Mock return values and side effects in PyTest

Introduction

We use mock return values and side effects to control what a fake function gives back during tests. This helps us test parts of code without running everything for real.

When a function calls an external service like a website or database and you want to test without using the real service.

When you want to check how your code behaves if a function returns different results.

When you want to simulate errors or special cases from a function to see how your code handles them.

When the real function is slow or changes data and you want fast, safe tests.

When you want to test only one part of your code and ignore others.

Syntax

PyTest

from unittest.mock import Mock

mock_function = Mock()
mock_function.return_value = value

# or for side effects
mock_function.side_effect = function_or_exception_or_list

return_value sets what the mock gives back when called.

side_effect can be a function to run, an error to raise, or a list of values/errors for each call.

Examples

This mock always returns 10 when called.

PyTest

mock = Mock()
mock.return_value = 10
print(mock())  # prints 10

The mock runs the side effect function and returns its result.

PyTest

def side_effect_func():
    return 'hello'

mock = Mock()
mock.side_effect = side_effect_func
print(mock())  # prints 'hello'

The mock returns different values on each call from the list.

PyTest

mock = Mock()
mock.side_effect = [1, 2, 3]
print(mock())  # prints 1
print(mock())  # prints 2
print(mock())  # prints 3

The mock raises an exception when called.

PyTest

mock = Mock()
mock.side_effect = Exception('Error!')
try:
    mock()
except Exception as e:
    print(e)  # prints 'Error!'

Sample Program

This example shows a real function and a mock version. The mock first returns a set value, then raises an error using side_effect.

PyTest

from unittest.mock import Mock

def greet():
    return 'Real Greeting'

mock_greet = Mock()
mock_greet.return_value = 'Mocked Greeting'

print(greet())        # Calls real function
print(mock_greet())   # Calls mock with return_value

# Using side_effect to simulate error
mock_greet.side_effect = Exception('No greeting available')
try:
    print(mock_greet())
except Exception as e:
    print(e)

Important Notes

Use mocks to isolate the part of code you want to test.

Remember to reset mocks if you reuse them in multiple tests.

Side effects can simulate complex behaviors like errors or changing return values.

Summary

Mocks let you control what functions return during tests.

Use return_value for simple fixed returns.

Use side_effect to simulate errors or dynamic behavior.

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