Testing exception chains in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of chained exception test with pytest

What is the output of this pytest test when run?

PyTest

import pytest

def func():
    try:
        raise ValueError("Initial error")
    except ValueError as e:
        raise RuntimeError("Chained error") from e

def test_exception_chain():
    with pytest.raises(RuntimeError) as excinfo:
        func()
    assert isinstance(excinfo.value.__cause__, ValueError)
    assert str(excinfo.value) == "Chained error"

A Test fails because excinfo.value.__cause__ is None

B Test fails because str(excinfo.value) is 'Initial error'

C Test passes with no assertion errors

D Test raises an unexpected TypeError during execution

💡 Hint

Remember that 'raise ... from ...' sets the __cause__ attribute of the exception.

The test checks that the RuntimeError was raised and that its __cause__ is the original ValueError. The assertions match the expected exception chain, so the test passes.

❓ assertion

intermediate

Correct assertion to verify exception cause in pytest

Which assertion correctly verifies that a caught exception has a cause of type KeyError?

PyTest

import pytest

with pytest.raises(RuntimeError) as excinfo:
    raise RuntimeError("Error") from KeyError("missing key")

A assert isinstance(excinfo.value.__cause__, KeyError)

B assert excinfo.value.__cause__ == KeyError

C assert excinfo.value.__context__ is None

D assert excinfo.value.__cause__.args == ('Error',)

💡 Hint

The __cause__ attribute holds the original exception instance, not the class.

Option A correctly checks that the __cause__ attribute is an instance of KeyError. Option A compares the instance to the class, which is false. Option A checks args incorrectly. Option A is unrelated.

🔧 Debug

advanced

Why does this pytest test fail to detect the cause exception?

Given this test code, why does the assertion fail?

PyTest

import pytest

def func():
    try:
        raise IndexError("Index problem")
    except IndexError:
        raise RuntimeError("Runtime problem")

def test_chain():
    with pytest.raises(RuntimeError) as excinfo:
        func()
    assert excinfo.value.__cause__ is not None

A Because the RuntimeError was raised without 'from', so __cause__ is None

B Because the test should check __context__ instead of __cause__

C Because the original exception was swallowed and replaced, causing a TypeError

D Because pytest.raises does not capture __cause__ attribute

💡 Hint

Check how exception chaining works when 'from' is omitted.

When raising a new exception without 'from', Python sets __cause__ to None. The original exception is stored in __context__, but __cause__ remains None, so the assertion fails.

🧠 Conceptual

advanced

Understanding exception chaining attributes

Which statement about Python exception chaining is TRUE?

A Using 'raise' without 'from' clears both __cause__ and __context__

B __context__ is always None when an exception is raised inside except block

C __cause__ and __context__ always point to the same exception instance

D __cause__ is set when an exception is raised with 'from', otherwise None

💡 Hint

Think about how explicit and implicit chaining differ.

Option D is correct: __cause__ is set explicitly by 'raise ... from ...'. __context__ is set automatically when an exception occurs during handling another exception. Other options are false.

❓ framework

expert

Testing nested exception chains with pytest

Consider this nested exception chain. Which pytest assertion correctly verifies the root cause exception type?

PyTest

import pytest

def nested_func():
    try:
        raise KeyError("key missing")
    except KeyError as e:
        raise ValueError("value error") from e

def test_nested_chain():
    with pytest.raises(ValueError) as excinfo:
        nested_func()
    # Which assertion below correctly checks the root cause is KeyError?

A assert isinstance(excinfo.value.__cause__.__cause__, KeyError)

B assert isinstance(excinfo.value.__cause__, KeyError)

C assert excinfo.value.__context__.__cause__ is None

D assert excinfo.value.__cause__.args[0] == 'value error'

💡 Hint

Check the chain depth and which exception is caught by pytest.raises.

The ValueError is raised from KeyError, so excinfo.value.__cause__ is the KeyError instance. __cause__.__cause__ is None. Option B correctly checks the immediate cause.