Mock return values and side effects in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this pytest mock with return_value?

Consider the following pytest test code using mock. What will be printed when the test runs?

PyTest

from unittest.mock import Mock

def fetch_data():
    return 'real data'

mock_fetch = Mock(return_value='mocked data')

result = mock_fetch()
print(result)

A mocked data

B real data

C None

D TypeError

💡 Hint

The mock object returns what you set as return_value when called.

The mock object mock_fetch is set to return 'mocked data' whenever called. So calling mock_fetch() prints 'mocked data'.

❓ assertion

intermediate

Which assertion correctly tests a mock's side effect raising an exception?

You have a mock object with side_effect set to raise a ValueError. Which assertion correctly verifies that calling the mock raises this error?

PyTest

from unittest.mock import Mock
import pytest

mock_func = Mock(side_effect=ValueError('error'))

A

with pytest.raises(ValueError):
    mock_func()

B assert mock_func() == ValueError

C assert mock_func.side_effect == ValueError

D assert mock_func() raises ValueError

💡 Hint

Use pytest.raises context manager to check exceptions.

The pytest.raises context manager is used to assert that a block of code raises a specific exception. Calling mock_func() inside it will raise ValueError as set by side_effect.

🔧 Debug

advanced

Why does this mock side_effect list cause an error on the third call?

Examine the code below. What error occurs on the third call to mock_func and why?

PyTest

from unittest.mock import Mock

mock_func = Mock(side_effect=[1, 2])
print(mock_func())
print(mock_func())
print(mock_func())

A StopIteration because side_effect list is exhausted

B IndexError because list index out of range

C TypeError because side_effect is not callable

D No error, prints 1, 2, None

💡 Hint

side_effect list acts like an iterator and raises StopIteration when empty.

When side_effect is a list, mock uses it as an iterator. After the last item, calling the mock raises StopIteration.

🧠 Conceptual

advanced

What is the difference between return_value and side_effect in mocks?

Choose the best explanation of how return_value and side_effect differ in mock objects.

A return_value sets the value returned by the mock; side_effect can be a function, exception, or iterable to control behavior

B return_value raises exceptions; side_effect returns fixed values

C return_value is used only for async mocks; side_effect only for sync mocks

D return_value and side_effect are interchangeable and do the same thing

💡 Hint

Think about how you can simulate different behaviors with side_effect.

return_value sets a fixed return value. side_effect can be a function to run, an exception to raise, or an iterable to return different values on each call.

❓ framework

expert

How to mock a function to return different values on consecutive calls in pytest?

You want to mock a function so that it returns 10 on the first call, 20 on the second, and then raises RuntimeError on the third call. Which mock setup achieves this?

A mock_func = Mock(side_effect=[10, 20, RuntimeError('fail')])

B mock_func = Mock(side_effect=[10, 20, lambda: (_ for _ in ()).throw(RuntimeError('fail'))])

C mock_func = Mock(side_effect=[10, 20, RuntimeError])

D mock_func = Mock(return_value=[10, 20, RuntimeError('fail')])

💡 Hint

side_effect list can include values or callables; exceptions must be raised, not returned.

Option B uses a lambda that raises RuntimeError on call, so the third call raises the exception. Option A returns the exception object instead of raising it. Option C uses the exception class, which is not raised. Option D sets return_value to a list, so all calls return the list.