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.

Practice

1. What is the main purpose of testing exception chains in pytest?

easy

A. To test if exceptions are ignored during execution

B. To check if the test function runs without any exceptions

C. To ensure the program does not raise any exceptions

D. To verify that an exception was raised and linked to its cause

Solution

1. Step 1: Understand exception chains

   Exception chains link one error to another, showing the root cause of a failure.

2. Step 2: Purpose of testing chains in pytest

   Testing exception chains ensures both the raised exception and its cause are correct.

3. Final Answer:

   To verify that an exception was raised and linked to its cause -> Option D

4. Quick Check:

   Exception chains test = verify raised exception and cause [OK]

Hint: Check both exception and cause to test chains correctly [OK]

Common Mistakes:

• Ignoring the cause of the exception
• Only checking if any exception was raised
• Assuming no exceptions means success

2. Which of the following is the correct syntax to test an exception chain using pytest?

easy

A. with pytest.raises(ValueError) as excinfo:
raise TypeError() from ValueError()

B. with pytest.raises(TypeError) as excinfo:
raise TypeError() from ValueError()

C. with pytest.raises(TypeError) as excinfo:
raise ValueError() from None

D. with pytest.raises(TypeError) as excinfo:
raise ValueError() from TypeError()

Solution

1. Step 1: Understand exception chaining syntax

   Python uses raise NewException() from OriginalException() to chain exceptions.

2. Step 2: Match pytest.raises with the raised exception

   The pytest.raises() context must match the exception actually raised, here TypeError.

3. Final Answer:

   with pytest.raises(TypeError) as excinfo: raise TypeError() from ValueError() -> Option B

4. Quick Check:

   pytest.raises matches raised exception, not cause [OK]

Hint: pytest.raises matches raised exception, not the cause [OK]

Common Mistakes:

• Matching pytest.raises to the cause exception
• Swapping raised and cause exceptions
• Using None as cause incorrectly

3. Given the code below, what will excinfo.value.__cause__ be?

def func():
    try:
        raise ValueError("original error")
    except ValueError as e:
        raise TypeError("new error") from e

def test_func():
    with pytest.raises(TypeError) as excinfo:
        func()

medium

A. None

B. AttributeError

C. ValueError('original error')

D. TypeError('new error')

Solution

1. Step 1: Analyze exception chaining in func()

   ValueError is raised first, caught, then TypeError is raised from it, linking ValueError as cause.

2. Step 2: Understand excinfo.value.__cause__

   It holds the original exception that caused the current one, so it will be the ValueError instance.

3. Final Answer:

   ValueError('original error') -> Option C

4. Quick Check:

   __cause__ stores original exception in chain [OK]

Hint: __cause__ holds the original exception object [OK]

Common Mistakes:

• Expecting __cause__ to be None
• Confusing __cause__ with the raised exception
• Assuming __cause__ is a string

4. Identify the error in this pytest test for exception chains:

def test_chain():
    with pytest.raises(TypeError) as excinfo:
        raise TypeError("error") from ValueError("cause")
    assert isinstance(excinfo.value.__cause__, TypeError)

medium

A. The assertion checks wrong exception type for __cause__

B. The exception chaining syntax is invalid

C. The raised exception and pytest.raises do not match

D. No error, test is correct

Solution

1. Step 1: Check raised exception and pytest.raises

   pytest.raises expects TypeError, but TypeError is raised from ValueError, so this is valid.

2. Step 2: Analyze assertion on __cause__

   __cause__ is the original exception, here ValueError, but assertion checks for TypeError incorrectly.

3. Final Answer:

   The assertion checks wrong exception type for __cause__ -> Option A

4. Quick Check:

   __cause__ is original exception, check its correct type [OK]

Hint: Assert __cause__ type matches original exception, not raised one [OK]

Common Mistakes:

• Confusing raised exception with __cause__
• Assuming __cause__ is None
• Ignoring mismatch between pytest.raises and raised exception

5. You want to test a function that raises a RuntimeError caused by a KeyError. Which pytest test code correctly verifies both exceptions?

hard

A. with pytest.raises(RuntimeError) as excinfo:
func()
assert isinstance(excinfo.value.__cause__, KeyError)

B. with pytest.raises(KeyError) as excinfo:
func()
assert isinstance(excinfo.value.__cause__, RuntimeError)

C. with pytest.raises(RuntimeError) as excinfo:
func()
assert excinfo.value.__cause__ is None

D. with pytest.raises(Exception) as excinfo:
func()
assert isinstance(excinfo.value.__cause__, RuntimeError)

Solution

1. Step 1: Match pytest.raises to the raised exception

   The function raises RuntimeError, so pytest.raises must expect RuntimeError.

2. Step 2: Check the cause exception

   The cause is KeyError, so assert __cause__ is instance of KeyError.

3. Final Answer:

   with pytest.raises(RuntimeError) as excinfo: func() assert isinstance(excinfo.value.__cause__, KeyError) -> Option A

4. Quick Check:

   pytest.raises matches raised, assert __cause__ matches cause [OK]

Hint: pytest.raises for raised, assert __cause__ for cause exception [OK]

Common Mistakes:

• Swapping raised and cause exceptions in pytest.raises
• Asserting __cause__ is None incorrectly
• Using too broad exception in pytest.raises