Testing exception chains in PyTest

Introduction

Sometimes one error causes another error. Testing exception chains helps check that the right errors happen in the right order.

When a function catches one error and raises a new error based on it.

When you want to make sure the original error is not lost after a new error is raised.

When debugging complex code that handles multiple errors.

When you want to confirm the full error history is available for logging or user messages.

Syntax

PyTest

with pytest.raises(ExpectedError) as exc_info:
    function_that_raises()

assert isinstance(exc_info.value.__cause__, OriginalError)

Use pytest.raises to catch the expected error.

Check exc_info.value.__cause__ to see the original error that caused it.

Examples

This example raises a ValueError caused by a KeyError. The test checks the cause.

PyTest

with pytest.raises(ValueError) as exc_info:
    raise ValueError('New error') from KeyError('Original error')

assert isinstance(exc_info.value.__cause__, KeyError)

This example converts a string to int, catches ValueError, then raises RuntimeError with the original error as cause.

PyTest

with pytest.raises(RuntimeError) as exc_info:
    try:
        int('abc')
    except ValueError as e:
        raise RuntimeError('Failed conversion') from e

assert isinstance(exc_info.value.__cause__, ValueError)

Sample Program

This test calls func() which raises a RuntimeError caused by a ValueError. The test confirms the cause is correct.

PyTest

import pytest

def func():
    try:
        int('not a number')
    except ValueError as e:
        raise RuntimeError('Conversion failed') from e

def test_func_exception_chain():
    with pytest.raises(RuntimeError) as exc_info:
        func()
    assert isinstance(exc_info.value.__cause__, ValueError)

if __name__ == '__main__':
    pytest.main([__file__])

Important Notes

Always check __cause__ to verify the original exception in a chain.

Exception chaining helps keep error context clear for debugging.

Summary

Testing exception chains ensures the right error causes are preserved.

Use pytest.raises and check exc_info.value.__cause__.

This helps find where errors really start in your code.

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