Testing exception chains in PyTest - Build an Automation Script

Verify exception chaining in a function

Preconditions (2)

Step 1: Call the function that raises the chained exception

Step 2: Catch the exception using pytest.raises

Step 3: Verify that the exception is of the expected type

Step 4: Verify that the exception's __cause__ attribute is set to the original exception

Step 5: Verify the messages of both exceptions

✅ Expected Result: The test should pass confirming the exception chain is correctly raised and accessible

Automation Requirements - pytest

Assertions Needed:

Assert the raised exception type

Assert the __cause__ attribute is not None

Assert the message of the original exception matches expected text

Assert the message of the raised exception matches expected text

Best Practices:

Use pytest.raises context manager to catch exceptions

Check exception chaining via __cause__ attribute

Use clear and descriptive assertion messages

Keep test code simple and readable

Automated Solution

PyTest

import pytest

class CustomError(Exception):
    pass

def function_that_raises():
    try:
        raise ValueError("Original error message")
    except ValueError as e:
        raise CustomError("Chained error message") from e


def test_exception_chain():
    with pytest.raises(CustomError) as exc_info:
        function_that_raises()
    raised_exc = exc_info.value
    # Assert the raised exception message
    assert str(raised_exc) == "Chained error message", "Raised exception message mismatch"
    # Assert the cause is set
    assert raised_exc.__cause__ is not None, "Exception cause should not be None"
    # Assert the original exception type and message
    assert isinstance(raised_exc.__cause__, ValueError), "Cause exception type mismatch"
    assert str(raised_exc.__cause__) == "Original error message", "Cause exception message mismatch"

This test defines a function function_that_raises which raises a CustomError chained from a ValueError. The test test_exception_chain uses pytest.raises to catch the CustomError. It then asserts the message of the raised exception, checks that the __cause__ attribute is set (meaning the exception was chained), and verifies the type and message of the original exception. This confirms the exception chaining behavior works as expected.

Common Mistakes - 4 Pitfalls

Not using pytest.raises context manager and catching exceptions manually

Checking exception chaining by comparing string messages only

Not verifying the original exception type in the chain

{'mistake': "Raising exceptions without chaining (missing 'from' keyword)", 'why_bad': "Without 'from', the exception chain is lost and __cause__ is None, so tests for chaining will fail.", 'correct_approach': "Use 'raise NewException() from original_exception' to preserve chaining."}

Bonus Challenge

Now add data-driven testing with 3 different original exception messages and verify chaining for each

Show Hint

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