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skills/python-testing/pytest-configuration.md
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skills/python-testing/pytest-configuration.md
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# Pytest Configuration Guide
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Pytest is a powerful testing framework for Python that makes it easy to write simple and scalable tests.
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## Installation
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```bash
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uv add --dev pytest
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uv add --dev pytest-cov # For coverage
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uv add --dev pytest-mock # For mocking
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uv add --dev pytest-asyncio # For async tests
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```
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## Basic Configuration
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Add to `pyproject.toml`:
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```toml
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[tool.pytest.ini_options]
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testpaths = ["tests"]
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python_files = ["test_*.py", "*_test.py"]
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python_classes = ["Test*"]
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python_functions = ["test_*"]
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addopts = [
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"--strict-markers",
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"--strict-config",
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"--showlocals",
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"-ra",
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]
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markers = [
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"slow: marks tests as slow (deselect with '-m \"not slow\"')",
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"integration: marks tests as integration tests",
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"unit: marks tests as unit tests",
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]
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```
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## Directory Structure
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```
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project/
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├── src/
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│ └── project_name/
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│ └── module.py
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├── tests/
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│ ├── __init__.py
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│ ├── conftest.py # Shared fixtures
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│ ├── test_module.py # Unit tests
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│ ├── integration/
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│ │ ├── __init__.py
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│ │ └── test_api.py # Integration tests
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│ └── fixtures/
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│ └── sample_data.json
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```
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## Writing Tests
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### Basic Test Function
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```python
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def test_simple_addition():
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"""Test that addition works correctly."""
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result = add(2, 3)
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assert result == 5
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```
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### Test Class
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```python
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class TestCalculator:
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"""Tests for Calculator class."""
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def test_addition(self):
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calc = Calculator()
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assert calc.add(2, 3) == 5
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def test_subtraction(self):
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calc = Calculator()
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assert calc.subtract(5, 3) == 2
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```
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### Using Assertions
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```python
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def test_assertions():
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# Equality
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assert result == expected
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# Boolean
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assert is_valid()
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assert not is_invalid()
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# Membership
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assert item in collection
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assert key in dictionary
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# Type checking
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assert isinstance(obj, MyClass)
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# Exceptions
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with pytest.raises(ValueError):
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raise_error()
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# Approximate equality (floats)
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assert result == pytest.approx(expected, rel=1e-5)
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```
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## Fixtures
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### Basic Fixture
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```python
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import pytest
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@pytest.fixture
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def sample_user():
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"""Create a sample user for testing."""
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return User(name="Alice", age=30)
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def test_user_greeting(sample_user):
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assert sample_user.greet() == "Hello, I'm Alice"
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```
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### Fixture Scopes
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```python
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@pytest.fixture(scope="function") # Default, new instance per test
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def function_scope():
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return setup()
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@pytest.fixture(scope="class") # One instance per test class
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def class_scope():
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return setup()
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@pytest.fixture(scope="module") # One instance per module
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def module_scope():
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return setup()
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@pytest.fixture(scope="session") # One instance per test session
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def session_scope():
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return setup()
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```
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### Fixture Cleanup
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```python
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@pytest.fixture
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def resource():
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# Setup
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res = acquire_resource()
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yield res
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# Teardown
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res.cleanup()
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# Or using context manager
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@pytest.fixture
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def database():
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with create_database() as db:
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yield db
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# Automatic cleanup
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```
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### Fixture Dependencies
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```python
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@pytest.fixture
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def database():
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return Database()
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@pytest.fixture
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def user_repository(database):
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return UserRepository(database)
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def test_find_user(user_repository):
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user = user_repository.find(1)
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assert user is not None
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```
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## Parametrized Tests
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### Basic Parametrization
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```python
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@pytest.mark.parametrize("input,expected", [
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(2, 4),
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(3, 9),
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(4, 16),
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(5, 25),
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])
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def test_square(input, expected):
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assert square(input) == expected
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```
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### Multiple Parameters
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```python
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@pytest.mark.parametrize("a,b,expected", [
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(1, 1, 2),
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(2, 3, 5),
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(10, -5, 5),
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])
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def test_addition(a, b, expected):
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assert add(a, b) == expected
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```
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### Parametrizing Fixtures
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```python
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@pytest.fixture(params=[1, 2, 3])
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def number(request):
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return request.param
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def test_with_different_numbers(number):
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assert number > 0
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```
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## Test Markers
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### Built-in Markers
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```python
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@pytest.mark.skip(reason="Not implemented yet")
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def test_future_feature():
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pass
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@pytest.mark.skipif(sys.version_info < (3, 10), reason="Requires Python 3.10+")
