Python fastapi ddd testing skill

Skill iktakahiro/python-fastapi-ddd-skill/skills/python-fastapi-ddd-testing-skill

Guides unit testing for Python DDD + Onion Architecture apps (Domain Entities/Value Objects and UseCases) using pytest and repository mocks, based on the dddpy reference. Use when adding tests, choosing what to mock, or structuring test folders for a DDD FastAPI project.From its SKILL.md

Install

npx -y skills add iktakahiro/python-fastapi-ddd-skill --skill python-fastapi-ddd-testing-skill

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SKILL.md

1.9 KB, 321 tokens by cl100k_base, as published. Nobody here has run it

Testing DDD Layers with pytest (Domain / UseCase)

This skill focuses on unit tests for the inner layers:

Domain: Value Objects + Entities (pure business rules)
UseCase: application workflows that orchestrate Domain + Repository interfaces

It intentionally avoids full HTTP/API tests unless explicitly requested.

Test strategy (recommended)

Domain tests: no mocks, assert invariants and state transitions.
UseCase tests: mock repository interfaces (Mock(spec=...)), assert:
- correct repository method calls
- correct domain exceptions raised
- correct entity state transitions
Infrastructure/Presentation: add integration tests separately (optional).

Folder structure

Mirror the Onion layers:

tests/
  domain/{aggregate}/...
  usecase/{aggregate}/...
  infrastructure/...   # optional integration tests
  presentation/...     # optional API tests

UseCase testing pattern (dddpy-based)

Use unittest.mock.Mock(spec=RepoInterface) so typos fail fast.

from unittest.mock import Mock
import pytest

@pytest.fixture
def todo_repository_mock():
    return Mock(spec=TodoRepository)

def test_create_todo_calls_save(todo_repository_mock):
    usecase = CreateTodoUseCaseImpl(todo_repository_mock)
    title = TodoTitle("Test")

    todo = usecase.execute(title=title)

    todo_repository_mock.save.assert_called_once_with(todo)

For more examples (Value Object validation, entity lifecycle tests, and exception cases), read references/TESTING.md.

What ships with it: 2 files

3.1 KB alongside SKILL.md

agents/

openai.yaml280 B

references/

TESTING.md2.8 KB

Gives 0 of the 12 instructions most test skills give in 321 tokens

Counted across 1,201 of the 2,096 authors here whose files we hold, read 2026-09-06

Write a failing test before writing codein 43 of 1201, across 36 files
Run the full test suitein 36 of 1201, across 35 files
Test only one variable per experimentin 34 of 1201, across 17 files
Read product marketing context before asking questionsin 34 of 1201, across 14 files
Mock external dependenciesin 34 of 1201, across 30 files
Define primary, secondary, and guardrail metricsin 33 of 1201, across 16 files
Pre-determine sample size before startingin 31 of 1201, across 14 files
Test behavior rather than implementationin 31 of 1201, across 29 files
Formulate a hypothesis before designing a testin 30 of 1201, across 13 files
Document every test hypothesis, variant, and resultin 29 of 1201, across 11 files
Use descriptive test function namesin 25 of 1201, across 21 files
Commit to the methodology without stopping earlyin 24 of 1201, across 8 files

Said here and by no other author read

Test domain entities without mocks
Assert domain invariants and state transitions
Mock repository interfaces in usecase tests
Use spec in mock objects to fail fast
Assert repository method calls in usecase tests
Assert domain exceptions in usecase tests

Grouped from the skills themselves: near-identical wordings counted once, and counted by distinct author, so one author publishing three of these counts once. Length counted with cl100k_base; the agent that loads this file may tokenize it differently.