Checking exception attributes in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this pytest exception attribute check?

Consider the following pytest test code. What will be the value of exc_info.value.args[0] after the test runs?

PyTest

import pytest

def test_divide_by_zero():
    with pytest.raises(ZeroDivisionError) as exc_info:
        result = 1 / 0
    assert exc_info.value.args[0] == 'division by zero'
    print(exc_info.value.args[0])

A "division by zero"

B "ZeroDivisionError"

C "float division by zero"

D IndexError

💡 Hint

Check the message stored in the exception's args attribute.

The ZeroDivisionError exception's first argument contains the error message string 'division by zero'. Accessing exc_info.value.args[0] retrieves this message.

❓ assertion

intermediate

Which assertion correctly checks the exception message in pytest?

You want to verify that a ValueError is raised with the message 'invalid input'. Which assertion line is correct inside a pytest test using pytest.raises?

PyTest

import pytest

def test_invalid_input():
    with pytest.raises(ValueError) as exc_info:
        raise ValueError('invalid input')
    # Which assertion below is correct?

A assert exc_info.value.args == ('invalid input',)

B assert exc_info.value.message == 'invalid input'

C assert str(exc_info.value) == 'invalid input'

D assert exc_info.message == 'invalid input'

💡 Hint

Use the string representation of the exception to check the message.

The message attribute does not exist on exception instances. The correct way is to convert the exception to string using str() and compare it to the expected message.

🔧 Debug

advanced

Why does this pytest exception attribute check fail?

This test is supposed to check the exception message but fails. What is the cause?

PyTest

import pytest

def test_key_error():
    with pytest.raises(KeyError) as exc_info:
        raise KeyError('missing_key')
    assert exc_info.value.args[0] == 'missing_key'
    assert str(exc_info.value) == 'missing_key'

# The test fails on the second assertion.

A The args attribute is empty, so args[0] raises IndexError

B The string representation of KeyError includes quotes, so str(exc_info.value) is "'missing_key'" not "missing_key"

C KeyError does not have args attribute

D The exception was not raised, so exc_info is None

💡 Hint

Print the string form of the exception to see its exact content.

The string form of a KeyError includes quotes around the key, so str(exc_info.value) returns "'missing_key'" with quotes. The assertion comparing to 'missing_key' without quotes fails.

🧠 Conceptual

advanced

What attribute of the pytest exception info object holds the caught exception instance?

When using with pytest.raises(SomeError) as exc_info:, which attribute of exc_info contains the actual exception instance?

A exc_info.value

B exc_info.type

C exc_info.message

D exc_info.args

💡 Hint

The exception instance is stored in an attribute named 'value'.

The exc_info object has a value attribute that holds the caught exception instance. Other attributes like type hold the exception class, not the instance.

❓ framework

expert

How to correctly check multiple exception attributes in pytest?

You want to test that a function raises a RuntimeError with message 'fail' and has a custom attribute code equal to 500. Which pytest test code is correct?

PyTest

import pytest

class CustomError(RuntimeError):
    def __init__(self, message, code):
        super().__init__(message)
        self.code = code

def func():
    raise CustomError('fail', 500)

# Which test below is correct?

A

def test_func():
    with pytest.raises(RuntimeError) as exc_info:
        func()
    assert exc_info.value.args[0] == 'fail'
    assert exc_info.code == 500

B

def test_func():
    with pytest.raises(CustomError) as exc_info:
        func()
    assert exc_info.value.args == ('fail', 500)
    assert exc_info.value.code == 500

C

def test_func():
    with pytest.raises(CustomError) as exc_info:
        func()
    assert exc_info.message == 'fail'
    assert exc_info.value.code == 500

D

def test_func():
    with pytest.raises(CustomError) as exc_info:
        func()
    assert str(exc_info.value) == 'fail'
    assert exc_info.value.code == 500

💡 Hint

Check the exception instance's string and custom attribute separately.

Option D correctly asserts the string message via str(exc_info.value) and accesses the custom attribute code on the exception instance. Option D wrongly accesses exc_info.code which does not exist. Option D uses exc_info.message which is invalid. Option D wrongly expects args to contain both message and code.