Pytest

Tests should fail for exactly one reason - aim for a single assert per test

Python clean test tip:

Aim for a single assert per test. Tests will be more readable and it's easier to locate a defect when a test is failing.

Example:

import pytest


class User:
    def __init__(self, username):
        if len(username) < 1:
            raise Exception("Username must not be empty.")
        self._username = username

    @property
    def username(self):
        return self._username


# BAD
def test_user():
    username = "johndoe"
    assert User(username).username == username

    username = ""
    with pytest.raises(Exception):
        User(username)


# GOOD
def test_user_with_valid_username_can_be_initialized():
    username = "johndoe"
    assert User(username).username == username


def test_user_with_empty_username_cannot_be_initialized():
    username = ""
    with pytest.raises(Exception):
        User(username)

It's fine to deviate from this, to include multiple asserts per test as long as you're testing the same concept.