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

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

I'm fixing a bug

Step-by-step:
```
workflows.md#fixing-a-bug
```

Patterns:

patterns.md#constructor-injection-for-fakes

Examples:

quick-reference.md#example-tests-to-reference

I'm adding/changing a gateway method

Understanding:
```
gateway-architecture.md
```
Step-by-step:
```
workflows.md#adding-a-gateway-method
```

Checklist:

quick-reference.md#quick-checklist-adding-a-new-gateway-method

Avoid:

anti-patterns.md#not-updating-all-layers

I don't know where my test should go

Decision tree:
```
quick-reference.md#decision-tree
```
Detailed guide:
```
testing-strategy.md
```

Examples:

quick-reference.md#example-tests-to-reference

I need to implement a pattern

All patterns:
```
patterns.md
```
Examples:
```
quick-reference.md#common-test-patterns
```

I think I'm doing something wrong

Anti-patterns:
```
anti-patterns.md
```
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

Thin gateway layer: Wrap external state, push complexity to business logic
Fast tests over fakes: 70% of tests should use in-memory fakes
Defense in depth: Fakes → sanity tests → pure unit → business logic → integration
Test what you're building: No speculative tests, only active work
Update all layers: When changing interfaces, update ABC/real/fake/dry-run
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

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

fake-driven-testing

NPX Install

Tags

SKILL.md Content

Fake-Driven Testing Architecture for Python

Overview

Quick Decision: What Should I Read?

When to Read Each Reference Document

📖
`gateway-architecture.md`

📖
`testing-strategy.md`

📖
`python-specific.md`

📖
`workflows.md`

📖
`patterns.md`

📖
`anti-patterns.md`

📖
`quick-reference.md`

Quick Navigation by Task

I'm adding a new feature

I'm fixing a bug

I'm adding/changing a gateway method

I don't know where my test should go

I need to implement a pattern

I think I'm doing something wrong

Visual Layer Guide

Key Principles

Layer Selection Guide

Default Testing Strategy

Summary