macalbert/envilder/python-test-doubles

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Test doubles with unittest.mock (Mock, AsyncMock, patch) and Mother pattern. Use when creating test data, mocking dependencies, or setting up fixtures in Python tests.

Install

npx vskill@latest install macalbert/envilder/python-test-doubles

SKILL.md

Test Doubles (Python)

This skill defines how to use test doubles in Python (unittest.mock + pytest).

Test Doubles Types

1. Mock: Mock / AsyncMock

Use Mock and AsyncMock to create mock objects for protocol interfaces.

Purpose: Replace real dependencies with controllable test doubles.

python
from unittest.mock import Mock, AsyncMock
@pytest.fixture()
def secret_provider() -> Mock:
    provider = Mock(spec=ISecretProvider)
    provider.get_secret = Mock(return_value=None)
    return provider
@pytest.fixture()
def async_provider() -> AsyncMock:
    provider = AsyncMock(spec=ISecretProvider)
    provider.get_secret = AsyncMock(return_value=None)
    return provider

MANDATORY: Always use spec=InterfaceClass to catch typos at test time.

2. Stub: return_value / side_effect

Use return_value or side_effect to configure stubs.

Purpose: Predefined responses without caring about call verification.

python
# Simple stub
provider.get_secret.return_value = "secret-value"
# Sequential returns
provider.get_secret.side_effect = ["first", "second", None]
# Conditional stub
def resolve_secret(name: str) -> str | None:
    secrets = {"/app/db": "postgres://...", "/app/key": "abc123"}
    return secrets.get(name)
provider.get_secret.side_effect = resolve_secret
3. Spy: wraps

Use wraps to observe calls on real objects without replacing behavior.

Purpose: Verify interactions while preserving real implementation.

python
real_parser = MapFileParser()
spy_parser = Mock(wraps=real_parser)
sut = EnvilderClient(provider, spy_parser)
sut.resolve_secrets(map_file)
spy_parser.parse.assert_called_once_with(map_file)
4. Module Patch: patch / patch.object

Use patch to replace module-level objects or class methods.

Purpose: Replace external dependencies (boto3, file I/O, etc.).

python
from unittest.mock import patch
@patch("boto3.client")
def Should_CallSSM_When_AwsProviderUsed(mock_boto: Mock) -> None:
    # Arrange
    mock_ssm = Mock()
    mock_boto.return_value = mock_ssm
    mock_ssm.get_parameter.return_value = {"Parameter": {"Value": "secret"}}
    # Act
    sut = AwsSsmSecretProvider()
    actual = sut.get_secret("/app/key")
    # Assert
    assert actual == "secret"
    mock_ssm.get_parameter.assert_called_once()
5. Error Simulation: side_effect with Exception

Use side_effect with an exception to simulate failures.

Purpose: Test error paths and exception handling.

python
provider.get_secret.side_effect = ClientError(
    {"Error": {"Code": "ParameterNotFound"}}, "GetParameter"
)

Verification Patterns

Basic Verification
python
provider.get_secret.assert_called_once_with("/ssm/path")
provider.get_secret.assert_called_with("/ssm/path")
logger.info.assert_called_once()
Not Called
python
provider.get_secret.assert_not_called()
logger.error.assert_not_called()
Call Count
python
assert provider.get_secret.call_count == 3
Argument Inspection
python
from unittest.mock import call
provider.get_secret.assert_has_calls([
    call("/app/db"),
    call("/app/key"),
], any_order=True)
Async Verification
python
provider.get_secret.assert_awaited_once_with("/ssm/path")
provider.get_secret.assert_awaited()
provider.get_secret.assert_not_awaited()

Pytest Fixtures as Factories

Use fixtures to build test doubles with proper lifecycle:

python
@pytest.fixture()
def secret_provider() -> Mock:
    provider = Mock(spec=ISecretProvider)
    provider.get_secret.return_value = None
    return provider
@pytest.fixture()
def logger() -> Mock:
    return Mock(spec=ILogger)
@pytest.fixture()
def sut(secret_provider: Mock, logger: Mock) -> EnvilderClient:
    return EnvilderClient(provider=secret_provider, logger=logger)

Mother Pattern

Use factory functions or classes for reusable test data:

python
from dataclasses import dataclass
from typing import Optional
from uuid import UUID, uuid4
class MapFileMother:
    @staticmethod
    def create(
        provider: str = "aws",
        mappings: Optional[dict[str, str]] = None,
    ) -> ParsedMapFile:
        return ParsedMapFile(
            config=MapFileConfig(provider=provider),
            mappings=mappings or {"DB_URL": "/app/db"},
        )
class EnvilderOptionsMother:
    @staticmethod
    def create(
        provider: SecretProviderType = SecretProviderType.AWS,
        profile: Optional[str] = None,
        vault_url: Optional[str] = None,
    ) -> EnvilderOptions:
        return EnvilderOptions(
            provider=provider,
            profile=profile,
            vault_url=vault_url,
        )

Usage:

python
# Arrange
map_file = MapFileMother.create(mappings={"API_KEY": "/prod/key"})

Summary

Double TypePython APIPurpose
MockMock(spec=X)Controllable replacement
Stub.return_value / .side_effectPredefined responses
SpyMock(wraps=real)Observe real objects
Patch@patch("module.obj")Replace module-level deps
Error sim.side_effect = Exception(...)Failure paths

When writing tests:

  1. Create port mocks with Mock(spec=Interface) in fixtures
  2. Configure stubs with .return_value in Arrange
  3. Always verify mock interactions in Assert
  4. Use Mother pattern for complex test data
  5. Always use `spec=` to catch typo bugs
  6. Prefer fixture injection over inline Mock() creation

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Source

Popularity

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4
7-day trend score
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