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def test_new_feature():
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pass
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@pytest.mark.xfail(reason="Known bug")
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def test_buggy_feature():
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pass
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```
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### Custom Markers
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```python
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# Define in pyproject.toml
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# markers = ["slow: marks tests as slow"]
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@pytest.mark.slow
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def test_expensive_operation():
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# Long-running test
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pass
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# Run with: pytest -m "not slow"
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```
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## Exception Testing
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```python
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def test_raises_value_error():
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with pytest.raises(ValueError):
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raise ValueError("Invalid value")
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def test_raises_with_message():
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with pytest.raises(ValueError, match="Invalid"):
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raise ValueError("Invalid value")
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def test_exception_info():
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with pytest.raises(ValueError) as exc_info:
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raise ValueError("Invalid value")
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assert "Invalid" in str(exc_info.value)
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```
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## Mocking
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### Using pytest-mock
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```python
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def test_with_mock(mocker):
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# Mock a function
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mock = mocker.patch('module.function')
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mock.return_value = 42
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result = call_function()
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assert result == 42
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mock.assert_called_once()
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def test_mock_method(mocker):
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# Mock a method
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mock = mocker.patch.object(MyClass, 'method')
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mock.return_value = "mocked"
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obj = MyClass()
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assert obj.method() == "mocked"
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```
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### Using unittest.mock
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```python
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from unittest.mock import Mock, patch
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def test_with_mock():
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with patch('module.function') as mock:
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mock.return_value = 42
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result = call_function()
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assert result == 42
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```
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## Async Tests
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```python
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import pytest
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@pytest.mark.asyncio
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async def test_async_function():
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result = await async_operation()
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assert result == expected
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@pytest.fixture
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async def async_client():
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client = AsyncClient()
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yield client
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await client.close()
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@pytest.mark.asyncio
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async def test_with_async_fixture(async_client):
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result = await async_client.get("/endpoint")
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assert result.status_code == 200
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```
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## Running Tests
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```bash
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# All tests
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pytest
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# Specific file
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pytest tests/test_module.py
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# Specific test
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pytest tests/test_module.py::test_function
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# Specific class
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pytest tests/test_module.py::TestClass
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# Pattern matching
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pytest -k "test_user"
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# Markers
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pytest -m slow # Only slow tests
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pytest -m "not slow" # Exclude slow tests
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# Last failed
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pytest --lf
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# Failed first
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pytest --ff
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# Stop on first failure
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pytest -x
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# Stop after N failures
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pytest --maxfail=3
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# Verbose output
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pytest -v
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# Show print statements
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pytest -s
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# Show locals in tracebacks
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pytest --showlocals
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# Parallel execution (requires pytest-xdist)
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pytest -n auto
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```
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## conftest.py
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Shared configuration and fixtures:
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```python
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# tests/conftest.py
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import pytest
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@pytest.fixture(scope="session")
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def database():
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"""Create database for entire test session."""
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db = create_test_database()
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yield db
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db.drop()
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@pytest.fixture(autouse=True)
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def reset_database(database):
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"""Reset database before each test."""
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database.clear()
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def pytest_configure(config):
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"""Pytest configuration hook."""
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config.addinivalue_line(
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"markers", "integration: mark test as integration test"
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)
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```
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## Best Practices
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1. **One assertion per test**: Keep tests focused
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2. **Descriptive names**: Use `test_user_creation_with_valid_data`
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3. **AAA pattern**: Arrange, Act, Assert
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4. **Use fixtures**: Avoid duplicating setup code
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5. **Test edge cases**: Not just happy paths
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6. **Fast tests**: Keep unit tests under 100ms
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7. **Independent tests**: Tests should not depend on each other
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8. **Clear assertions**: Make failures obvious
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## Example Test File
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```python
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"""Tests for user module."""
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import pytest
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from project_name.models import User
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@pytest.fixture
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def valid_user_data():
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"""Valid user data for testing."""
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return {
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"name": "Alice",
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"email": "alice@example.com",
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"age": 30
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}
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class TestUser:
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"""Tests for User model."""
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def test_create_user(self, valid_user_data):
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"""Test creating a user with valid data."""
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# Arrange & Act
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user = User(**valid_user_data)
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# Assert
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assert user.name == "Alice"
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assert user.email == "alice@example.com"
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assert user.age == 30
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def test_user_greeting(self, valid_user_data):
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"""Test user greeting message."""
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user = User(**valid_user_data)
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assert user.greet() == "Hello, I'm Alice"
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@pytest.mark.parametrize("age", [-1, 0, 151])
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def test_invalid_age(self, valid_user_data, age):
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"""Test that invalid ages raise ValueError."""
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valid_user_data["age"] = age
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with pytest.raises(ValueError, match="Invalid age"):
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User(**valid_user_data)
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```
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