Mock return values and side effects in PyTest - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a mock's return value.

PyTest

from unittest.mock import Mock
mock_obj = Mock()
mock_obj.method.return_value = [1]
assert mock_obj.method() == 10

Drag options to blanks, or click blank then click option'

A10

B'10'

CNone

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '10' instead of integer 10

Setting return_value to None

The return_value sets what the mock method returns. Here, it should be the integer 10, not a string.

2fill in blank

medium

Complete the code to set a mock's side effect to raise an exception.

PyTest

from unittest.mock import Mock
mock_obj = Mock()
mock_obj.method.side_effect = [1]
try:
    mock_obj.method()
except ValueError:
    caught = True
else:
    caught = False
assert caught is True

Drag options to blanks, or click blank then click option'

AException('error')

BRuntimeError('error')

CValueError('error')

DTypeError('error')

Attempts:

3 left

💡 Hint

Common Mistakes

Using Exception instead of ValueError

Using a different exception type

The side_effect can be set to an exception instance to make the mock raise it when called. The test expects a ValueError.

3fill in blank

hard

Fix the error in setting a mock's side effect to return different values on each call.

PyTest

from unittest.mock import Mock
mock_obj = Mock()
mock_obj.method.side_effect = [1]
assert mock_obj.method() == 1
assert mock_obj.method() == 2
assert mock_obj.method() == 3

Drag options to blanks, or click blank then click option'

A1, 2, 3

B(1, 2, 3)

C{1, 2, 3}

D[1, 2, 3]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a set which is unordered

Using a tuple which works but list is preferred for mutability

The side_effect can be set to a list to return each value on successive calls. A list preserves order and allows repeated calls.

4fill in blank

hard

Fill both blanks to create a mock that returns 5 and then raises an exception on the second call.

PyTest

from unittest.mock import Mock
mock_obj = Mock()
mock_obj.method.side_effect = [[1], [2]]
assert mock_obj.method() == 5
try:
    mock_obj.method()
except ValueError:
    caught = True
else:
    caught = False
assert caught is True

Drag options to blanks, or click blank then click option'

A5

BValueError('fail')

CValueError

DException('fail')

Attempts:

3 left

💡 Hint

Common Mistakes

Using exception type instead of instance in side_effect list

Mismatch between exception raised and caught

The side_effect list must have the return value 5 first, then the exception instance. The except block catches the exception type, which is ValueError.

5fill in blank

hard

Fill all three blanks to create a mock that returns the length of input string, then raises TypeError, then returns 'done'.

PyTest

from unittest.mock import Mock
mock_obj = Mock()
mock_obj.method.side_effect = [lambda s: len(s), [1], [2]]
assert mock_obj.method('test') == 4
try:
    mock_obj.method('fail')
except [3]:
    caught = True
else:
    caught = False
assert caught is True
assert mock_obj.method('end') == 'done'

Drag options to blanks, or click blank then click option'

ATypeError('wrong type')

BValueError('error')

Cdone

DTypeError

Attempts:

3 left

💡 Hint

Common Mistakes

Using exception type instead of instance in side_effect list

Mismatch between exception raised and caught

The side_effect list has a lambda returning length, then a TypeError instance, then a string 'done'. The except block catches TypeError type.

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