer 4: Business logic over fakes (majority of tests)
• •Layer 5: Business logic integration tests (smoke tests over real systems)
• •Decision tree: where should my test go?
• •Test distribution examples

📖 python-specific.md

Read when:

• •Working with pytest fixtures
• •Need Python mocking patterns
• •Testing Flask/FastAPI/Django applications
• •Understanding Python testing tools
• •Need Python-specific commands

Contents:

• •pytest fixtures and parametrization
• •Mocking with unittest.mock and pytest-mock
• •Testing web frameworks (Flask, FastAPI, Django)
• •Python testing commands
• •Type hints in tests
• •Python packaging for test utilities

📖 workflows.md

Read when:

• •Adding a new feature (step-by-step)
• •Fixing a bug (step-by-step)
• •Adding a gateway method (complete checklist)
• •Changing an interface (what to update)
• •Managing dry-run features

Contents:

• •Adding a new feature (TDD workflow)
• •Fixing a bug (reproduce → fix → regression test)
• •Adding a gateway method (8-step checklist with examples)
• •Changing an interface (update all layers)
• •Managing dry-run features (wrapping pattern)
• •Testing with builder patterns

📖 patterns.md

Read when:

• •Implementing constructor injection for fakes
• •Adding mutation tracking to fakes
• •Using CliRunner for CLI tests
• •Building complex test scenarios with builders
• •Testing dry-run behavior
• •Need code examples of specific patterns

Contents:

• •Constructor injection (how and why)
• •Mutation tracking properties (read-only access)
• •Using CliRunner (not subprocess)
• •Builder patterns for complex scenarios
• •Simulated environment pattern
• •Error injection pattern
• •Dry-run testing pattern

📖 anti-patterns.md

Read when:

• •Unsure if your approach is correct
• •Want to avoid common mistakes
• •Reviewing code for bad patterns
• •Debugging why tests are slow/brittle

Contents:

• •❌ Testing speculative features
• •❌ Hardcoded paths in tests (catastrophic)
• •❌ Not updating all layers
• •❌ Using subprocess in unit tests
• •❌ Complex logic in gateway classes
• •❌ Fakes with I/O operations
• •❌ Testing implementation details
• •❌ Incomplete test coverage for gateways

📖 quick-reference.md

Read when:

• •Quick lookup for file locations
• •Finding example tests to reference
• •Looking up common fixtures
• •Need command reference
• •Want test distribution guidelines

Contents:

• •Decision tree (where to add test)
• •File location map (source + tests)
• •Common fixtures (tmp_path, CliRunner, etc.)
• •Common test patterns (code snippets)
• •Example tests to reference
• •Useful commands (pytest, ty, etc.)
• •Quick checklist for adding gateway methods

Quick Navigation by Task

I'm adding a new feature

1. •Quick start: quick-reference.md → Decision tree
2. •Step-by-step: workflows.md#adding-a-new-feature
3. •Patterns: patterns.md (CliRunner, builders)
4. •Avoid: anti-patterns.md (speculative tests, hardcoded paths)

I'm fixing a bug

1. •Step-by-step: workflows.md#fixing-a-bug
2. •Patterns: patterns.md#constructor-injection-for-fakes
3. •Examples: quick-reference.md#example-tests-to-reference

I'm adding/changing a gateway method

1. •Understanding: gateway-architecture.md
2. •Step-by-step: workflows.md#adding-a-gateway-method
3. •Checklist: quick-reference.md#quick-checklist-adding-a-new-gateway-method
4. •Avoid: anti-patterns.md#not-updating-all-layers

I don't know where my test should go

1. •Decision tree: quick-reference.md#decision-tree
2. •Detailed guide: testing-strategy.md
3. •Examples: quick-reference.md#example-tests-to-reference

I need to implement a pattern

1. •All patterns: patterns.md
2. •Examples: quick-reference.md#common-test-patterns

