Testing exception chains in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an exception chain in Python testing?

An exception chain happens when one exception causes another. It shows the sequence of errors leading to the final failure. Testing exception chains helps check if the right errors happen in the right order.

intermediate

How do you test exception chains using pytest?

Use pytest.raises() with the 'match' parameter to check the final exception message. To check the chain, access the '__cause__' or '__context__' attributes of the caught exception inside the 'as excinfo' block.

intermediate

What is the difference between __cause__ and __context__ in exception chaining?

__cause__ is set when an exception is raised directly from another using 'raise ... from ...'. __context__ is set automatically when an exception occurs during handling another exception without 'from'.

intermediate

Write a simple pytest test snippet to check an exception chain where ValueError causes a RuntimeError.

def test_exception_chain():
    import pytest
    with pytest.raises(RuntimeError) as excinfo:
        try:
            raise ValueError("bad value")
        except ValueError as e:
            raise RuntimeError("runtime failed") from e
    assert isinstance(excinfo.value.__cause__, ValueError)
    assert str(excinfo.value.__cause__) == "bad value"

beginner

Why is testing exception chains important in software testing?

It ensures that errors are handled properly and the root cause is not lost. This helps developers understand what really went wrong and fix bugs faster.

In pytest, which attribute holds the original exception when using 'raise ... from ...'?

A __cause__

B __context__

C __traceback__

D __exception__

The __cause__ attribute stores the original exception when an exception is raised from another using 'raise ... from ...'.

What does pytest.raises() do in a test?

A Catches all exceptions silently

B Checks if a block of code raises a specific exception

C Prevents exceptions from happening

D Logs exceptions to a file

pytest.raises() verifies that the code inside its block raises the expected exception.

If an exception occurs during handling another exception without 'from', which attribute links them?

A __cause__

B __linked__

C __handler__

D __context__

__context__ links exceptions when one happens during handling another without explicit chaining.

Which pytest feature helps check the message text of an exception?

A exception_message()

B assertRaisesMessage()

C match parameter in pytest.raises()

D check_message()

The 'match' parameter in pytest.raises() lets you check if the exception message matches a pattern.

Why should you test exception chains instead of just the final exception?

A To verify the root cause and error flow

B To make tests run faster

C To avoid writing assertions

D To skip error handling

Testing chains helps confirm the root cause and the sequence of errors, improving debugging.

Explain how to test an exception chain in pytest with an example.

Think about how to access the original exception inside the pytest.raises block.

Describe the difference between __cause__ and __context__ in Python exception chaining.

One is set explicitly, the other automatically.