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

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