Mock return values and side effects in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mock return value in pytest?

A mock return value is a preset value that a mocked function returns when called during a test. It helps simulate behavior without running the real code.

intermediate

What does the side_effect attribute do in a pytest mock?

The side_effect attribute lets you define custom behavior for a mock, like raising exceptions or returning different values on each call.

intermediate

How do you set a mock to return different values on consecutive calls?

Use side_effect with a list of values. Each call to the mock returns the next value in the list.

beginner

Example: What will this mock return on the first and second call?

mocked_function.side_effect = [10, 20]

On the first call, it returns 10. On the second call, it returns 20. After that, it raises StopIteration.

intermediate

Why use side_effect to raise an exception in a mock?

To test how your code handles errors without causing real failures. It simulates exceptions from dependencies.

In pytest, how do you set a mock to always return the value 5?

A mock.return_value = 5

B mock.side_effect = 5

C mock.return = 5

D mock.value = 5

Use return_value to set a fixed return value for the mock.

What happens if side_effect is set to a list of values?

A The mock returns values from the list on each call, then raises StopIteration.

B The mock always returns the whole list.

C The mock raises an exception immediately.

D The mock ignores the list and returns None.

side_effect as a list makes the mock return each value in order, then raises StopIteration when exhausted.

How can you simulate an exception being raised by a mock?

A Set side_effect to the exception class or instance.

B Set return_value to the exception.

C Call mock.raise_exception()

D Set mock.error = True

Setting side_effect to an exception causes the mock to raise it when called.

Which attribute would you use to make a mock return different values on multiple calls?

A side_effect

B return_value

C call_count

D mock_values

side_effect can be a list to return different values on each call.

If you want a mock to behave like a function that sometimes raises an error, what should you do?

A Use side_effect with a function that raises the error conditionally.

B Set return_value to None.

C Set side_effect to a list of only values.

D Use return_value with an error message.

side_effect can be a function that raises exceptions or returns values based on logic.

Explain how to use mock return_value and side_effect in pytest to control mock behavior.

Think about how to simulate normal and error cases with mocks.

Describe a real-life scenario where you would use side_effect to raise an exception in a mock.

Imagine testing how your app reacts when a service is down.

Practice

1. What does setting mock.return_value do in a pytest mock object?

easy

A. It delays the mock response by a set time.

B. It defines a fixed value that the mock returns when called.

C. It records the number of times the mock was called.

D. It raises an exception when the mock is called.

Solution

1. Step 1: Understand the role of return_value

   The return_value attribute sets a fixed return value for the mock when it is called.

2. Step 2: Differentiate from other mock features

   Unlike side_effect which can raise exceptions or return different values, return_value always returns the same fixed value.

3. Final Answer:

   It defines a fixed value that the mock returns when called. -> Option B

4. Quick Check:

   return_value = fixed return [OK]

Hint: return_value sets fixed output for mock calls [OK]

Common Mistakes:

• Confusing return_value with side_effect
• Thinking return_value raises exceptions
• Assuming return_value tracks call count

2. Which of the following is the correct way to set a mock to raise an exception when called?

easy

A. mock.side_effect = Exception('Error')

B. mock.return_value = Exception('Error')

C. mock.raise_exception = Exception('Error')

D. mock.throw = Exception('Error')

Solution

1. Step 1: Identify how to raise exceptions with mocks

   Setting side_effect to an exception causes the mock to raise that exception when called.

2. Step 2: Check syntax correctness

   Only mock.side_effect = Exception('Error') is valid syntax to raise an exception on call.

3. Final Answer:

   mock.side_effect = Exception('Error') -> Option A

4. Quick Check:

   side_effect raises exceptions [OK]

Hint: Use side_effect to raise exceptions in mocks [OK]

Common Mistakes:

• Using return_value to raise exceptions
• Using non-existent attributes like raise_exception
• Confusing syntax for exception raising

3. What will be the output of this pytest mock code?

from unittest.mock import Mock
mock = Mock()
mock.side_effect = [1, 2, 3]
print(mock())
print(mock())
print(mock())

medium

A. 1 2 3

B. [1, 2, 3] [1, 2, 3] [1, 2, 3]

C. None None None

D. Error on third call

Solution

1. Step 1: Understand side_effect as a list

   When side_effect is a list, each call to the mock returns the next item in the list.

2. Step 2: Trace each call output

   First call returns 1, second returns 2, third returns 3, so printed outputs are 1, 2, 3.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   side_effect list returns items in order [OK]

Hint: side_effect list returns next item each call [OK]

Common Mistakes:

• Expecting entire list returned each call
• Confusing return_value with side_effect list
• Assuming calls after list ends raise error

4. Identify the error in this pytest mock setup:

from unittest.mock import Mock
mock = Mock()
mock.side_effect = Exception('Fail')
mock.side_effect = [1, 2, 3]
print(mock())

medium

A. side_effect cannot be assigned twice

B. Exception object should be raised, not assigned to side_effect

C. The second assignment overwrites the first, no error occurs

D. Mock must be called before setting side_effect

Solution

1. Step 1: Analyze multiple side_effect assignments

   The second assignment to side_effect replaces the first one; no error occurs.

2. Step 2: Understand mock call behavior

   After setting side_effect = [1, 2, 3], the mock returns 1 on first call, so print(mock()) outputs 1.

3. Final Answer:

   The second assignment overwrites the first, no error occurs -> Option C

4. Quick Check:

   Last side_effect assignment wins [OK]

Hint: Last side_effect assignment replaces previous ones [OK]

Common Mistakes:

• Thinking multiple side_effect assignments cause errors
• Expecting exception to be raised immediately
• Confusing assignment with raising exceptions

5. You want a mock to return 10 on the first call, raise a ValueError on the second, and return 20 on the third call. Which setup achieves this?

hard

A. mock.side_effect = [10, ValueError('bad'), 20]

B. mock.return_value = 10
mock.side_effect = ValueError('bad')
mock.return_value = 20

C. mock.side_effect = lambda: [10, ValueError('bad'), 20]

D. mock.side_effect = [10, lambda: (_ for _ in ()).throw(ValueError('bad')), 20]

Solution

1. Step 1: Understand side_effect list behavior

   When side_effect is a list, the mock returns successive items from the list on each call. Non-callable items like numbers or exception instances are returned directly.

2. Step 2: Recognize exception raising in side_effect list

   Placing an exception instance directly returns it as a value, not raises it. To raise, use a generator expression that throws the exception.

3. Step 3: Analyze options

   mock.side_effect = [10, ValueError('bad'), 20] returns the exception object instead of raising it. mock.side_effect = [10, lambda: (_ for _ in ()).throw(ValueError('bad')), 20] uses a generator expression to raise ValueError on second call, returning 10 and 20 on first and third calls respectively.

4. Final Answer:

   mock.side_effect = [10, lambda: (_ for _ in ()).throw(ValueError('bad')), 20] -> Option D

5. Quick Check:

   Use generator throw to raise exceptions in side_effect list [OK]

Hint: Use generator throw to raise exceptions in side_effect list [OK]

Common Mistakes:

• Putting exception instance directly in side_effect list
• Trying to set multiple return_values
• Using lambda returning list instead of generator throw