Using pytest-assume for Assumption‑Based Testing in Python

pytest‑assume is a useful plugin that enables assumption mechanisms in test cases, allowing multiple related conditions to be checked while preserving earlier results when a later assumption fails.

1. Installation

Install the plugin via pip:

pip install pytest-assume

2. Basic Usage

Import and use the assume function in your tests:

import pytest
from pytest_assume.plugin import assume
def test_example():
assume(1 + 1 == 2)
assume(2 + 2 == 4)
assume(3 + 3 == 6)
assume(4 + 4 == 8)

3. Handling Failures

If an assumption fails, subsequent assumptions are skipped but earlier results are kept. Example with an intentional failure:

import pytest
from pytest_assume.plugin import assume
def test_example():
assume(1 + 1 == 2)
assume(2 + 2 == 4)
assume(3 + 3 == 7)  # intentional error
assume(4 + 4 == 8)

Run the test with pytest test_example.py to see failure details.

4. Advanced Usage – Decorator

The assume function can also be used as a decorator to mark specific tests:

import pytest
from pytest_assume.plugin import assume
@pytest.mark.assume
def test_example():
assume(1 + 1 == 2)
assume(2 + 2 == 4)
assume(3 + 3 == 6)
assume(4 + 4 == 8)

Execute with pytest test_example.py to see a passing result.

5. Combining with Regular Assert

You can mix assume with normal assert statements:

import pytest
from pytest_assume.plugin import assume
def test_example():
assume(1 + 1 == 2)
assume(2 + 2 == 4)
assume(3 + 3 == 6)
assert 4 + 4 == 8

6. Conditional Assumptions

Assumptions can be placed inside conditional blocks:

import pytest
from pytest_assume.plugin import assume
def test_example():
assume(1 + 1 == 2)
assume(2 + 2 == 4)
assume(3 + 3 == 6)
assume(4 + 4 == 8)
if assume(5 + 5 == 10):
assume(6 + 6 == 12)

7. Custom Error Messages

Provide a custom reason for each assumption:

import pytest
from pytest_assume.plugin import assume
def test_example():
assume(1 + 1 == 2, reason="First assumption failed")
assume(2 + 2 == 4, reason="Second assumption failed")
assume(3 + 3 == 6, reason="Third assumption failed")
assume(4 + 4 == 8, reason="Fourth assumption failed")

8. Summary

The article covered installation, basic usage, failure handling, advanced decorator usage, mixing with asserts, conditional checks, and custom error messages, demonstrating how pytest‑assume enhances Python testing flexibility.