pytest-idempotent 1.3.1

Description:

pytestidempotent 1.3.1

pytest-idempotent






Pytest plugin for testing the idempotency of a function.
Usage
pip install pytest-idempotent

Documentation
Suppose we had the following function, that we (incorrectly) assumed was idempotent. How would we write a test for this?
First, we can label the function with a decorator:
# abc.py
from pytest_idempotent import idempotent # or use your own decorator! See below.

@idempotent
def func(x: list[int]) -> None:
x += [9]

Note: this function is not idempotent because calling it on the same list x grows the size of x by 1 each time. To be idempotent, we should be able to run func more than once without any adverse effects.
We can write an idempotency test for this function as follows:
# tests/abc_test.py
import pytest

@pytest.mark.idempotent
def test_func() -> None:
x: list[int] = []

func(x)

assert x == [9]

Adding the @pytest.mark.idempotent mark automatically splits this test into two - one that tests the regular behavior, and one that tests that the function can be called twice without adverse effects.
❯❯❯ pytest

================= test session starts ==================
platform darwin -- Python 3.9.2, pytest-6.2.5
collected 2 items

tests/abc_test.py .F [100%]

===================== FAILURES ========================
------------- test_func[idempotency-check] -------------

@pytest.mark.idempotent
def test_func() -> None:
x: list[int] = []

func(x)

> assert x == [9]
E assert [9, 9] == [9]
E Left contains one more item: 9
E Use -v to get the full diff

tests/abc_test.py:19: AssertionError
=============== short test summary info ================
FAILED tests/abc_test.py::test_func[idempotency-check]
- assert [9, 9] == [9]
============= 1 failed, 1 passed in 0.16s ==============

How It Works
Idempotency is a difficult pattern to enforce. To solve this issue, pytest-idempotent takes the following approach:


Introduce a decorator, @idempotent, to functions.

This decorator serves as a visual aid. If this decorator is commonly used in the codebase, it is much easier to consider idempotency for new and existing functions.
At runtime, this decorator is a no-op.
At test-time, if the feature is enabled, we will run the decorated function twice with the same parameters in all test cases.
We can also assert that the second run returns the same result using an additional parameter to the function's decorator: @idempotent(equal_return=True).



For all tests marked using @pytest.mark.idempotent, we run each test twice: once normally, and once with the decorated function called twice.

Both runs need to pass all assertions.
We return the first result because the first run will complete the processing. The second will either return exact the same result or be a no-op.
To disable idempotency testing for a test or group of tests, add the Pytest marker:
@pytest.mark.idempotent(enabled=False)



Enforcing Tests Use @pytest.mark.idempotent
By default, any test that calls an @idempotent function must also be decorated with the marker @pytest.mark.idempotent.
To disable idempotency testing for a test or group of tests, use:
@pytest.mark.idempotent(enabled=False), or add the following config to your project:
def pytest_idempotent_enforce_tests() -> bool:
return False

To disable enforced idempotency testing for a specific function, you can also pass the flag into the decorator:
# abc.py
from pytest_idempotent import idempotent

@idempotent(enforce_tests=False)
def func() -> None:
return


Or, you can automatically add the marker based on the test name by adding to conftest.py:
# conftest.py
def pytest_collection_modifyitems(items):
for item in items:
if "idempotent" in item.nodeid:
item.add_marker(pytest.mark.idempotent)

@idempotent decorator
By default, the @idempotent decorator does nothing during runtime. We do not want to add overhead to production code to run tests.
from typing import Any, Callable, TypeVar

_F = TypeVar("_F", bound=Callable[..., Any])


def idempotent(func: _F) -> _F:
"""
No-op during runtime.
This marker allows pytest-idempotent to override the decorated function
during test-time to verify the function is idempotent.
"""
return func

To use your own @idempotent decorator, you can override the pytest_idempotent_decorator function in your conftest.py to return the module path to your implementation.
# conftest.py
# Optional: you can define this to ensure the plugin is correctly installed
pytest_plugins = ["pytest_idempotent"]


def pytest_idempotent_decorator() -> str:
# This links to my custom implementation of @idempotent.
return "src.utils.idempotent"