I think I'm doing something wrong

1. •Anti-patterns: anti-patterns.md
2. •Correct approach: workflows.md

Visual Layer Guide

┌──────────────────────────────────────────────────────────────┐
│ Layer 5: Business Logic Integration Tests (5%)               │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Real database, filesystem, APIs, actual subprocess        │ │
│ │ Purpose: Smoke tests, catch integration issues           │ │
│ │ When: Sparingly, for critical workflows                  │ │
│ │ Speed: Seconds per test                                   │ │
│ │ Location: tests/e2e/ or tests/integration/               │ │
│ └──────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────┘

┌──────────────────────────────────────────────────────────────┐
│ Layer 4: Business Logic Tests (70%) ← MOST TESTS HERE       │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ FakeDatabase, FakeApiClient, FakeFileSystem              │ │
│ │ Purpose: Test features and business logic extensively    │ │
│ │ When: For EVERY feature and bug fix                      │ │
│ │ Speed: Milliseconds per test                              │ │
│ │ Location: tests/unit/, tests/services/, tests/commands/  │ │
│ └──────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────┘

┌──────────────────────────────────────────────────────────────┐
│ Layer 3: Pure Unit Tests (10%)                               │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Zero dependencies, no fakes, no mocks                    │ │
│ │ Purpose: Test isolated utilities and helpers             │ │
│ │ When: For pure functions, data structures, parsers       │ │
│ │ Speed: Milliseconds per test                              │ │
│ │ Location: tests/unit/                                     │ │
│ └──────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────┘

┌──────────────────────────────────────────────────────────────┐
│ Layer 2: Integration Sanity Tests (10%)                      │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ RealDatabase with mocked connections                     │ │
│ │ Purpose: Quick validation, catch syntax errors           │ │
│ │ When: When adding/changing real implementation           │ │
│ │ Speed: Fast (mocked)                                      │ │
│ │ Location: tests/integration/test_real_*.py               │ │
│ └──────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────┘

┌──────────────────────────────────────────────────────────────┐
│ Layer 1: Fake Infrastructure Tests (5%)                      │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Test FakeDatabase itself                                 │ │
│ │ Purpose: Verify test infrastructure is reliable          │ │
│ │ When: When adding/changing fake implementation           │ │
│ │ Speed: Milliseconds per test                              │ │
│ │ Location: tests/unit/fakes/test_fake_*.py               │ │
│ └──────────────────────────────────────────────────────────┘ │
└──────────────────────────────────────────────────────────────┘

Key Principles

1. •Thin gateway layer: Wrap external state, push complexity to business logic
2. •Fast tests over fakes: 70% of tests should use in-memory fakes
3. •Defense in depth: Fakes → sanity tests → pure unit → business logic → integration
4. •Test what you're building: No speculative tests, only active work
5. •Update all layers: When changing interfaces, update ABC/real/fake/dry-run
6. •Gateways vs Backends: Gateways have fakes; backends compose gateways and do NOT have fakes

Layer Selection Guide

Distinguishing Layer 3 (Pure Unit) from Layer 4 (Business Logic):

• •

  Layer 3 (Pure Unit Tests): ZERO dependencies - no fakes, no mocks, no external state

  • •Testing string utilities: sanitize_branch_name("feat/FOO") → "feat-foo"
  • •Testing parsers: parse_git_status("## main") → {"branch": "main"}
  • •Testing data structures: LinkedList.append() without any external dependencies
• •

  Layer 4 (Business Logic Tests): Uses fakes for external dependencies

  • •Testing commands: create_worktree(fake_git, name="feature")
  • •Testing workflows: submit_pr(fake_gh, fake_git, ...)
  • •Testing business logic that coordinates multiple integrations

If your test imports a Fake*, it belongs in Layer 4, not Layer 3.

Default Testing Strategy

When in doubt:

• •Write test over fakes (Layer 4) for business logic
• •Write pure unit test (Layer 3) for utilities/helpers with no dependencies
• •Use pytest with fixtures
• •Use tmp_path fixture (not hardcoded paths)
• •Follow examples in quick-reference.md

Summary

For quick tasks: Start with quick-reference.md

For understanding: Start with testing-strategy.md or gateway-architecture.md

For step-by-step guidance: Use workflows.md

For implementation details: Use patterns.md

For validation: Check anti-patterns.md

For Python specifics: Check python-specific.md