Testing exception chains in PyTest - Test Execution Trace

Test Overview

This test checks that a function raises a specific exception and that this exception is caused by another exception, verifying the exception chain.

Test Code - pytest

PyTest

import pytest

def func_that_raises():
    try:
        raise ValueError("Initial error")
    except ValueError as e:
        raise RuntimeError("Secondary error") from e

def test_exception_chain():
    with pytest.raises(RuntimeError) as exc_info:
        func_that_raises()
    assert isinstance(exc_info.value.__cause__, ValueError)
    assert str(exc_info.value) == "Secondary error"
    assert str(exc_info.value.__cause__) == "Initial error"

Execution Trace - 8 Steps

Step	Action	System State	Assertion	Result
1	Test starts	pytest test runner initialized	-	PASS
2	Calls func_that_raises() inside pytest.raises context	Function executes, raises ValueError internally	-	PASS
3	func_that_raises catches ValueError and raises RuntimeError from it	RuntimeError with cause set to ValueError is raised	-	PASS
4	pytest.raises captures RuntimeError exception	exc_info contains RuntimeError instance with __cause__ attribute	Check exception type is RuntimeError	PASS
5	Assert that exc_info.value.__cause__ is instance of ValueError	Exception cause is ValueError	assert isinstance(exc_info.value.__cause__, ValueError)	PASS
6	Assert that RuntimeError message is 'Secondary error'	RuntimeError message matches	assert str(exc_info.value) == 'Secondary error'	PASS
7	Assert that ValueError message is 'Initial error'	ValueError message matches	assert str(exc_info.value.__cause__) == 'Initial error'	PASS
8	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: func_that_raises does not raise RuntimeError or the cause is not ValueError

Execution Trace Quiz - 3 Questions

What exception does the test expect func_that_raises() to raise?

A RuntimeError

B ValueError

C TypeError

D KeyError

Key Result

Always verify exception chains by checking both the raised exception and its cause to ensure error context is preserved.