Testing exception chains in PyTest - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check that a ValueError is raised.

PyTest

import pytest

def test_value_error():
    with pytest.raises([1]):
        int('abc')

Drag options to blanks, or click blank then click option'

AKeyError

BValueError

CTypeError

DIndexError

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong exception type like TypeError or KeyError.

Not using pytest.raises context manager.

The int('abc') call raises a ValueError because 'abc' cannot be converted to an integer.

2fill in blank

medium

Complete the code to check that a KeyError is raised with the correct message.

PyTest

import pytest

def test_key_error():
    with pytest.raises(KeyError) as exc_info:
        {}['missing']
    assert exc_info.value.args[0] == [1]

Drag options to blanks, or click blank then click option'

A"key"

B"missing"

C'key'

D'missing'

Attempts:

3 left

💡 Hint

Common Mistakes

Using double quotes instead of single quotes (both work but be consistent).

Checking the wrong attribute of exc_info.

The KeyError message is the missing key as a string. It is stored in exc_info.value.args[0].

3fill in blank

hard

Fix the error in the test to correctly check the exception chain.

PyTest

import pytest

def test_exception_chain():
    with pytest.raises(RuntimeError) as exc_info:
        try:
            int('abc')
        except ValueError as e:
            raise RuntimeError('Failed') from [1]

Drag options to blanks, or click blank then click option'

Ae

Bexc_info

CRuntimeError

DValueError

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable name in the 'from' clause.

Using the exception class instead of the instance.

The from keyword requires the original exception instance, which is e here.

4fill in blank

hard

Fill both blanks to assert the original exception type and message in the exception chain.

PyTest

import pytest

def test_exception_chain_details():
    with pytest.raises(RuntimeError) as exc_info:
        try:
            int('abc')
        except ValueError as e:
            raise RuntimeError('Failed') from e
    original = exc_info.value.__cause__
    assert isinstance(original, [1])
    assert str(original) == [2]

Drag options to blanks, or click blank then click option'

AValueError

BTypeError

C'invalid literal for int() with base 10: \'abc\''

D'Failed'

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the RuntimeError message with the original exception message.

Using the wrong exception type in isinstance check.

The original exception is a ValueError with the message about invalid literal.

5fill in blank

hard

Fill all three blanks to write a test that verifies the exception chain and message.

PyTest

import pytest

def test_full_exception_chain():
    with pytest.raises([1]) as exc_info:
        try:
            float('xyz')
        except [2] as e:
            raise [3]('Conversion failed') from e

Drag options to blanks, or click blank then click option'

ARuntimeError

BValueError

DTypeError

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up exception types in the raises or except clauses.

Not using the 'from' keyword to chain exceptions.

The test expects a RuntimeError raised from a caught ValueError when converting 'xyz' to float.

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