Mock call assertions in PyTest - Build an Automation Script

Verify mock function call and arguments using pytest

Preconditions (2)

Step 1: Create a mock object for the dependent function

Step 2: Call the main function that uses the mock

Step 3: Assert that the mock was called exactly once

Step 4: Assert that the mock was called with the expected arguments

✅ Expected Result: Test passes confirming the mock function was called once with the correct arguments

Automation Requirements - pytest with unittest.mock

Assertions Needed:

assert mock_function.called is True

assert mock_function.call_count == 1

assert mock_function.call_args == expected_args

Best Practices:

Use patch decorator or context manager to mock dependencies

Use explicit assertions for call count and call arguments

Keep tests isolated and independent

Automated Solution

PyTest

from unittest.mock import Mock, patch
import pytest

# Function to test

def greet_user(user_service, user_id):
    user_name = user_service.get_user_name(user_id)
    return f"Hello, {user_name}!"

# Test function
@patch('test_module.UserService')
def test_greet_user_calls_get_user_name(mock_user_service_class):
    # Arrange
    mock_user_service = Mock()
    mock_user_service.get_user_name.return_value = 'Alice'
    mock_user_service_class.return_value = mock_user_service
    user_id = 42

    # Act
    from test_module import greet_user
    result = greet_user(mock_user_service, user_id)

    # Assert
    assert mock_user_service.get_user_name.called is True
    assert mock_user_service.get_user_name.call_count == 1
    mock_user_service.get_user_name.assert_called_with(user_id)
    assert result == 'Hello, Alice!'

The test uses patch to replace UserService with a mock object.

We create a mock instance and set the return value for get_user_name.

We call the greet_user function with the mock and a user ID.

Assertions check that get_user_name was called once with the correct argument.

Finally, we verify the returned greeting string is as expected.

Common Mistakes - 3 Pitfalls

Not asserting the call count, only checking if called

Using hardcoded arguments in assertions without matching the actual call

Not using patch or mock properly, leading to real function calls

Bonus Challenge

Now add data-driven testing with 3 different user IDs and expected names

Show Hint

Practice

1. What does the assert_called_with method check in pytest mocks?

easy

A. It checks if the mock was called exactly twice.

B. It checks if the mock was never called.

C. It checks if the mock returned a specific value.

D. It checks if the last call to the mock was with specific arguments.

Solution

1. Step 1: Understand the purpose of assert_called_with

   This method verifies that the last call to the mock was with the exact arguments provided.

2. Step 2: Differentiate from other assertions

   Unlike assert_called_once_with, it does not require the call to be exactly once, just that the last call had those arguments.

3. Final Answer:

   It checks if the last call to the mock was with specific arguments. -> Option D

4. Quick Check:

   assert_called_with = last call with args [OK]

Hint: assert_called_with checks the last call's args [OK]

Common Mistakes:

• Confusing assert_called_with with assert_called_once_with
• Thinking it checks call count exactly
• Assuming it checks return values

2. Which of the following is the correct syntax to assert a mock was called exactly once with arguments 5 and 'test'?

easy

A. mock.assert_called_once_with(5, 'test')

B. mock.assert_called_with(5, 'test')

C. mock.assert_called_once(5, 'test')

D. mock.assert_called(5, 'test')

Solution

1. Step 1: Identify the correct assertion method

   The method to check a single call with specific arguments is assert_called_once_with.

2. Step 2: Verify syntax correctness

   The syntax requires calling this method on the mock object with the expected arguments inside parentheses.

3. Final Answer:

   mock.assert_called_once_with(5, 'test') -> Option A

4. Quick Check:

   assert_called_once_with = exactly one call with args [OK]

Hint: Use assert_called_once_with(args) for exact single call [OK]

Common Mistakes:

• Using assert_called_with instead of assert_called_once_with
• Calling assert_called_once without _with
• Passing arguments to assert_called which takes no args

3. Given the code:

from unittest.mock import Mock
mock = Mock()
mock(1, 2)
mock(3, 4)
mock.assert_called_with(3, 4)

What will happen when the last line runs?

medium

A. The assertion passes because the last call was with (3, 4).

B. The assertion fails because the first call was with (1, 2).

C. The assertion fails because assert_called_with checks all calls.

D. The code raises a TypeError.

Solution

1. Step 1: Understand assert_called_with behavior

   This assertion checks that the last call to the mock was with the specified arguments.

2. Step 2: Analyze the calls made

   The mock was called twice: first with (1, 2), then with (3, 4). The last call matches the assertion arguments.

3. Final Answer:

   The assertion passes because the last call was with (3, 4). -> Option A

4. Quick Check:

   assert_called_with checks last call args [OK]

Hint: assert_called_with checks last call args by default [OK]

Common Mistakes:

• Thinking assert_called_with checks all calls
• Confusing call order importance
• Expecting failure due to earlier calls

4. Identify the error in this test code snippet:

from unittest.mock import Mock
mock = Mock()
mock(10)
mock.assert_called_once(10)

medium

A. assert_called_once should be called before the mock call.

B. Mock object was never called, so assertion fails.

C. assert_called_once does not accept arguments; use assert_called_once_with instead.

D. The mock call should use keyword arguments.

Solution

1. Step 1: Check method usage

   assert_called_once checks the call count but does not accept arguments to check call parameters.

2. Step 2: Correct method for argument check

   To check the call arguments and count, assert_called_once_with should be used instead.

3. Final Answer:

   assert_called_once does not accept arguments; use assert_called_once_with instead. -> Option C

4. Quick Check:

   assert_called_once no args; assert_called_once_with with args [OK]

Hint: Use assert_called_once_with(args) to check call count and args [OK]

Common Mistakes:

• Passing arguments to assert_called_once
• Confusing call order
• Assuming assert_called_once checks arguments

5. You have a mock that was called three times with arguments:
(1, 'a'), (2, 'b'), (3, 'c')
Which assertion correctly verifies all calls in order?

hard

A. mock.assert_called()

B. mock.assert_has_calls([call(1, 'a'), call(2, 'b'), call(3, 'c')], any_order=False)

C. mock.assert_called_once_with(3, 'c')

D. mock.assert_called_with(1, 'a') and mock.assert_called_with(3, 'c')

Solution

1. Step 1: Understand verifying multiple calls

   To check multiple calls in order, assert_has_calls with a list of expected calls and any_order=False is used.

2. Step 2: Analyze other options

   mock.assert_called() only checks if it was called at least once without verifying arguments or order. Chaining mock.assert_called_with(1, 'a') and mock.assert_called_with(3, 'c') checks some arguments but not all calls, order, or completeness. mock.assert_called_once_with(3, 'c') checks only that there was exactly one call with those arguments.

3. Final Answer:

   mock.assert_has_calls([call(1, 'a'), call(2, 'b'), call(3, 'c')], any_order=False) -> Option B

4. Quick Check:

   assert_has_calls with any_order=False checks ordered calls [OK]

Hint: Use assert_has_calls with any_order=False for ordered multiple calls [OK]

Common Mistakes:

• Using assert_called_with for multiple calls
• Ignoring call order
• Not importing call from unittest.mock