Asserting exceptions (pytest.raises) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of pytest.raises in testing?

pytest.raises is used to check if a specific exception is raised during a test. It helps verify that your code correctly handles error cases.

beginner

How do you use pytest.raises as a context manager?

You wrap the code that should raise an exception inside a with pytest.raises(ExceptionType): block. If the exception occurs, the test passes; if not, it fails.

beginner

Example: Write a test that checks if ValueError is raised when converting 'abc' to int.

<pre>import pytest

def test_value_error():
    with pytest.raises(ValueError):
        int('abc')</pre>

beginner

What happens if the expected exception is not raised inside pytest.raises block?

The test will fail because pytest.raises expects the specified exception to occur. Not raising it means the code did not behave as expected.

intermediate

Can you capture the exception object using pytest.raises? How?

<p>Yes. Use <code>as exc_info</code> to capture the exception object for further checks, like message content.</p><pre>import pytest

with pytest.raises(ValueError) as exc_info:
    int('abc')
assert 'invalid literal' in str(exc_info.value)</pre>

What does pytest.raises(ValueError) check in a test?

A That the test is skipped

B That no exceptions are raised

C That a ValueError exception is raised

D That the code runs without errors

pytest.raises(ValueError) expects a ValueError to be raised. If it is not, the test fails.

How do you write a test to check if a function raises an exception using pytest.raises?

A Call the function normally and catch exceptions manually

B Use <code>with pytest.raises(ExceptionType):</code> around the code

C Use <code>assertRaises</code> from unittest

D Use <code>try-except</code> without pytest

The recommended way in pytest is to use with pytest.raises(ExceptionType): as a context manager.

What happens if the expected exception is not raised inside the pytest.raises block?

A The test fails

B The test passes

C The test is skipped

D The test is marked as xfail

The test fails because the expected exception did not occur.

How can you check the message of an exception raised inside pytest.raises?

A Use <code>assertRaisesMessage</code>

B Use <code>print</code> inside the block

C You cannot check the message

D Capture the exception with <code>as exc_info</code> and check <code>exc_info.value</code>

Using as exc_info captures the exception object for message assertions.

Which of these is a correct use of pytest.raises?

A with pytest.raises(ValueError): int('abc')

B with pytest.raises(IndexError): [1,2,3][1]

C with pytest.raises(TypeError): int('123')

D with pytest.raises(KeyError): {'a':1}['a']

Only int('abc') raises a ValueError. The others do not raise the exceptions listed.

Explain how to use pytest.raises to test that a function raises an exception.

Think about wrapping the code that should fail inside a special block.

Describe how to capture and check the exception message using pytest.raises.

You can save the exception to a variable for more checks.